6 min readConcept sheetA 2-mark or 3-mark reasoning question; often a 4-mark case-based question

Scattering of light: Tyndall effect and the blue sky

Tiny particles scatter light in all directions, making a beam visible: the Tyndall effect. Air molecules scatter blue light most, so the clear sky looks blue.

Tyndall effectScattering of light by colloidal particles, which makes the path of a beam visible

Very fine particlesScatter mainly blue light (shorter wavelength)

Large particlesScatter all colours, so the light looks white

Red vs blueThe wavelength of red light is about 1.8 times that of blue

What scattering is

Our air is a mixture of gas molecules, dust, smoke and tiny water droplets. When light strikes such small particles, they send it off in all directions. This is scattering of light. Which colour is scattered depends on the size of the particles. Very fine particles, such as air molecules, scatter mainly blue light, which has a shorter wavelength (the length of one light wave). Larger particles scatter longer wavelengths as well. Particles that are big enough scatter all the colours, so the scattered light looks white.

The Tyndall effect

The Tyndall effect is the scattering of light by colloidal particles, which makes the path of a light beam visible. Colloidal particles are bigger than molecules but too small to see.
A beam of sunlight entering a dark, smoky or dusty room through a small hole or a gap in the roof shows a bright path.
Sunlight coming through the canopy of a dense forest shows beams, because tiny water droplets in the mist scatter it.
The beam of light from a projector in a dark hall shows up because dust and smoke particles scatter it.
The headlight beams of vehicles look like bright cones on a foggy night.

Why the clear sky is blue

1
Sunlight contains all the colours, from violet to red.
2
Air molecules and other fine particles in the atmosphere are smaller than the wavelength of visible light.
3
Such tiny particles scatter shorter wavelengths (the blue end) much more strongly than longer wavelengths (the red end). The wavelength of red light is about 1.8 times that of blue.
4
So, as sunlight passes through the air, blue light is scattered all over the sky.
5
This scattered blue light reaches our eyes from every direction, so the clear sky looks blue.

With air and without air

Seen from the groundAir scatters blue sunlight in all directions, so the clear daytime sky looks blue.

Seen from space or at very high altitudeThere is little or no air to scatter light, so the sky looks dark, even in the daytime. Passengers flying at very high altitudes see a darker sky.

If the Earth had no atmosphere, there would be no scattering, and the sky would look dark.

In real life

Light an agarbatti in a dim puja room and let a sunbeam come in through the window: the smoke and dust show up clearly in the beam. That is the Tyndall effect at home.

Exam angle

1 marks: “Which end of the spectrum do the molecules of air scatter more strongly, and what colour does this give the clear sky?” The blue end (shorter wavelengths), so the clear sky looks blue.

2 marks: “What is the Tyndall effect? Give two examples from daily life.” Scattering of light by colloidal particles, which makes the path of a beam visible. Examples: a sunbeam in a smoky room; sunlight through a misty forest.

3 marks: “Why does the clear sky appear blue? How would it look to an astronaut in space?” Air molecules, smaller than the wavelength of light, scatter blue (short wavelength) light more than red; the scattered blue reaches the eye. In space there is no air to scatter light, so the sky looks dark.

Remember it

Small particles scatter short waves (blue). Big particles scatter every colour (white).