Diffraction Grating

A diffraction grating is a series of narrow, parallel slits. They usually have around 500 slits per mm.

When light shines on the diffraction grating several bright sharp lines can be seen as shown in the diagram to the right.

The first bright line (or interference maximum) lies directly behind where the light shines on the grating. We call this the zero-order maximum. At an angle of θ from this lies the next bright line called the first-order maximum and so forth.

The zero-order maximum (n=0)

There is no path difference between neighbouring waves. They arrive in phase and interfere constructively.

The first-order maximum (n=1)

There is a path difference of 1 wavelength between neighbouring waves. They arrive in phase and interfere constructively.

The second-order maximum (n=2)

There is a path difference of 2 wavelengths between neighbouring waves. They arrive in phase and interfere constructively.

Between the maxima

The path difference is not a whole number of wavelengths so the waves arrive out of phase and interfere destructively.