Displacement: There are several different types of displacement reactions, including single, double, and acid-base.
An example of a single substitution reaction (A + BC AC + B) occurs when zinc combines with hydrochloric acid. The zinc replaces the hydrogen: Zn + 2 HCl ZnCl2 + H2
This reaction happens when simple compounds come together to form a more complicated compound.
The iron (Fe) in a nail combines with oxygen (O2) to form rust, also called iron oxide (Fe2O3).
2Fe + O2 Fe2O3
We’re about to do a synthesis reaction with sulfur. Sulfur is element #6 on the periodic table. Sulfur is used in fertilizer, black powder, matches, and insecticides. In pioneer times sulfur was put into patent medicines and used as a laxative.
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A combustion reaction gives off energy, usually in the form of heat and light. The reaction itself includes oxygen combining with another compound to form water, carbon dioxide, and other products.
A campfire is an example of wood and oxygen combining to create ash, smoke, and other gases. Here’s the reaction for the burning of methane (CH4) which gives carbon dioxide (CO2) and water (H2O):
CH4 + 2 O2 CO2 + 2 H2O
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If you guessed that electrochemistry has to do with electricity and chemistry, you’re right! But you might wonder how they work together. Back in 1800, William Nicholson and Johann Ritter were the first ones to split water into hydrogen and oxygen using electrolysis. (Soon afterwards, Ritter went on to figure out electroplating.) They added energy in the form of an electric current into a cup of water and captured the bubbles forming into two separate cups, one for hydrogen and other for oxygen.
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Chemical equilibrium is the condition that happens when the concentration of the reactants and products don’t have any net change over time. This doesn’t mean that the reaction stops, just that the producing and consuming of the molecules is in balance.
Most chemical reactions are reversible, just like phases changes. Do you remember the hot icicle experiment? Do you remember how to get it back to the starting point? You have to add energy to the solid sodium acetate to turn it back into a liquid, so it can turn back into a solid again. Then let that experiment sit for a bit (overnight or about 12 hours) and in the morning, you’ll have crystals growing on your pipe cleaner. Now if you want to reverse this reaction, all you have to do is add energy to the system and the crystals will dissolve back into the solution. You can heat it up in the microwave or in a pot of water on the stove, and the crystals will disappear.
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Plasma makes up a very large percentage of the matter in the universe. Not much of it is on Earth and the plasma that is here is very short lived or stuck in a tube. Plasma is basically what happens when you add enough energy to a gas that the atoms move and vibrate around so energetically that they smack into each other and rip electrons off each other, so you have positively charged atoms (called ions) that lost their electrons, and also the electrons themselves which are negatively charged, all zinging around in the gas.
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Solids
What makes the solids, liquids, gases etc. different is basically the energy (motion) of the atoms. From BEC, where they are so low energy that they are literally blending into one another, to plasma, where they are so high energy they can emit light. Solids are the lowest energy form of matter that exist in nature (BEC only happens under laboratory conditions).
In solids, the atoms and molecules are bonded (stuck) together in such a way that they can’t move easily. They hold their shape. That’s why you can sit in a chair. The solid molecules hold their shape and so they hold you up. The typical characteristics that solids tend to have are they keep their shape unless they are broken and that they do not flow.
A liquid has a definite volume (meaning that you can’t compress or squish it into a smaller space), but takes the shape of its container. Think of a water-filled balloon. When you smoosh one end, the other pops out. Liquids are generally incompressible, which is what hydraulic power on heavy duty machinery (like excavators and backhoes) is all about.
Here are the most important things about gases to remember:
By knowing the value of the bond energy, we can predict if a chemical reaction will be exothermic or endothermic. If the bonds in the products are stronger than the bonds in the reactants, then the products are more stable and the reaction will give off heat (exothermic).
Exothermic chemical reactions release energy as heat, light, electrical or sound (or all four). Usually when someone says it’s an exothermic reaction, they usually just mean energy is being released as heat.
