The Human Eye and the Colourful World · Lesson 7 of 8
Scattering of Light
“Tiny particles redirect light and paint the sky without using a brush.”
• Explain how particles make a light beam visible by scattering. • Distinguish the passage of light through a true solution and a colloidal medium. • Describe the Tyndall effect in familiar situations. • Relate particle size to the colour of scattered light. • Explain why the clear sky appears blue and the high-altitude sky appears dark. • Explain why red light is used for danger signals.
A beam from a projector can seem invisible in clean air but becomes obvious when dust, mist or smoke drifts through it. We see the beam sideways because particles redirect some of its light towards our eyes. This redirection, called scattering, is responsible for several striking natural effects, including the blue sky and the reddish appearance of the Sun near the horizon.
The redirection of light in different directions when it interacts with particles of a medium.
The path of a beam through a true solution is ordinarily not visible because its dissolved particles are extremely small and do not scatter enough light into the observer’s eye. In a colloidal medium, relatively larger dispersed particles scatter light more effectively. The path then appears as a bright cone or shaft even when viewed from the side.
| Medium | Relative particle size | Visibility of beam path |
|---|---|---|
| True solution | Extremely small dissolved particles | Usually not visible |
| Colloidal medium | Larger dispersed particles | Visible because light is scattered |
Tyndall Effect
The visibility of a light beam caused by scattering from colloidal particles along its path.
The atmosphere is a heterogeneous mixture containing molecules of air, smoke, tiny water droplets and suspended dust. When a narrow beam meets suitable fine particles, light is scattered into many directions. Some of it reaches an observer who is not standing directly in the original beam, making the beam path visible.
A fine shaft of sunlight entering a smoke-filled room through a small opening shows the Tyndall effect. It also appears when sunlight passes through a dense forest canopy and tiny water droplets in mist scatter the light. The particles become visible because they redirect light; the beam does not make them luminous on their own.
The colour of scattered light depends on particle size. Very fine particles scatter mainly shorter wavelengths such as blue. Larger particles can scatter longer wavelengths more effectively. If particles are large enough to scatter the visible wavelengths more nearly together, the scattered light can appear whitish, as in many clouds or dense mists.
Problem
A narrow sunbeam is visible from the side in a dusty room. What phenomenon is being observed?
- 1.Dust particles lie in the beam path.
- 2.They redirect part of the beam towards the observer’s eye.
- 3.The visible path is an example of the Tyndall effect produced by scattering.
Problem
Why is a beam generally invisible in a true solution but visible in a colloid?
- 1.The dissolved particles in a true solution are extremely small and do not produce sufficient visible scattering.
- 2.Colloidal particles are relatively larger and scatter an observable amount of light sideways.
- 3.Therefore the path, rather than merely the final illuminated spot, becomes visible in the colloid.
Why Is The Colour Of The Clear Sky Blue?
Molecules of air and other very fine atmospheric particles are smaller than the wavelengths of visible light. They scatter shorter wavelengths at the blue end more effectively than longer wavelengths at the red end. Red light has a wavelength about 1.8 times greater than blue light, so it is scattered much less strongly by these very fine particles.
Sunlight entering the atmosphere contains all visible colours. Fine particles redirect a large amount of its blue component across the sky. When an observer looks away from the direct Sun, scattered blue light enters the eyes from many directions, making the clear sky appear blue. The sky is not a blue surface; its colour is an effect of selectively scattered sunlight.
If Earth had no atmosphere, there would be no significant atmospheric scattering and the sky would look dark. At very high altitudes, the atmosphere is much thinner and scattering is less prominent, so the sky appears darker to passengers or observers above much of the air. Beyond the atmosphere, the background of space appears dark even when the Sun is bright.
Problem
An observer rises to a very high altitude on a clear day. Predict the change in sky appearance and justify it.
- 1.At high altitude, less atmosphere lies above and around the observer.
- 2.Fewer particles are available to scatter sunlight into the line of sight.
- 3.The blue glow becomes less prominent and the sky appears darker.
- 4.This does not mean sunlight has vanished; direct sunlight can remain intense while the surrounding sky darkens.
Red Light And Distant Visibility
Danger signals are red because red light is scattered least by fog or smoke among the visible colours discussed here. A larger fraction continues along the intended direction and can be recognised from farther away. Blue light would be redirected more strongly and lose more of its direct intensity in the same fine-particle conditions.
The reddish appearance of the Sun near sunrise and sunset is connected to the same selective scattering. Sunlight then travels through a longer atmospheric path. Much of the shorter-wavelength light is scattered away from the direct line of sight, leaving a greater proportion of red and orange light to reach the observer. This complements atmospheric refraction: refraction changes the apparent position of the Sun, while scattering changes the colour reaching the eye.
Refraction changes a light ray’s direction because it passes through regions with different refractive properties. Scattering redirects light through interaction with particles. Both occur in the atmosphere, but twinkling and advance sunrise are explained mainly by refraction, while the blue sky and red danger signals are explained by scattering.
Quiz
What makes the path of a beam visible in a colloidal medium?
Which particles mainly scatter blue light?
Why does a clear sky appear blue?
Why does the sky appear dark at very high altitude?
Why is red chosen for danger signals?
Practice Problems
- Distinguish a true solution from a colloid using the path of a narrow light beam. Answer: The path is ordinarily not visible in a true solution because its particles are extremely small. It becomes visible in a colloid because larger dispersed particles scatter light sideways.
- Give two natural or everyday examples of the Tyndall effect and explain them. Answer: A sunbeam in a smoky room and sunlight through a misty forest canopy are examples. Smoke or water droplets scatter light into the observer’s eye, revealing the path.
- Why can very large suspended particles make scattered light appear white? Answer: When particles are large enough to scatter the visible wavelengths more nearly together, no single colour dominates strongly, so the combined scattered light appears whitish.
- Explain why the sky would appear dark on an airless world. Answer: Without an atmosphere there are essentially no air molecules or suspended fine particles to scatter sunlight across the sky. Only direct light from luminous objects reaches the eye, leaving the surrounding sky dark.
- Separate the causes of a reddish Sun and delayed sunset. Answer: Selective scattering removes more shorter-wavelength light from the long atmospheric path, making the Sun appear reddish. Atmospheric refraction bends the Sun’s rays and keeps its apparent image above the horizon after actual sunset.
Key Takeaways
• Scattering redirects light when it interacts with particles in a medium. • The Tyndall effect is the visibility of a beam path due to scattering by colloidal particles. • Very fine particles scatter shorter blue wavelengths more strongly. • Larger particles scatter longer wavelengths, and sufficiently large particles can produce whitish scattering. • Scattered blue sunlight entering the eye from many directions makes the clear sky appear blue. • With little or no atmosphere, scattering is weak and the sky appears dark. • Red light is scattered less by fog and smoke, so it is used for danger signals. • Atmospheric refraction and scattering explain different effects and should not be confused.