What is the Raman effect? Explain the origin of Stokes and anti-Stokes lines in terms of energy, and give one use of the effect.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Define the effect. When light is scattered by a transparent substance, a small part of it comes out with a changed frequency.
2 · The unshifted line. If no energy is exchanged, the scattered light keeps the frequency ν.
3 · Stokes lines. The light gives energy to the molecule; since E = hν, its frequency falls.
4 · Anti-Stokes lines. The light takes energy from an already excited molecule, so its frequency rises.
5 · Draw the spectrum. [Frequency axis; tall unshifted line at ν; shorter Stokes lines at lower frequency; still shorter anti-Stokes lines at higher frequency]
6 · Real life. Raman instruments identify medicines and gemstones without damaging them.
7 · Close in one line. So, the Raman spectrum reveals the energy levels of the molecules that scatter the light.
Scoring tip: Link frequency to energy with E = hν; that one sentence earns the explanation mark.
