Question

Why are diffraction gratings used to measure wavelength of light instead of double slits?

Answer 1

The interference fringes are much sharper ( narrow lines instead of smeared out blobs ) resulting in massively increased resolution. And the lines are much brighter ( light that would lie between the fringes is shifted into the fringe by the diffraction grating )

A diffraction grating can be used to split light into different wavelengths with a high degree of accuracy, much more so than glass prisms. A diffraction grating usually consists of a piece of glass with very closely spaced lines ruled on it. A transmission grating has clear spaces between the lines so that light can pass through it. A reflection grating has a shiny surface between the lines so that light gets reflected off it. A compact disc acts as a reflection grating.

The diffraction grating has the advantage over the double slit method of measuring wavelength in that:

• the maxima are more sharply defined;

• the beam passes through more slits than two, so the intensity is brighter;

• the angles are larger so that they can be measured with greater precision.

The derivation of the diffraction grating formula is on the syllabus, but experience has shown that most students struggle with it. It is more important that they know how to use the formula. If you want to see the derivatio. The formula is:

d sin q = nl

The term n is called the spectrum order. If n = 1, we have the first diffraction maximum. The other physics codes:

• q is the angle,

• l is the wavelength,

• and d is the slit width

Sin q can never be greater than 1, so there is a limit to the number of spectra that can be obtained.