Diffraction Gratings

When coherent light passes through two slits we get an interference pattern consisting of fringes.

• What difference would it make if there were more than two slits?
• What difference would it make if we had lots of slits very close together?

A diffraction grating consists of many slits, typically 300 per mm, etched onto a small piece of glass or perspex.

( 300 lines / mm  =  300, 000 lines / m          therefore          d = 1 / 300, 000 = 3.33 x 10^-6 m )

a diffraction grating	Light through a diffraction grating	A spectrometer measures the angles accurately

When coherent light falls on a diffraction grating we get very bright, distinct maxima at very definite angles.

Using a machine called a spectrometer we can very accurately measure the angles at which these maxima occur. We the use the equation below to calculate the wavelength of the light.

There are many situations where we need to accurately measure the wavelength of radiation.

Spectroscopy is the analysis of spectra. All chemical elements emit particular wavelengths of light when their atoms are exited.

Below is the emission spectrum for Hydrogen. Is like a fingerprint. If a substance emits light of these wavelengths then it must contain hydrogen.

By analyzing the light from stars astronomers can find out what elements they contain and how much of each. Much of what we know about our Sun and other stars comes from the study of spectra.