Diffraction & the Wave Nature of Light

You’ll be able to

Describe diffraction and the condition under which it is pronounced
Explain how diffraction gratings and single slits spread light
Identify interference and diffraction as evidence that light is a wave

Waves bend around edges

Diffraction is the bending and spreading of waves as they pass through an opening or around an obstacle. The effect is most pronounced when the opening is comparable to or smaller than the wavelength; when the opening is much larger than the wavelength, the wave passes nearly straight through with little spreading. This is why you can hear someone around a corner (sound’s long wavelength diffracts well) but cannot see them (light’s tiny wavelength barely diffracts through everyday openings).

Gratings, single slits, and the wave verdict

A diffraction grating — thousands of finely spaced slits — spreads light into sharp, widely separated bright lines, splitting white light into its spectrum. A single slit produces its own pattern of a wide central bright band flanked by weaker fringes. Both effects, along with double-slit interference, can only be explained if light is a wave: particles would cast simple sharp shadows, not produce alternating bright and dark bands. The photoelectric effect (next unit) shows light’s particle side, but diffraction and interference are the wave side.

Diffraction grating (bright lines)

d · sinθ = m · λ

Same form as the double slit, where d is the spacing between adjacent slits and m is the order. Smaller slit spacing d spreads the orders farther apart.

Worked example

Explain why sound easily bends around a doorway but light passing through the same doorway does not visibly spread.

1.Diffraction is significant when the opening is about the size of the wavelength or smaller.
2.Audible sound has wavelengths from centimeters to meters — comparable to a doorway (~1 m), so it diffracts strongly and bends around the frame.
3.Visible light has wavelengths near 500 nm (5 × 10⁻⁷ m), millions of times smaller than the doorway.
4.Because the opening is enormous compared with light’s wavelength, the light passes essentially straight through with negligible spreading.

Answer: Sound diffracts around the doorway because its wavelength is comparable to the opening, while light’s wavelength is far too small relative to the doorway to spread noticeably.

Checkpoint

Diffraction (the spreading of waves through an opening) is most pronounced when the opening is:

Watch out

Diffraction and refraction are different phenomena. Refraction is bending because a wave changes speed crossing a boundary; diffraction is spreading as a wave passes an edge or opening. Both bend light, but for different reasons.

Checkpoint

Which observation is the strongest direct evidence that light behaves as a wave?

On the exam

Sort the evidence: interference and diffraction demonstrate light’s wave nature; the photoelectric effect and Compton scattering demonstrate its particle nature. Light is both — the exam expects you to name which experiment reveals which side.

Answer the 2 checkpoints as you read.