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Recreate one of the most important experiments in the history of physics–the two-slit experiment–by shining a laser pointer through two narrow slits and observing the interference pattern on a distant screen.
Look at the pattern produced when the light goes through the two slits and shines on the distant screen. Notice there are alternating dark and light regions.
Use the razor blade or knife to block the light from going through one of the slits. Notice that some of the dark bands vanish. Try blocking the light from going through the other slit.
Remove the razor blade and notice that when you allow light from both slits to shine on the screen, the dark bands re-appear.
When light goes through a slit, diffraction causes it to bend and spread across the screen, making a predictable banded pattern. When light goes through two slits, new dark regions appear. The dark and light regions are produced by interference of the light passing through the slits.
As light coming through one slit reaches the screen, it overlaps with light coming through the other slit. When the crest of one wave of light overlaps with the crest of another wave, the two waves combine to make a bigger wave and you see a bright blob of light. When the trough of one wave overlaps with the crest of another wave, the waves cancel each other out and you see a dark band. The appearance of dark bands when two light sources strike a screen shows that light is a wave phenomenon.
This experiment was first performed in 1801 by Thomas Young, and it provided support for a wave theory of light.
The angle, T, between two adjacent dark bands in the interference pattern and the slits depends on the distance between the centers of the slits, d, and the wavelength of light, L. For small angles, the angle is inversely proportional to the distance between the slits and proportional to the wavelength of the light:
T = L/d
If light of multiple wavelengths is shone through two or more slits, each wavelength will be bent at a different angle. This spreads light out into a spectrum, with red light bent at a larger angle than blue light. This is the principle behind a diffraction grating.
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Attribution: Exploratorium Teacher Institute