Reflection Diffraction Gratings

(not on the syllabus!)

Transmission Diffraction Gratings

A transmission diffraction grating is a slide with large number of parallel, closely spaced slits (transparent spaces) drawn on it. Early ones were carbon covered glass slides etched by a needle point - now they tend to be printed onto a slide. It is excellent at separating the colors in incident light because different wavelengths are diffracted at different angles, according to the grating relationship:

d sin = n
	where d is the distance between the slits is the angle of diffraction is the wavelength of the light n is the order of diffraction

The separation being so small makes the angle large. The diffraction grating is an immensely useful tool for the separation of the spectral lines associated with atomic transitions. It separates the different colors of light much more than the dispersion effect in a prism is able to - it used diffraction not refraction to do it!

Note that from the equation you can see that, the bigger the wavelength the more diffraction. This means that it is the red end of the visible spectrum that is diffracted the most. In the prism the red end of the spectrum is refracted the least!

The condition for maximum intensity is the same as that for the double slit or multiple slits, but with a large number of slits the intensity maximum is very sharp and narrow, providing the high resolution for spectroscopic applications. The peak intensities are also much higher for the grating than for the double slit.

When monochromatic light (light of a single wavelength - like the 632.8nm red light from a helium-neon laser) strikes a diffraction grating it is diffracted to each side in multiple orders. The condition for maximum intensity is the same as that for a double slit. However, angular separation of the maxima is generally much greater because the slit spacing is so small for a diffraction grating.

You can make a simple grating at home - see this link

See also diffraction through slits