Sir Isaac Newton allowed white light from the sun to enter a dark room through a small aperture in a window and places a glass prism in the path of light rays. The light coming out of the prism was received on a white screen. He observed that on the screen a colored patch like a rainbow was formed. This patch was termed a spectrum. Starting from the side of the base of the prism the colors in the spectrum on the screen are in the following order:
violet, indigo, blue, green, yellow, orange, and red.
Spectrum is the colored band obtained on a screen when the white light is passing through a prism. For this experiment, Newton concluded that white light is a mixture of seven colors. Prism is not producing the colors but is simply separating the colors which already exist in white light. So, when white light is passed through a prism, it splits into different colors. The splitting of white light into different colors is called dispersion of light.
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1. Dispersion of light occurs only at the first surface of the prism. 2. Refraction of light rays occurs at both surfaces of the prism. 3. Prism does not produce colors, it only splits the various colors present in the light incident on it. 4. In the spectrum, each color is mixed with the other color, i.e. there is no sharp boundary line separating the colors. In the diagram, colors are shown widely separated just for clarity. The total spread of colors is much less than that shown in the diagram. Different colors have different widths on the screen. 5. In the spectrum of white light, the red color has the longest wavelength of 8000 Å( or 8 × 10-7m ) or the lowest frequency of 3.74 × 1014Hz, and the violet color has the shortest wavelength of 4000 Å ( or 4 X 10-7m ) or highest frequency 7.5 X 1014Hz. From the violet end to the red end of the spectrum, the wavelength increases while the frequency decreases. |
The white light of the sun is composed of seven prominent colors namely violet, indigo, blue, green, yellow, orange, and red. The speed of light of all colors in air or vacuum is the same but it differs in a transparent medium like glass or water. The speed of violet light is minimum and of red light is maximum therefore the refractive index of the transparent medium is different for different colors.
Since µ = (speed of light in the air) ∕ (speed of light in medium)
So, the refractive index of a medium is maximum for violet light and minimum for red light. Therefore when white light enters a prism, it splits into its constituent colors with refraction on the first surface of the prism. When these rays strike the second prism surface they refract further and these colors get farther separated from each other.
When light from the sun enters the earth's atmosphere, it gets scattered (i.e. the light spreads in all directions) by the dust particles and air molecules present in the earth's atmosphere. The scattering of light was first studied by the scientist Rayleigh.
Scattering is the process of absorption and then re-emission of light energy by the dust particles and air molecules present in the atmosphere.
The air molecules of size smaller than the wavelength of incident light absorb the energy of incident light and then re-emit it without a change in its wavelength. The scattering of light is not the same for all wavelengths of the incident light. The intensity of scattered light is inversely proportional to the fourth power of the wavelength of light \(I \propto 1/_{\lambda^4}\).
As the wavelength of violet light is the least and red light the most, the violet light is scattered the most, and red light is scattered the least (violet light is scattered nearly 16 times more than red light). This means the light from the sun when reaching the earth's surface has less intensity of the light of the violet end and more intensity of the light of the red end. The air molecule of size bigger than the wavelength of incident light scatters the light of all wavelengths of white light to the same extent.
The light from the sun has to travel a long distance from the earth's atmosphere before it reaches the earth's surface. As light travels through the atmosphere it gets scattered in different directions by the air molecules. Blue or violet light due to its shorter wavelength is scattered more as compared to other light colors. The light reaching our eye directly from the sun is rich in red color while the light that reaches our eye from all other directions is mainly blue light. Therefore the sky in a direction other than the direction of the sun is seen as blue.
The formation of a rainbow is a natural example of the dispersion of white light. Following a shower of rain, a large number of water droplets remain suspended in the air. Every droplet acts as a prism. As sunlight falls on these droplets, it splits into seven colors. The dispersed light from a large number of droplets forms a rainbow
Use of red light for danger signal
As the wavelength of red light is the longest, therefore, the light is scattered the least by the air molecules of the atmosphere. Hence the light of red color as compared to the light of other colors can penetrate to a longer distance without becoming weak. Hence red light is used for danger signals so that the signal can be visible from a large distance even in fog etc.
Take a circular disc of cardboard and divide it into seven sectors. Then paint the sectors with the seven colors, violet, indigo, blue, green, yellow, orange, and red.
Rotate the disc rapidly, you will notice that the disc appears white!
This shows that violet, indigo, blue, green, yellow, orange, and red is the seven constituent colors of white light and when combined produce the white effect.
