Human Eye and The Colorful World

Atmospheric Refraction and Scattering of Light

The sight of sunrise and sunset is the prettiest. Isn’t it? But if you notice carefully, you’ll notice the sun appears red during sunrise and sunset. Do you know, why? Well, it’s because of ‘Scattering of Light’. But what does one mean when they say scattering of light? Or, why do stars appear higher than they actually are? Does this have something to do with the scattering of light? Well, the answer to this is ‘No’. Stars appear higher than they actually are because of ‘Atmospheric Refraction’. Now, what is atmospheric refraction? Let us find out below.

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What is Atmospheric Refraction?

The earth’s atmosphere consists of different layers of air. From these, some layers of air with low densities are hot, while the others with high densities are cold. The hot layers of air act as the optically rarer medium for the rays of light, whereas the cold layers of air act as the optically denser medium for rays of light. Hence, when rays of light are emitted by an object in the atmosphere, these rays pass through the atmosphere having varying layers of air with varied densities and are refracted by the atmosphere.

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Therefore, the refraction of light triggered by the earth’s atmosphere is termed as ‘Atmospheric Refraction’

Examples of Atmospheric Refraction

a. Twinkling of Stars

After passing through the earth’s atmosphere having different layers of air with different optical densities, the rays of light coming from the stars finally reach our eyes. But, due to the change in the temperature conditions, the optical densities of different layers of air keep changing continuously. Because of which, the rays of light coming from a star are refracted to different degrees at different moments of time, and the path of refracted rays keeps changing. Hence, sometimes due to the amount of light refracted towards our eyes is more stars appear bright to us, on the other hand sometimes since less light is refracted towards our eyes, the stars appear dim to us. This gives us the twinkling of stars.

b. Sun is visible 2 minutes before the actual sunrise and 2 minutes after the actual sunset

Due to the atmospheric refraction, we can see the sun 2 minutes before and after actual sunrise and sunset. The rays of light emitted by the sun are refracted towards the ground while travelling through the optically rarer layers of air in the atmosphere; when the Sun is slightly below the horizon. This results in making the sun appear slightly raised above the horizon and hence the sun is visible 2 minutes before and after actual sunrise and sunset.

c. Mirage

In summers in desserts, due to atmospheric refraction is an optical illusion known as ‘Mirage’ occurs. During the mirage, an object such as a tree appears to be inverted and appears to be on the bank of a pond. On a hot summer day, since the layers of air near earth’s surface become hot, these layers act optically rarer medium.Whereas, the layers of air above the earth’s surface are cooler and therefore act as the optically denser medium. Light, therefore, refracts when it travels through a sudden transition between layers of cold and hot air.

Scattering of Light

A part of light gets absorbed by the particles of any medium ahead of it’s following radiation in a particular direction when light travels from one medium to another medium. This is called as the ‘Scattering of Light’. During the scattering of light; it in all directions.

Without a change in its wavelength, the absorption and re-emission of light energy occurs; wherein the molecules of air smaller than the incident light’s wavelength absorb the incident light’s energy and a re-emit the light energy. Scattered light’s ‘intensity’ is a function of the wavelength of the ray of light.

\( I \times \frac{1}{\lambda}\)

Tyndall Effect

Tyndall effect is the scattering of a beam of light as it passes through a colloid (a medium containing suspended particles like smoke or dust in a room) wherein the suspended particles scatter and reflect the beam of light, making it visible. What differentiates a colloid from a true solution is the size of the particles, i.e. the particle size for a colloid must be between 1-1000 nanometers.

Tyndall Effect was named after the physicist John Tyndall after discovering it in the 19th century.

Why does the sky appear blue?

When white light (sunlight) falls in the atmosphere, rays of light with longer wavelengths don’t get scattered by the air molecules. Since only blue light has the shortest wavelength, it is scattered by air molecules in the atmosphere resulting in the sky appearing blue.

Sun appears red at sunset and sunrise. Why?

During the sunset and sunrise, since all the blue light is scattered away already and is out of our sight, the light reaching is red which has a longer wavelength.

Solved Example For You

Q. What characteristic property of light is responsible for the blue color of the sky?

a. Interference

b. Dispersion

c. Scattering

d. Diffraction

Sol: c. Scattering

A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. When we look towards the sun at sunset, we see red and orange colors because the blue light has been scattered out and away from the line of sight.
Hence, scattering is the characteristic property of light which is responsible for the blue color of the sky.

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