Some release heat gradually (for example, a disposable hand-warmer), while others are more explosive (like burning magnesium). The energy comes from breaking the bonds within the chemical reaction.
A molecule is the smallest unit of a compound that still has the compound’s properties attached to it. Molecules are made up of two or more atoms held together by covalent bonds.
In the space where electrons from different atoms interact with each other, chemical bonds form. The electrons in the outermost shell are the ones that form the bonds with other atoms.
When the atoms share the electron(s), a covalent bond is formed. Electrons aren’t perfect, though, and usually an electron is more attracted to one atom than another, which forms a polar covalent bond between atoms (like in water, H2O).
While it may seem a bit random right now, with a little bit of study, you’ll find you can soon understand how molecules are formed and the shapes they choose once you figure out the types of bonds that can form.
There are different kinds of bonds that can form in a molecule. When two atoms approach each other close enough for their electron clods to interact, the electrons of one repels the electrons in the other, and the same thing happens within the nucleus of the atoms. At the same time, each atom’s negatively charged electron is attracted to the other atom’s positively charged nucleus. If the atoms still come closer, the attractive forces offset the repulsive and the energy of the atom decreases and bonds are formed – the atom sticks together. When the energy decrease is small, the bonds are van der Waals. When the energy decrease is larger, we have chemical bonds, either ionic or covalent.
You’re going to try to determine what is happening during the flame test when you see different colors. Think about what particles are found in the chemicals you’re using, and why the different chemicals emit different colors of light? Where else have you seen colorful light emissions?
Did you aim your razor slit at a light source such as a fluorescent light, neon sign, sunset, light bulb, computer screen, television, night light, candle, fireplace… ? Make sure that the diffraction grating does right up to your eye. Move the spectrometer around until you can get the rainbow to be on the scale inside the tube.
Once you’ve got the hang of it, you might be wondering, wow – cool… but what am I looking at exactly? Ok – so those lines you saw inside the tube – those are spectral lines. Can you see how there are brighter lines? Which frequencies are those? Well we need a ruler to measure those. Can you see how if we lined up a ruler as could tell what the frequencies are?
Energy can take one of two forms: matter and light (called electromagnetic radiation). Light is energy that can travel through space. When you feel the warmth of the sun on your arm, that’s energy from the sun that traveled through space as infrared radiation (heat). When you see a tree or a bird, that’s light from the sun that traveled as visible light (red, orange… the whole rainbow) reflecting and bouncing off objects to get to your eye. Light can travel through objects sometimes… like the glass in a window.
Light can take the form of either a wave or a particle, depending on what you’re doing with it. It’s like a reversible coat – fleece on the inside, windbreaker on the outside. It can adapt to whatever environment you put it in.
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One of the dreams of early chemists was to figure out how to transform lead into gold. Lead has 82 protons in its core whereas gold contains only 79. So conceivably all you’d need to do is remove three protons and presto! So how do you do that? Since protons can’t be stripped off with a chemical reaction, you need to smack it hard with something to knock off just the right amount. Lead, however, if a very stable element, so it’s going to require a lot of energy to remove three protons. How about a linear accelerator?
In a linear accelerator, a charged particle moves through a series of tubes that are charged by electrical and/or magnetic fields. The accelerated particle smacks the target, knocking free protons or neutrons and making a new element (or isotope). Glenn Seaborg (I actually met him!), 1951 Nobel Laureate in Chemistry, actually succeeded in transmuting a tiny quantity of lead into gold in 1980. He actually discovered (or helped discover) 10 elements on the periodic table, 100 new isotopes, and while he was still living (which usually doesn’t happen), they named an element after him (Seaborgium – 106).
Naturally radioactive elements emit energy without absorbing it first. Fluorescence for example – the atom absorbs a photon before emitting another photon. You have to “charge it up” or mix chemicals together before light comes out. With radioactive materials, they emit energy on their own, sometimes in the form of light, but sometimes they emit other particles. Let me explain.
Chemical reactions usually deal with only electron or atom exchanges. Nuclear reactions deal with changes inside the nucleus of an atom.
