Diffraction is how light bends as it passes through very narrow slits or around very thin objects like a hair. When light travels around a hair, two wave patterns form, and those waves interfere with each other constructively (they add together to form a bright region) or destructively (the cancel each other out and leave a dark spot).
This experiment looks at the light and dark areas of interference to determine the wavelength of a laser. You can do this for lasers that don’t have labels on them, so you really don’t know what wavelength they are!
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There are white light lasers (which started back in the 1970s), but yes you’re right – white is a combination of colors, which means it’s not monochromatic (one wavelength). The range of the spectrum is 400-700 nm.
You can buy a white light laser here.
You can read more about white lasers here:
http://www.leica-microsystems.com/science-lab/white-light-laser/
http://www.gizmag.com/test-subjects-dont-mind-laser-light/20300/
http://en.wikipedia.org/wiki/Ion_laser
You can make white light by combining different colors through a prism. You can’t make a true white laser because a laser has a coherent beam created by stimulated emission that consists of one wavelength (or a very small range of wavelengths), and a white light laser has a large spectrum of wavelengths.
The basic idea is that you have to mix four different wavelengths together to get a white light laser, or you can use a special mix of Argon and Krypton to produce light that appears to be white using a super-continuum white light generation technique. The application for white lasers so far is a perfect light source for special experiments.
If white lasers exist, will they show a spectrum if you shine then through a diffraction grating