Diffraction gratings and line spectra

What you will learn

Formation and observation of line emission spectra with gratings.

Specification mapping: A1.3.2

Prerequisites: A1.3, A1.3.1

Physical principles

A diffraction grating contains many equally spaced narrow apertures or grooves. Diffraction spreads light from each aperture; interference selects angles at which the contributions reinforce. With several wavelengths present, each has its own reinforcement angles. A slit and collimator give a narrow, approximately parallel incident beam so individual lines can be resolved.

The central, zero-order image is undeviated and overlaps all incident wavelengths. First and higher orders appear on both sides. Within a given non-zero order under normal incidence, longer wavelengths have larger deviation angles. A monochromatic source gives repeated images of one wavelength, whereas a gas discharge gives separate coloured lines from its discrete emitted wavelengths.

Use a spectrometer to locate line angles relative to the central direction. Subtract angular readings rather than using an arbitrary scale reading as deviation. Measuring corresponding left and right orders can reduce alignment bias. Gratings separate frequencies already present; they do not create the atomic spectrum. Never look into a laser or use an unfiltered solar source.

Scientific diagram and interpretation

Worked examples and applications

Common misconceptions

Diffraction spreading and interference selection both contribute; refraction in a prism is a different mechanism.

Examination technique

Distinguish the light source, the grating and the detector in a labelled arrangement.

For an explanation, connect the physical cause to its effect. For a calculation, state the model, rearrange before substitution, retain intermediate digits, and finish with units. For a data question, refer to the actual readings and distinguish a trend from proof of a cause.

Check your understanding

Five questions. Choose an option, then Check Answer. Read the feedback and use Try Again to reset. Use Tab between controls, arrow keys within radio groups, and Enter or Space on buttons. These local practice answers are not saved.