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Which one of these things you see on the screen now is radioactive? Most kids think that anything that glows must be radioactive, but it turns out that there’s a lot of things that glow that aren’t radioactive at all. Many minerals (called phosphors) glow after being exposed to sunlight which contains UV light. In 1897, Henri Becquerel was studying phosphorescence when he accidentally discovered radioactivity. Naturally radioactive elements emit energy without absorbing it first. Let me explain…
Cold light refers to the light from a glow stick, called luminescence. A chemical reaction (chemiluminescence) starts between two liquids, and the energy is released in the form of light. On the atomic scale, the energy from the reaction bumps the electron to a higher shell, and when it relaxes back down it emits a photon of light. Glow sticks generate light with very little heat, just like the glow you see from fireflies, jellyfish, and a few species of fungi. Chemiluminescence means light that comes from a chemical reaction.
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Okay, so now I want you to imagine a room full of ping pong balls that can bounce all by themselves. They go zipping all over the place all on their own. Now take those ping pong balls and add energy to them so now they bounce twice as fast. Got it?
Now what happens if we take away energy from them? Do they bounce slower? Yup!
Okay, now get them back to their original bouncing speed. Now take the room and make it smaller, like half it’s size, but keep the ping pong ball speed the same. Do they hit the walls more or less frequently? More! Are they speeding up or slowing down? Speeding up!
Find a low pressure (like the pressure you feel right now – it’s called 1 atm). Put your finder on the 1 mark on the vertical side (next to the “P”, which stands for Pressure) and follow the dashed line straight across. As you move across, so you notice how at low temperatures you’re in the ice region, but when you hit zero, you turn to water, and for temperatures below 100 deg C you’re only in the liquid water phase?
When something changes state, goes from like a liquid to a solid, all of the substance must change to the next state. For example, at 100° C all the water must change from a liquid to a gas. The temperature stays constant until it’s completely changed state. It’s kind of weird when you think about it.
If you were able to take the temperature of water as it changed from a solid (ice) to a liquid you would notice that the temperature stays at 32° F until that piece of ice was completely melted. The temperature would not increase at all.
Even if that ice was in an oven, the temperature would stay the same. Once all the solid ice had disappeared, then you would see the temperature of the puddle of water increase.
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Do you remember when I said that heat and temperature are two different things? Heat is energy – it is thermal energy. It can be transferred from one object to another.
Here’s what you do:
They can have a thermal energy but they can’t have heat. Heat is really the transfer of thermal energy. Or, in other words, the movement of thermal energy from one object to another.
If you put an ice cube in a glass of lemonade, the ice cube melts. Which way does heat flow?
The thermal energy from your lemonade moves to the ice cube.
The movement of thermal energy is called heat. The ice cube receives heat from your lemonade. Your lemonade gives heat to the ice cube.
Thermal energy is how much the molecules are moving inside an object. The faster molecules move, the more thermal energy it has.
Objects whose molecules are moving very quickly are said to have high thermal energy or high temperature. Like a cloud of steam, for example. The higher the temperature, the faster the molecules are moving.
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Energy is the capacity to do work or to transfer heat. You do work when you walk up a flight of stairs. You can feel the heat from the sun when you step in the sunlight. Both are energy.
Heat is associated with changing the temperature of an object. The temperature changes because energy is being transferred to it. Another word for heat is thermal energy.
Thermochemistry is the science of heat or thermal energy transfer and how to use it with chemical reactions.
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Your silver turns black because of the presence of sulfur in food. Here’s how the cleaning works: The tarnished spoon has silver sulfide on it, and when you put it in the solution, the silver sulfide combines with the baking soda and salt in the water solution to break apart into sulfur (which gets deposited on the foil) and silver (which goes back onto the spoon). Using the heat from your stove, you’ve just relocated the tarnish from the spoon to the foil. Just rinse clean and wipe dry!
