When light bends as it passes from one medium to another, interesting things happen. A glass prism and the tiny droplets and molecules in our atmosphere both bend and split light, and that is why we see rainbows, blue skies and red sunsets. This topic ties refraction to the colours of the world around us.
Refraction through a prism: A prism has two triangular ends and three rectangular faces. When a ray of light enters one refracting face, it bends towards the normal; on leaving the other face it bends away from the normal. Because the two faces are not parallel, the ray emerges bent towards the thicker base of the prism. The angle between the original direction of the ray and the emergent ray is the angle of deviation.
Dispersion of white light: White light is a mixture of seven colours. When it passes through a prism, each colour bends by a slightly different amount because each has a different wavelength. Violet bends the most, red bends the least. So the colours spread out into a band called the spectrum in the order VIBGYOR (Violet, Indigo, Blue, Green, Yellow, Orange, Red). This splitting of white light into its colours is called dispersion. Newton showed that a second, inverted prism can recombine the spectrum back into white light, proving white light is truly a mixture.
Rainbow: A rainbow is a natural spectrum. Sunlight is refracted, internally reflected and refracted again inside tiny raindrops, which act like prisms. A rainbow always appears in the part of the sky opposite the Sun after rain.
Scattering of light (Tyndall effect): When light strikes very small particles, it changes direction; this is scattering. The scattering of light by colloidal particles (as in smoke, fog or a colloidal solution) is the Tyndall effect. The colour of scattered light depends on the size of the particles: very fine particles scatter shorter (blue) wavelengths more strongly.
- Why the sky is blue: Air molecules scatter blue light much more than red, so the scattered blue light reaches our eyes from all directions.
- Why the Sun looks red at sunrise/sunset: Near the horizon sunlight travels through a thicker layer of air; most blue is scattered away and mainly red light reaches us.
- Twinkling of stars: Star light is refracted by ever-changing layers of the atmosphere, so its apparent brightness flickers. Planets are nearer and look like extended sources, so they do not twinkle.
- Advance sunrise and delayed sunset: Atmospheric refraction bends the Sun's light, so we see the Sun about two minutes before actual sunrise and about two minutes after actual sunset.
Arrange the colours of white light in order of increasing deviation when passing through a prism.
Solution- The amount of bending (deviation) depends on the wavelength of each colour.
- Red has the longest wavelength and bends the least; violet has the shortest wavelength and bends the most.
- So in increasing order of deviation the colours are: Red, Orange, Yellow, Green, Blue, Indigo, Violet.
Answer: Red < Orange < Yellow < Green < Blue < Indigo < Violet.
Why does the sky appear blue on a clear day?
Solution- Sunlight passing through the atmosphere is scattered by tiny air molecules.
- Shorter wavelengths (blue) are scattered much more strongly than longer wavelengths (red).
- This scattered blue light reaches our eyes from every direction, so the sky looks blue.
Answer: Air molecules scatter blue light more than red, making the sky look blue.
Explain why the Sun appears reddish at sunrise and sunset.
Solution- At sunrise and sunset, sunlight has to travel through a much thicker layer of the atmosphere to reach us.
- Most of the shorter blue wavelengths are scattered away during this long path.
- Mainly the longer red wavelengths reach our eyes, so the Sun looks reddish.
Answer: The longer path scatters blue away, leaving mostly red light, so the Sun looks red.
What is meant by recombination of white light, and which scientist demonstrated it?
Solution- When white light passes through a prism it disperses into the seven colours of the spectrum.
- If a second, identical but inverted prism is placed after the first, the separated colours combine again.
- The emergent light is once more white. Isaac Newton demonstrated this with two prisms.
Answer: A second inverted prism recombines the spectrum into white light; shown by Newton.
Why do stars twinkle but planets generally do not?
Solution- Star light passes through the atmosphere, whose layers keep changing, so the light is refracted by varying amounts.
- Stars are so far away they act as point sources; tiny changes make their brightness flicker, i.e. twinkle.
- Planets are much closer and appear as collections of many point sources, so the flickers average out and they do not twinkle.
Answer: Atmospheric refraction makes point-like stars twinkle; nearer planets appear extended and do not.
Define the Tyndall effect and give one everyday example.
Solution- When a beam of light passes through a medium containing very fine (colloidal) particles, the particles scatter the light.
- This scattering of light by colloidal particles is called the Tyndall effect.
- A common example is the visible path of sunlight streaming through smoke or through a small gap into a dusty room.
Answer: Tyndall effect is scattering of light by colloidal particles; e.g. a sunbeam visible in smoke/dust.