# Concepts on diffraction

1. Sep 7, 2004

### franz32

Hello.

What is all about that interference pattern formed by a diffraction grating? Also, are there factors that affect the kind of pattern that a kind of diffraction grating forms?

2. Sep 7, 2004

### Claude Bile

A diffraction grating is basically a series of parallel ridges that act as point sources when light is incident upon them. The waves from each 'source' will interfere when the Bragg condition is satisfied;

$$2dsin\theta = \lambda$$

Where d is the seperation of the ridges, theta is the angle of incidence and lambda is the wavelength. Clearly lambda is a function of theta, that is, different wavelengths will reflect at different angles. The diffraction grating thus seperates the wavelengths of an incident source, giving its spectrum as a function of angle (most spectrometers work in this fashion).

The critical parameter that will affect the output of a diffraction grating is the line density, i.e. the number of ridges per unit length. More ridges per unit length give greater resolution, but reduces the free spectral range of the output.

The diffraction pattern itself will depend on the incidident light of course, but will possibly contain several orders, since the Bragg condition is satisfied for integer multiples of the wavelength as well. The higher orders will have the same pattern, but with a reduced intensity.

Claude.