The oxidation number of an element is the charge the atom has
I. For an atom in its elemental form the oxidation number is zero.
II. For any monatomic ion the oxidation number equals the charge of the ion.
III. For nonmetals the oxidation number is usually negative.
a) Oxygen is usually -2 in all compounds.
b) Fluorine is -1 in all compounds.
c) Hydrogen is +1 when bonded to nonmetals and -1 when bonded to metals (metal hydrides).
IV. The sum of the oxidation numbers for all atoms is zero for neutral
compounds or equals the charge for polyatomic ions.
Sterling silver is an alloy (a solid solution) of silver and copper. In order to find the percent of silver, you have to break it apart from the copper, which will make it an ion floating around in a liquid. Then you will need to bond it to something that will make it turn back into a solid so you can measure it.
In order to mix up chemicals in the right amounts (so we get the right amount out of the reaction), we have to figure out how much of a chemical to put in in the first place. Sometimes chemists have this problem: they need for example 2.0 L of 1.5 M solution of Na2CO3 (sodium carbonate). They find a bottle of Na2CO3 on the shelf, some distilled water, and a 2.00L flask. How much Na2CO3 do they put in the flask with the water?
Gas forming reactions are also exchange reactions. The best example I can think of for this type of reaction is what happens when you put a piece of chalk in a cup of vinegar. The chalk, which is mostly CaCO3 (calcium carbonate) and vinegar (acetic acid) forms calcium chloride and carbonic acid, which isn’t stable and quickly turns into water and carbon dioxide. A faster version of this experiment is what happens when you take an effervescent tablet, like alka seltzer, and stick it in water, because the tablet is actually a solid form of baking soda and vinegar put together. What happens when you mix baking soda and vinegar together?
Strong acids and strong bases (which we’ll talk about in a minute) all have one thing in common: they break apart (completely dissociate) into ions when placed in water. This means that once you dunk the acid molecule in water, it splits apart and does not exist as a whole molecule in water. Strong acids form H+ and a negative ion
The seven strong acids are: hydrochloric acid (HCl), nitric acid (HNO3) used in fireworks and explosives, sulfuric acid (H2SO4) which is the acid in your car battery, hydrobromic acid (HBr), hydroiodic acid (HI), and perchloric acid (HClO4). The record-holder for the world’s strongest acid are the carborane (CAR-bor-ane) superacids (over a million times stronger than concentrated sulfuric acid).
Precipitate reactions are like watching a snow globe, but the snow appears out of nowhere.
For example, you can combine two liquid solutions that are totally clear and when you put them together, they each break apart into ions and then recombine in a way that looks like white snow in your test tube. Basically precipitate reactions make it possible to see the ions in a solution because they form a salt that’s not soluble – it doesn‘t dissolve in the solution. You can also get different colors of the precipitate snow, depending on which reactants you start out with. If you were to use potassium bromide (KBr) with silver nitrate, you’d find a yellowish snowstorm of silver bromide (AgBr).
When a substance is mixed with water it’s called an aqueous solution. Solubility is a property that a solid, liquid, or gases has when mixed with a solvent. If it can dissolve into the solvent, then it’s soluble. Dissolving marbles in water is a physical change. The marbles don’t break apart in the water to form new molecules with the water.
A decomposition reaction breaks a complicated molecule into simpler ones usually by heating, but not always. In fact, if you leave a bottle of hydrogen peroxide on the counter, it decomposes into water (H2O) and oxygen (O2) without any heating at all. 2H2O 2O2 + 2H2
A very common type of decomposition is shown by the chemistry of metal carbonates. Calcium, one of the most abundant elements on earth, usually is locked up in limestone, called calcium carbonate. CaCO3. When heated to about 1000 degrees C, it decomposes to make lime (a solid metal oxide) and CO2 gas. Chemical engineers make more then 348 million tonnes of lime to make steel, cement and other chemicals.
If you have one element, like sulfur, which is S, and it’s a negative ion, just add “ide” to the end, like sulfide. Or if you have a carbon ion, it’s carbide. Nitrogen would be nitride, chlorine would be chloride.
