Physics

# Dispersion and Scattering of Sun Light

## Cause of dispersion and scattering of Sunlight:

By the prism formula [δm = (n-1) A] clear that the deviation of a ray of light of a color by a prism depends on its refractive index n and prism angle A. We know that:
Refractive index (n) = velocity (v) / wavelength (λ).
It is clear that the color which has the highest wavelength will have the lowest refractive index and the one with the lowest refractive index will have the lowest deviation. Among the seven colors of sunlight, the deviation is the least due to the highest wavelength of red light and its deviation is the highest since the wavelength of a purple ray of light is the lowest. The rays of light of other colors deviate between the red and violet light rays according to their wavelength.

### The wavelength of different colors

Sl. Number     Color         Wavelength Range (in Å) {1 Å =10-10 meter}

1)                    Red            7800 – 6400 Å
2)                    Orange       6400 – 6000 Å
3)                    Yellow         6000 – 5700 Å
4)                    Green         5700 – 5000 Å
5)                    Blue           5000 – 4600 Å
6)                    Purple        4600 – 4300 Å
7)                    Violet         4300 – 4000 Å

## A natural example of dispersion of light: Rainbow

After the rain comes in the morning or evening, when the sunlight comes out, little drops of floating water in the atmosphere act as prisms. so the light falling on them is divided into 7 colors due to the color deflection and on the second surface of these drops due to the full internal reflection of light the colored concentric circular arc in the sky starts to appear in the opposite direction of the Sun, that is, if the Sun is in the east then it is in the west and if the Sun is in the west then it is seen in the east. The colored structure or arc is called a rainbow. This phenomenon can be well understood from the following picture-

In the rainbow, the outer arc is colored red and the inner arc is purple.

## Scattering of Sunlight:

When light passes through a transparent medium in which microscopic particles of dust, smoke, air, etc. are present, some part of the light is spread in all directions by those particles, this phenomenon is called scattering of light.
According to Lord Rally, “The amount of scattering of light is the wavelength of light that is inversely proportional to the fourth power.” Ie – scattering α 1 / λ4

Therefore, the scattering of red light is the lowest (due to the highest wavelength), and the scattering of violet light is the highest (due to the lowest wavelength).

### Natural phenomena based on the scattering of light:

#### 1) Blue appearance of the sky:

When the sunlight passes through the dust particles, gas molecules, etc., in the atmosphere,  it becomes scattering. In this blue and violet color scattering is about 16 times more than red, so blue and violet color are scattered around And this scattered color reaches our eyes, making us see the sky blue.

If there is no atmosphere, then the sky looks black as there is no atmosphere on the moon, so when viewed from the lunar surface, the sky appears black.

#### 2) Sun red appearance at sunrise and sunset:

At sunrise and sunset, the rays of light have to travel more distance than in the afternoon to reach our eyes. At this time only red rays from the light transmitted to our eye reach because of these rays Scattering is the lowest, that is why the sun appears red at sunrise and sunset.

While in the afternoon light rays have to travel less distance, so scattering is also less and the rays of light of all colors reach our eyes, hence the sun White appears.

#### 3) Red color of danger signal:

The scattering is the lowest due to the higher the wavelength of the red light rays, so it can be seen from a greater distance, hence the red color is used for danger signals.

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