Double Slit Experiment with Light

Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★☆ Some safety precautions required to perform safely

Categories: Electromagnetic Spectrum and Waves, Light

Alternative titles: Light Interference Experiment

Summary

The double slit experiment with light shows how photons exhibit both wave and particle properties. A monochromatic light source passing through two slits produces an interference pattern on a screen, even when photons are sent one at a time.

Procedure

Set up a coherent light source, such as a laser, pointing at a barrier with two narrow, closely spaced slits.
Place a screen or sensitive detector behind the barrier.
Turn on the laser and observe the pattern of alternating bright and dark fringes that appear, caused by constructive and destructive interference of light waves.
Reduce the light intensity so photons pass through the slits one at a time.
Use a sensitive detector with a long exposure to record the impacts. Initially, single photons appear as random points, but over time the familiar interference pattern emerges.

Links

Interference Demo: Double Slit - Physics Demos:

Simple double slit experiment at home - no81e:

📄 Double slit experiment - Anton Paar: https://wiki.anton-paar.com/au-en/double-slit-experiment/

Variations

Try using different wavelengths of light (red, green, blue lasers) to compare fringe spacing.
Use one slit covered to show the difference between single-slit diffraction and double-slit interference.

Safety Precautions

Do not look directly into the laser beam; it can damage eyes.
Secure the laser on a stable surface to prevent accidental movement.
Keep reflective surfaces (mirrors, watches, jewelry) away from the beam path to avoid stray reflections.

Questions to Consider

Why do bright and dark fringes appear on the screen? (Constructive interference produces bright fringes; destructive interference produces dark fringes.)
What happens when photons are sent one at a time? (They arrive as particles, but collectively form a wave-like interference pattern.)
How does this experiment support the idea of wave-particle duality? (Light acts like a wave when passing through slits and like a particle when detected.)
How would changing the wavelength affect the pattern? (Longer wavelengths produce wider spaced fringes; shorter wavelengths make fringes closer together.)