If there’s more than one atom, especially if one of them is oxygen, then they have special names. The one with more oxygen atoms is the “ate” and the one with less is the “ite”. Sulfate has 4 oxygen atoms, and sulfite only has 3. Nitrate has three oxygen, and nitrite has only 2.
If there’s more than two ions, the one with the largest number of atoms gets the “per” and “ate”, like perchlorate. And the smallest one gets the “hypo” and “ite”, like hypochlorite.
A lot of chemical reactions happen in a solution (it allows the chemicals to interact much more easily with each other when it is), so chemists define how much of the solute is in the solution by the term MOLARITY.
Molarity is a really convenient unit of concentration and it works like this. If I have 10 moles of solute in 10 liters of water, what’s the molarity? 10/10 = 1! So it’s a 1M solution. What if I have 20 moles in 10 liters? Then it’s a 2M solution. See how easy that is?
Mole means “heap” or “pile” and is a unit for measuring the amount of a pure substance. It’s a chemist’s dozen. It’s a lot bigger than 12 though. It’s 6.022 x 10^23. So if you had a mole of eggs, you’d have… that huge number at the bottom of the slide. The most confusing part is this…
Elements are arranged so that the ones with similar chemical and physical properties are stacked in vertical groups, and there are 8 groups (see the numbers at the top?) with either an A or B after the number? I know they’re written in Roman… just remember that IV means four, and VI means six. Sometimes you’ll see them numbered 1-18 starting with hydrogen on the left.
The rows are called periods. Now point to the metals… what colors are those? There are lots of them!
Atoms are made of protons, neutrons, and electrons. The protons and the neutrons make up the nucleus (the center) of the atom. The electron lives outside the nucleus in an electron cloud and are way too small to see. Protons and neutrons are made up of smaller little particles, which are made of smaller little particles and so on. Atoms can have anywhere from only one proton and one electron (a hydrogen atom) to over 300 protons, neutrons and electrons in one atom. It is the number of protons that determines the kind of atom an atom is, or in other words, the kind of element that atom is. How many protons does Zinc have?
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Matter that is made of only one kind of atom is an element, like helium. Helium likes to hang out in groups of two helium atoms.
An atom is the smallest particle of an element that still has its chemical properties. If you have a gold atom and you split it into smaller parts (which you can do), it won’t still act like it did chemically as it did when it was a whole atom.
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When doing your experiments, you’ll often repeat an experiment again and again for various reasons. One reason is to make sure the experiment you’re doing is repeatable – it’s not just a one-time thing. You might also be checking to be sure you’ve done it right, or written down the amounts of chemicals correctly, or need to observe something you didn’t previously.
Precision measures how well your answers agree with each other from experiment to the next.
Read the temperature from the thermometer… what do you get? This thermometer is reading in Celsius.
We’ll cover thermometers and the four temperature scales in a bit when we get to thermochemistry, but I just wanted to make sure we’re all on the same page when it comes to reading a thermometer, especially now that so many are digital and some kids may have not yet had the experience of reading a temperature scale.
If you’re going to do a chemistry experiment, you’re going to use chemicals. How much of each one you use is going to change the results you get, so it’s important to find a way to accurately measure out the same amount of chemical each time.
We already talked about how matter is anything that takes up space, like air, kittens, your left armpit… Mass can exist in different states. What are they?
Solid, liquid and gas. You also know about two more additional states: what are they? Plasma and BEC! Can matter exist in more than one state at a time? Sure – ever had a glass of water? That has liquid water and solid water molecules (ice) at the same time!
Glow sticks generate light with very little heat, just like the glow you see from fireflies, jellyfish, and a few species of fungi. Chemiluminescence means light that comes from a chemical reaction. When this happens in animals and plants, it’s called bioluminescence.
In a glow stick, when you bend it to activate it, you’re breaking a little glass tube inside which contains hydrogen peroxide (H2O2). The tube itself is filled with another chemical (phenyl oxalate ester and a fluorescent dye) that is kept separate from the H2O2, because as soon as they touch, they begin to react. The dye in the light stick is what gives the light its color.
