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Scattering of Light: What Causes Sky to Appear Blue or Red?

Light can be investigated completely from its source. When light passes from one medium to another, such as air or a glass of water, a portion of the light is absorbed by medium particles, followed by subsequent radiation in a specific direction. This phenomenon is known as scattering of light. The intensity of scattered light is determined by particle size and light wavelength. Because of the wave nature of light and its intersection with a particle, shorter wavelengths and higher frequencies scatter more. The more wavy the line, the more likely it will intersect with a particle.

Longer wavelengths, on the other hand, have a lower frequency and are straighter, so the chances of colliding with a particle are lower. In the afternoon, the bending of multicolored light can be seen due to refraction and total internal reflection of light. Sunlight’s wavelength produces different colors in different directions. Rayleigh scattering theory explains why the sun is red in the morning and the sky is blue.

What is scattering of light?

Scattering of Light: What Causes Sky to Appear Blue or Red?

When sunlight enters the earth’s atmosphere, the atoms and molecules of various gases in the air absorb the light. These atoms then re-emit light in all directions. This is referred to as scattering of light. Scatterers are atoms or particles that scatter light. When the particle (Scatterer) has a smaller diameter than the wavelength of light, the intensity of scattered light is inversely proportional to the fourth power of the wavelength of incident light.

When light encounters particles on its path, it is scattered by the particles. The intensity of the scattered light is determined by particle size and light wavelength.

Rayleigh theory of scattering of light

Scattering of Light: What Causes Sky to Appear Blue or Red?

The sky is blue because of Rayleigh theory, a theoretical description of light scattering involving particles much smaller than the wavelength of the light. According to this theory, when light strikes a molecule smaller than its wavelength, electric fields temporarily polarize it, redistributing electrons in the molecule so that one end has a weak positive charge and the other has a weak negative charge. A dipole moment is the separation of charges. Now, the molecule’s dipole moment oscillates as the oscillating electric fields interact with it, and the molecule re-radiates the light in all directions.

Difference between Effusion and Diffusion of Gases

Rayleigh scattering is elastic scattering because the photon energies of the scattered photons do not change. Molecules with sizes larger than the wavelength of light, experience scattering differently. They are subjected to what is known as the Mie Effect. The light appears white because the particles are larger. This is why clouds, which are made up of water droplets, appear white.

Probability of scattering

If we consider p to be the probability of scattering and is λ the wavelength of radiation:

P ∝ (1 / λ4)

The probability for scattering will give a high rise for shorter wavelength and it is inversely proportional to the fourth power of the wavelength of radiation.

Difference between scattering and reflection

Although it is a very little difference but it is very important to clear. Scattering occurs when a light particle is completely absorbed and then emitted, whereas reflection occurs when a wave or particle is simply reflected off the surface without interacting.

Scattering of Light: What Causes Sky to Appear Blue or Red?

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Difference between scattering and refraction

Refraction is the bending of light as it passes through different transparent/translucent mediums, with no change. So refraction does not cause any change in the properties of light. On the other hand, Light’s properties change as a result of scattering, as light interacts in case of scattering.

Why do shorter wavelengths scatter more than longer wavelengths?

Shorter wavelengths and higher frequencies scatter more because the more wavy the line is, the more likely it is to collide, with a particle. Longer wavelengths have a lower frequency and thus have fewer chances because they are straighter and have a lower chance of colliding or intersecting with a particle.

Applications of scattering of light

There are many applications of scattering of light, from which some are:

  • Blue color of the sky
  • Red sky and sun at sunrise and sunset
  • White color of clouds

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Why is the sky blue?

Scattering of Light: What Causes Sky to Appear Blue or Red?

Because of the shorter wavelength and the length of the path, the sky is blue. As the path distance changes, so does the color of the sky. When sunlight enters the atmosphere, it scatters. Red light has the longest wavelength and thus scatters the least. Violet rays are the most scattered, followed by blue, green, yellow, and orange. Because our eyes are more sensitive to blue light, we see the sky as blue.

Why sky is red at sunset and sunrise?

Scattering of Light: What Causes Sky to Appear Blue or Red?

The sun’s position is very far away from the earth at sunrise and sunset. The sun’s rays travel a greater distance through the atmosphere. Blue light scatters more than red light. As a result, more red light than blue light reaches our eyes. As a result, sunrise and sunset appear reddish.

Why clouds are white?

Scattering of Light: What Causes Sky to Appear Blue or Red?

Clouds are closer to the earth’s surface and contain dust particles and water molecules larger than the wavelength of visible light. Because these particles scatter all colors of incident white light from the sun to the same extent, the clouds appear white when the scattered light reaches our eyes.

What is Difference between Distance and Displacement?

What is Tyndall effect?

The phenomenon of light scattering by dust, particles, smoke, and water droplets suspended in colloidal form in air is known as the Tyndall effect or Tyndall Scattering. When the diameter of the dispersed particles is slightly smaller than the wavelength of light used, the Tyndall effect occurs. Light with a longer wavelength is more transmitted, while light with a shorter wavelength is more scattered.

What is Mie effect?

When the size of the molecules is larger than the wavelength of the incident light then the more scattering occurs. This effect is known as the Mie effect.

If you observe here, then both Tyndall effect and Mie effect are just opposite to each other. In Tyndall effect, more light gets transmitted while in the Mie effect the more light is scattered.

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