Rays and beams
Scattering and fluorescence
A beam in clear water is invisible from the side. Add a drop of milk and the fat globules scatter some light sideways, more for blue than for red. A dye absorbs the beam and glows at a longer wavelength: fluorescence.
What you are looking at
The green beam scatters in the milky cell and is absorbed in the rhodamine cell, which glows yellow-orange. A spectrometer beside the dye cell sees both the scattered green and the fluorescence.
How the bench is set up
A 532 nm laser, a 100 mm cell of water with milk, a 100 mm cell of rhodamine 6G and a spectrometer looking at the dye from the side.
The theory
Particles much smaller than the wavelength scatter with a coefficient proportional to . A dye absorbs by the Beer–Lambert law,
and re-emits a fraction (its quantum yield) at a longer wavelength: the Stokes shift.
What to measure
Scattering strength against λ⁻⁴; Stokes-shifted emission
- Read the spectrometer: a line at 532 nm and a band near 566 nm.
- Change the laser to 633 nm: the dye barely absorbs, and the glow disappears.
What the model leaves out
- Scattering and fluorescence are sent out along a few directions rather than every direction.
Things to try
- Use a blue laser at 450 nm with the milk cell and see it scatter more strongly.
References
- E. Hecht (2017), Optics, 5th edition, Pearson.
- G. G. Stokes (1852), On the change of refrangibility of light, Philosophical Transactions 142, 463.
More experiments, and how the lab works, are in the laser lab’s overview.