Student Physics FAQ: How Does a Diffraction…

How Does a Diffraction Grating Work? (HL)

A diffraction grating separates light into distinct directions because waves from many equally spaced slits interfere constructively only at particular angles. This is Additional Higher Level (AHL) content in C.3 Wave phenomena.

The Mechanism

Each slit diffracts the incident wave and acts as a source of a new wavelet. Because the slits have equal spacing and are illuminated by the same incident wave, they behave as coherent sources.

For light emerging at angle $\theta$ , the path difference between waves from adjacent slits is $d\sin\theta$ , where $d$ is the distance between adjacent slits. A bright principal maximum occurs when this equals a whole number of wavelengths:

How Does a Diffraction Grating Work? (HL)

The Mechanism

Each slit diffracts the incident wave and acts as a source of a new wavelet. Because the slits have equal spacing and are illuminated by the same incident wave, they behave as coherent sources.

Change	Effect on the pattern
Increase wavelength $\lambda$	Maxima occur at larger angles
Increase line density	Spacing $d$ decreases, so angles increase
Increase the number of illuminated slits	Principal maxima become narrower and more intense

How Does a Diffraction Grating Work? (HL)

The Mechanism

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How Does a Diffraction Grating Work? (HL)

The Mechanism

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