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Dispersion and Scattering of Light

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Dispersion and Scattering of Light - Lesson Summary

Rainbow is the natural phenomenon in which dispersion takes place. The cause of dispersion is that sun light consists of seven constituents (colours namely violet, indigo, blue, green, yellow, orange and red popularly referred to as VIBGYOR) that have different refractive index with respect to a medium. The wavelength of each colour is different that causes the difference in velocity of the corresponding light when passing from one medium to another.

This phenomenon can be observed in a lab environment using a triangular glass prism. It is a solid structure having three rectangular and two triangular surfaces. Any two rectangular faces are the refracting surfaces and the third one is the base. The angle between two refracting surfaces of a triangular glass prism is denoted by A, called the angle of the prism or the refracting angle.


The ray that deviates at the point of incidence due to a change in the medium is the refracted ray.
The angle formed between the incident ray and the normal at the point of incidence is known as the angle of incidence. 
The angle between the normal and the refracted ray is known as the angle of refraction.
The angle between the directions of the incident ray and that of the emergent ray is called the angle of deviation and is represented by Greek letter δ or qd.

        •  The splitting of white light into its constituent colours is called dispersion of light.
        •  Light disperses and creates a rainbow effect, when it propagates and refracts in a prism.
        •  Light disperses and creates a rainbow effect, when it propagates and refracts in a prism.

The order of colours in a rainbow is popularly identified using the acronym, VIBGYOR, each letter standing for a colour in order.
Atmospheric refraction refers to the apparent random wavering or flickering of objects due to inconsistency in the physical conditions of the refracting media such as air.
In scientific terms, the twinkling of stars is termed as astronomical scintillation. When the sun is just below the horizon, its rays enter earth’s atmosphere and are refracted towards the earth. The refracted rays reach the earth making it appear as if the sun has already risen above the horizon. This is apparent sunrise.
Apparent sunset occurs slightly later than the actual sunset, since the light from the sun is already below the horizon, it refracts through the atmosphere, enabling us to see the apparent sunset, even after the sun has already set.

Scattering of light is the deviation of light rays from its straight path. As light propagates through the atmosphere, it travels in a straight path until it is obstructed by bits of dust or gas molecules.

During sunrise and sunset, the sun is at the horizon and refractive index of the atmosphere of the earth decrease with height. Due to this, light reaching the earth's atmosphere from different parts of the vertical diameter of the sun enters at different heights in earth's atmosphere and so travels in media of different refractive indices at the same instant and hence, bend unequally. Due to this unequal bending of of light from the vertical diameter, the image of the sun gets destored and it apppears oval and larger. However, at noon when the sun is overhead, then due to normal incidence of light there is no bending of light and hence, the sun appears circular.
The sky appears blue because out of the seven colours of light, blue has the shortest wavelength, and therefore it experiences more scattering than other colours.
Scattering of light gives rise to many amazing and spectacular phenomena such as the Tyndall effect and the reddening of the sun at sunrise and sunset.
The Tyndall effect is the scattering of light by colloidal particles.

Sun Appears red at Sunrise and Sunset
The sun appears white at noon becuase the light from the sun overhead would travel relatively shorter distance. As only a little of the blue and violetcolours are scattered.

the light from the sun, near the horizon, passes through the thicker layers of air and covers a large distance in the earth's atmosphere before reaching our eyes.

Near the horizon, most of the blue light and other shorter wavelengths are scattered away by the particles. Therefore, the light that reaches our eyes is of longer wavelengths. This gives rise to the reddish appearance of the sun.

Clouds are White

Clouds are white because their water droplets or ice crystals are large enough to scatter the light of the seven wavelengths the component colours of white light(i.e red, orange, yellow, green, blue, indigo, and violet), which combine to produce white light.


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