Everyone old enough to remember the Rubik’s Cube craze of the 1980s in the USA also remembers how it was near impossible to solve the thing! Originally created by a professor of architecture Erno Rubik, it was sold to a toy company in 1980 as the “Magic Cube”.
To date, over 350 million cubes have been sold worldwide, making it the world’s top selling puzzle game, and most people think of it as the best-selling toy of all time as well.
The original goal of creating this object was to help teach his students how to create something that rotated independently in layers without falling apart. Rubik didn’t realize he had created a puzzle until he scrambled it, and it took him over a month to solve it the first time!
There are eight corners and twelve edges, and when you do the math to figure out the number of possible combinations the puzzle has, it’s about 43 quintillion, or:
43,252,003,274,489,856,000
So what do you do with this thing? How DO you solve it?
It has to do with identifying the different layers, and solving one layer at a time. Here’s how you can do it:
There’s also a World Cube Association where folks keep track of cube competitions and records. The fastest cube solve was set by Mats Valk in 2013 – he can solve it in under 6 seconds. Some of the more creative competitions include solving the cube while blindfolded (record is 23.8 seconds), with only one hand (record is 12.6 seconds), only using the feet (record is 27.93 seconds), and underwater using a single breath.
Kaleidocycles are a three-dimensional paper sculpture you can turn around and round! Flexagons were first created by Arthur Stone at Princeton University in 1939, which were later published in 1959 to the general public in Scientific American.
These are simple to make and fun to play with. When I first showed them to my own kids, they immediately made one for each kid in their class, and also stumped the teacher that day when they asked how it worked. Please login or register to read the rest of this content.
The video below is made by Vi Hart, a smart and spunky mathemusician who has made amazing videos about the history of hexaflexagons that are fast-paced and fun.
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By controlling how and when a circuit is triggered, you can easily turn a simple circuit into a burglar alarm – something that alerts you when something happens. By sensing light, movement, weight, liquids, even electric fields, you can trigger LEDs to light and buzzers to sound. Your room will never be the same.
Switches control the flow of electricity through a circuit. There are different kinds of switches. NC (normally closed) switches keep the current flowing until you engage the switch. The SPST and DPDT switches are NO (normally open) switches.
The pressure sensor we’re building is small, and it requires a fair amount of pressure to activate. Pressure is force (like weight) over a given area (like a footprint). If you weighed 200 pounds, and your footprint averaged 10” long and 2” wide, you’d exert about 5 psi (pounds per square inch) per foot.
However, if you walked around on stilts indeed of feet, and the ‘footprint’ of each stilt averaged 1” on each side, you’d now exert 100 psi per foot. Why such a difference?
The secret is in the area of the footprint. In our example, your foot is about 20 square inches, but the area of each stilt was only 1 square inch. Since you haven’t changed your weight, you’re still pushing down with 200 pounds, only in the second case, you’re pressing the same weight into a much smaller spot… and hence the pressure applied to the smaller area shoots up by a factor of 20.
So how do we use pressure in this experiment? When you squeeze the foam, the light bulb lights up! It’s ideal for under a doormat or carpet rug where lots of weight will trigger it.
This experiment is for advanced students. Hydrolysis is a chemical reaction that involves breaking a molecular bond using water. In chemistry, there are three different types of hydrolysis: sat hydrolysis, acid hydrolysis, and base hydrolysis. In nature, living organisms survive by making their energy from processing food. The energy converted from food is stored in ATP molecules. To release the energy stored in food, a phosphate group breaks off an ATP molecule (and becomes ADP) using hydrolysis and releases energy from the bonds.
Imagine you have a thin rope attached to a thick rope, and you jerk the thin rope so it creates a pulse that travels down the rope. When it hits the boundary between the two ropes, the wave just doesn’t stop and go away. Some of the energy from the wave is reflected back toward the source along the thin rope, and some of the energy is transmitted to the thicker (more dense) rope.
