Scattering of light: Rayleigh, Mie, Tyndall and Raman
Scattering is light sent off in all directions by particles in its path. Rayleigh scattering (S ∝ 1/λ⁴) makes the sky blue; Raman scattering changes the light's frequency.
What is scattering?
When sunlight enters the atmosphere, it meets atoms, molecules, dust and water droplets. These particles absorb the light and send it out again in all directions. This is scattering of light. If the scattered light has the same energy and frequency as the incident light, it is elastic scattering. If the energy and frequency change, it is inelastic scattering.
The four types of scattering
| Type | Caused by | Kind | Example |
|---|---|---|---|
| Rayleigh | Gas atoms and molecules, much smaller than the wavelength of light | Elastic | Blue sky; reddish Sun at sunrise and sunset |
| Mie | Dust, smoke, pollen and water droplets, about as large as the wavelength or larger | Elastic | White clouds |
| Tyndall | Colloidal particles (tiny particles spread through a liquid or gas) | Elastic | The path of a sunbeam coming through a window into a dusty room becomes visible |
| Raman | Molecules of a transparent liquid, solid or gas | Inelastic (frequency changes) | Raman lines used to study the structure of molecules |
Rayleigh scattering in the sky
- Rayleigh's law: the amount of scattering is inversely proportional to the fourth power of the wavelength: S ∝ 1/λ⁴.
- Blue light has a shorter wavelength than red, so it is scattered much more. Scattered blue light reaches our eyes from all over the sky, so the sky looks blue.
- At sunrise and sunset, sunlight travels a longer path through the air. Most blue light is scattered away on the way, so mostly red and orange light reaches us.
- Danger signals are red because red light is scattered the least, so it can be seen from far away.
- Clouds look white because their large droplets scatter all colours almost equally (Mie scattering).
Raman scattering
In 1928, Sir C. V. Raman passed monochromatic light (light of a single colour and frequency) through a transparent liquid. The scattered light had the original frequency and also new frequencies. This is the Raman effect. Lines with the same frequency as the incident light are Rayleigh lines. Lines with different frequencies are Raman lines. Raman lines of lower frequency are Stokes lines; those of higher frequency are anti-Stokes lines. Raman won the Nobel Prize in Physics in 1930. India marks National Science Day on 28 February for this discovery.
Activity: seeing scattering in milky water
- 1Fill a clear glass with water and stir in a few drops of milk.
- 2In a dark room, shine a torch through the side of the glass.
- 3★Observe: the path of the beam is visible inside the water. The tiny milk particles scatter the light (Tyndall effect).
- 4Look at the torch through the glass from the opposite side. The light looks slightly orange-red, because blue light has been scattered out of the beam.
★ key step
Sunrise over the sea at Marina Beach looks orange-red because blue light is scattered away on the long path through the air. In a dark cinema hall, you can see the projector's beam because dust scatters its light.
2 marks: “Why does the sky appear blue?” Air molecules scatter sunlight by Rayleigh scattering, S ∝ 1/λ⁴. Blue light (short wavelength) is scattered the most and reaches our eyes from all directions.
2 marks: “What are Stokes lines and anti-Stokes lines?” In Raman scattering, lines of lower frequency than the incident light are Stokes lines; lines of higher frequency are anti-Stokes lines.
4 marks: “What is scattering of light? Explain Rayleigh, Mie and Tyndall scattering with one example each.” Define scattering, elastic and inelastic; Rayleigh: gas molecules, S ∝ 1/λ⁴, blue sky; Mie: dust and droplets, white clouds; Tyndall: colloidal particles, sunbeam in a dusty room.
Really Many Tiny Rays: Rayleigh (molecules, blue sky), Mie (dust and droplets, white clouds), Tyndall (colloids, visible beam), Raman (frequency changes).