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Two-Slit Experiment

Science Snack
Two-Slit Experiment
Light plus light equals dark.
Two-Slit Experiment
Light plus light equals dark.

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.

Tools and Materials
  • Laser pointer
  • Lice comb (or, not quite as good, an eyelash comb with narrowly spaced metal teeth)
  • Black tape
  • White screen or 5 x 7-inch index card
  • Two large binder clips, 1 inch (2.5 cm) wide, 2 inches (5 cm) long
  • Two medium binder clips, 1/2 in (1 cm) wide, 1 inch (2.5 cm) long
  • A single-edge razor blade or straight-edge knife (not shown)
  1. Use the black tape to cover the teeth on the lice comb, leaving exposed only two slits between adjacent teeth.
  2. Insert the handle of the comb into a large binder clip and set the clip on its side on a table or other flat surface so the teeth of the comb are vertical (click to enlarge photo)
  3. .Clip the two medium binder clips to the barrel of the laser pointer and position them so the laser pointer rests horizontally.
  4. Attach the remaining large binder clip to the index card so that it creates a stand for the card.
  5. Set up the laser pointer and comb so that the laser beam shines through the two open slits on the comb onto the white screen. Position the screen so it is at least 4 feet (1.5 meters) from the two slits.
To Do and Notice

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.

What's Going On?

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.

Going Further

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:

$$T = \frac{L}{d}$$

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.