If you stick a pencil is a glass of water and look through the side of the glass, you’ll notice that the pencil appears shifted. The speed of light is slower in the water (140,000 miles per second) than in the air (186,282 miles per second), called optical density, and the result is bent light beams and broken pencils.
You’ll notice that the pencil doesn’t always appear broken. Depending on where your eyeballs are, you can see an intact or broken pencil. When light enters a new substance (like going from air to water) perpendicular to the surface (looking straight on), refraction does not occur.
However, if you look at the glass at an angle, then depending on your sight angle, you’ll see a different amount of shift in the pencil. Where do you need to look to see the greatest shift in the two halves of the pencil? (Hint: move the pencil back and forth slowly.)
We’re going to bend light to make objects disappear. You’ll need two glass containers (one that fits inside the other), and the smaller one MUST be Pyrex. It’s okay if your Pyrex glass has markings on the side. Use cooking oil such as canola oil, olive oil, or others to see which makes yours truly disappear. You can also try mineral oil or Karo syrup, although these tend to be more sensitive to temperature and aren’t as evenly matched with the Pyrex as the first choices mentioned above.
Here’s what you need:
Published value for light speed is 299,792,458 m/s = 186,282 miles/second = 670,616,629 mph
But why does light bend? You can imagine a toy car going from a wood floor to carpeting. One wheel hits the carpet first and slows down before the other, causing the toy to turn. The direction of the wave changes in addition to the speed. The slower speed must also shorten its wavelength since the frequency of the wave doesn’t change.
Do you remember the eye balloon that you made earlier? The white portion of the balloon represents your sclera, which you may have already guessed is also the white part of your eye. It is actually a coating made of protein that covers the various muscle in your eye and holds everything together.
Wowza… you’ve made it through the ENTIRE course in Advanced Physics! Wa-hoo!!
Time for one more video… ready?
If you’ve ever tried to skewer something under the water from above it, you know that you can’t aim directly at the object, because of the way light bends when it goes from a slower to a faster medium. Can you guess the one condition where light doesn’t bend as it crosses a boundary?
How much incident light bends as it crosses a boundary can be calculated and measured if we know about the mediums, including information about the index of refraction. Snell’s Law is a mathematical relationship between the refractive and incident angles of light and the optical density of the different mediums.
If you’re scratching your head during math class, wondering what you’ll ever use this stuff for, here’s a cool experiment that shows you how scientists use math to figure out the optical density of objects, called the “index of refraction”.
How much light bends as it goes through one medium to another depends on the index of refraction (refractive index) of the substances. There are lots of examples of devices that use the index of refraction, including fiber optics. Fiber optic cables are made out of a transparent material that has a higher index of refraction than the material around it (like air), so the waves stay trapped inside the cable and travel along it, bouncing internally along its length. Eyeglasses use lenses that bend and distort the light to make images appear closer than they really are.
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The Law of Reflection states that when light reflects off the surface, the angle of incidence is equal to the angle of reflection. Snell’s Law states that when light crosses into a new medium, the relationship between the angle of incidence (θi) and angle of refraction (θr) are related by the equation:
Fiber Optics are one application of total internal reflection. Optical fibers are flexible, transparent fibers made from plastic or glass about the size of a human hair that can serve as a "light pipe" to transmit light from one location to another. The bundle of fibers is used in medical applications where doctors can see inside the body by attaching a small camera to one side of the cable. To make the project below, you can order this Fiber Optics kit.
Dispersion is when visible light is separated into the colors that make up the light. We’ve already seen how optical density is a measure of how much a medium slows down light that travels through it. The index of refraction depends on the frequency of the light.
In this experiment, water is our prism. A prism un-mixes light back into its original colors of red, green, and blue. You can make prisms out of glass, plastic, water, oil, or anything else you can think of that allows light to zip through.
What’s a prism? Think of a beam of light. It zooms fast on a straight path, until it hits something (like a water drop). As the light goes through the water drop, it changes speed (refraction). The speed change depends on the angle that the light hits the water, and what the drop is made of. (If it was a drop of mineral oil, the light would slow down a bit more.) Okay, so when white light passes through a prism (or water drop), changes speed, and turns colors. So why do we see a rainbow, not just one color coming out the other side?
