Some natural phenomena of Dispersion of light : The Rainbow, Raddish appearance of horizon at Sunrise and Sunset, Scattering of light and see the Sky blue

16/07/2020 Vinod 0 Comments

Dispertion of light :-

Dispersion of light is the phenomenon of splitting of a beam of white light into its 7 constituent colours when passed through a transparent medium like water droplets or Prism .

It was discovered by Isaac Newton in 1666 . Newton discovered that light is made up of seven different colours . He passed a beam of Sunlight through a glass prism . The glass prism split the light into a band of Seven colours on the wall . He called this band of colours the spectrum and the phenomenon is called Dispersion of light .

There are following natural phenomena of dispersion of light .

  1. The Rainbow.
  2. Raddish appearance of horizon at sunrise and sunset.
  3. Scattering of light.
  4. See the sky blue.

1.The Rainbow :-

It is the phenomenon due to combined effect of dispersion of light . refraction and reflection of light by spherical water droplets of rain.

The conditions for observing a rainbow are that the sun should be shining in one part of the sky while it is raining in the opposite part of the sky. An observer can therefore see a rainbow only when his back is towards the sun.

In this phenomena the spectrum of light appearing in the sky, it takes the form of multicoloured circular arc due to the dispersion of light .

  • It has two parts – primary and secondary . Both Rainbows appear togather.

Primary rainbow :-

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Fig. 1. Formation of Primary Rainbow.

It is formed due to two refractions and one total internal reflection of light incident on the droplet. Sun light is first refracted as it enters a rainbow which cause different colours of light are bent through different angles. The reflected light is refracted again as it comes out of drop. Thus the observer see a natural phenomena with red colour on the top and violet on the bottom.

The It is the brightest band of colours. Both primary and secondary rainbows are appear togather but primary occurs inner to the rainbow.

Secondary rainbow :-

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Fig. 2. Formation of Secondary Rainbow.

It is formed due to two refractions and two total internal reflection of light incident on the water droplet.

The intensity of light is reduced at the second reflection and hence the secondary rainbow is fainter than the primary . It is the dimmer band of colours.

They appears outside of a primary . In secondary red light from the lower drop enters the observer’s eye and violet is not seen.

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Fig. 3. The Rainbow.

2. Raddish appearance of horizon at sunrise and sunset :-

It is a phenomenon of dispersion of light .

At sunset and sunrise, the Sun’s rays have to pass through a larger distance in the atmosphere. Most of the blue and other shorter wavelengths are removed by scattering. The least scattered light ( red ) reaching our eyes, therefore the Sun looks raddish.

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Fig. 4. Raddish appearance of horizon at sunrise and sunset.

3. Scattering of light :-

When the sun light travels through the earth’s atmosphere, it gets scattered ( change its direction ) by the atmospheric particles, then this phenomenon is called scattering of light.

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Fig. 5. Scattering of light by atmospheric particle.

Light of shorter wavelength is scattered much more than light of longer wavelengths.

Rayleigh scattering :-

The amount of scattering is inversely proportional to the fourth power of the wavelength.

4. See the sky blue :-

Due to the dispersion of light , white light is split into several colours such as violet , indigo , blue , green , yellow , orange and red .

Since blue has a shorter wavelength than red and is scattered much more strongly. In facts, violet gets scattered even more than blue having a shorter wavelength but our eyes are more sensitive to blue than violet. So we see the sky blue.

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Fig. 6. See the Sky blue.

Question : – Explain the formation of Rainbow .

Answer – The formation of rainbow by considering the process of dispersion of light . When it comes to the rainbow , we see it seven colours in the Sky just after the rain when the Sun shines .

Normally , after rainfall a large number of water droplets get suspended in the atmosphere , these droplets of water function as small Prisms .

Therefore , when the white light emitted by the Sun falls on the droplets , then the light gets split into several different colours due to the dispersion of light and rainbow get formed .

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