The diffraction of light

When a wave bypasses an obstacle

Diffraction is a phenomenon characteristic of waves: when a wave encounters an obstacle or an opening whose size is of the same order of magnitude as its wavelength, it spreads out beyond the obstacle rather than travelling in a straight line. This is direct evidence of the wave nature of light (a single ray of light should never deviate from a straight path).

A simple experiment

When a narrow slit of width a is illuminated by a laser (of known wavelength λ), what is observed on a screen is not a simple bright line, but a broad central spot surrounded by fainter secondary spots, separated by dark regions.

The angular diffraction angle

The angular deviation θ (in radians), for small θ, is given by:

θ ~ λ / a

where λ is the wavelength and a is the width of the slit (or the diameter of the aperture), in metres, both expressed in the same unit.

Example

Using a red laser with λ = 650 nm and a slit width a = 0.1 mm = 1.0 × 10⁻⁴ m: θ = (650 × 10⁻⁹) / (1.0 × 10⁻⁴) ~ 6.5 × 10⁻³ rad

Common pitfall

The narrower the slit (the smaller a), the more pronounced the diffraction (the larger theta): this is counter-intuitive; one might think that a wide aperture would spread the light out more; in fact, the opposite is true!