Spectrometers are used in chemistry and astronomy to measure light. In astronomy, we can find out about distant stars without ever traveling to them, because we can split the incoming light from the stars into their colors (or energies) and “read” what they are made up of (what gases they are burning) and thus determine their what they are made of. In this experiment, you’ll make a simple cardboard spectrometer that will be able to detect all kinds of interesting things!
SPECIAL NOTE: This instrument is NOT for looking at the sun. Do NOT look directly at the sun. But you can point the tube at a sheet of paper that has the sun’s reflected light on it.
Usually you need a specialized piece of material called a diffraction grating to make this instrument work, but instead of buying a fancy one, why not use one from around your house? Diffraction gratings are found in insect (including butterfly) wings, bird feathers, and plant leaves. While I don’t recommend using living things for this experiment, I do suggest using an old CD.
CDs are like a mirror with circular tracks that are very close together. The light is spread into a spectrum when it hits the tracks, and each color bends a little more than the last. To see the rainbow spectrum, you’ve got to adjust the CD and the position of your eye so the angles line up correctly (actually, the angles are perpendicular).
You’re looking for a spectrum (the rainbow image at left) – this is what you’ll see right on the CD itself. Depending on what you look at (neon signs, chandeliers, incandescent bulbs, fluorescent bulbs, Christmas lights…), you’ll see different colors of the rainbow. For more about how diffraction gratings work, click here.
Materials:
Like sound, light travels in waves. These waves of light enter your eyes through the pupil, which is the small black dot right in the center of your colored iris. Your lens bends and focuses the light that enters your eye. In this experiment, we will study this process of bending light and we will look at the difference between concave and convex lenses.
When I was in grad school, I needed to use an optical bench to see invisible things. I was trying to ‘see’ the exhaust from a new kind of F15 engine, because the aircraft acting the way it shouldn’t – when the pilot turned the controls 20o left, the plane only went 10o. My team had traced the problem to an issue with the shock waves, and it was my job to figure out what the trouble was. (Anytime shock waves appear, there’s an energy loss.)
Since shock waves are invisible to the human eye, I had to find a way to make them visible so we could get a better look at what was going on. It was like trying to see the smoke generated by a candle – you know it’s there, but you just can’t see it. I wound up using a special type of photography called Schlieren.
An optical table gives you a solid surface to work on and nails down your parts so they don’t move. This is an image taken with Schlieren photography. This technique picks up the changes in air density (which is a measure of pressure and volume).
The air above a candle heats up and expands (increases volume), floating upwards as you see here. The Schlieren technique shines a super-bright xenon arc lamp beam of light through the candle area, bounces it off two parabolic mirrors and passes it through a razor-edge slit and a neutral density filter before reaching the camera lens. With so many parts, I needed space to bolt things down EXACTLY where I wanted them. The razor slit, for example, just couldn’t be anywhere along the beam – it had to be right at the exact point where the beam was focused down to a point.
I’m going to show you how to make a quick and easy optical lab bench to work with your lenses. Scientists use optical benches when they design microscopes, telescopes, and other optical equipment. You’ll need a bright light source like a flashlight or a sunny window, although this bench is so light and portable that you can move it to garage and use a car headlight if you really want to get creative. Once your bench is set up, you can easily switch out filters, lenses, and slits to find the best combination for your optical designs. Technically, our setup is called an optical rail, and the neat thing about it is that it comes with a handy measuring device so you can see where the focal points are for your lenses. Let’s get started:
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Convex lenses bulge outwards, bending the light out in a spray (diverging beam). A hand-held magnifying glass is a single concave lens with a handle. These lenses have been used as ‘burning glasses’ for hundreds of years – by placing a small piece of paper at its focal point and using the sun as a light source, you can focus the light energy so intensely that you reach the flash point of the paper (the paper auto-ignites around 450oF).
