RAINBOW[THE RAINBOW COLORS]
the rainbow is an example of the dispersion of sunlight by the water drops in the atmosphere. This is a phenomenon due to the combined effect of dispersion, refraction, and reflection of sunlight by spherical water droplets of rain.
An observer can see a rainbow only when his back is towards the sun.
Some time two rainbows are seen. The inner rainbow is called the primary rainbow and the outer one is called the secondary rainbow.
The secondary rainbow is bigger than the primary rainbow.
a. Primary rainbow:-
The primary rainbow has a violet color on the inner edge and the red color on the outer edge of the rainbow
The primary rainbow is formed due to two refraction and one TIR of the incident light, as shown in fig. Violet is bent the most and so reaches the observer’s eye from droplets lower in the sky. On the other hand, red is bent the least so that it reaches the observer’s eye from droplets higher in the sky. Other colors of the light are seen at intermediate angles. The result is that the top of the rainbow appears red and the inner arc appears violet. It is found that the violet light emerges at an angle of related to the incoming sunlight (horizontal) and red light emerges at an angle of
.
b. Secondary rainbow:-
the secondary rainbow has red color on the inner edge and violet color on the outer edge of the rainbow.
The secondary rainbow is formed due to two refraction and two TIR of light incident on the raindrop as shown in fig.
The order of colors of the secondary rainbow is just reverse, this is due to the fact that light enters from the bottom of the raindrop.
The intensity of light is reduced at the second reflection and hence the secondary rainbow is fainter than the primary rainbow. the secondary rainbow is larger than the primary rainbow because it is found that the violet light emerges at an angle of and red light emerges at an angle of
related to the incoming sunlight (horizontal)
SCATTERING OF LIGHT
If the molecules of a medium after absorbing incoming radiation( light), emit them in all possible direction, except its incident direction. This is known as the scattering of light.
Rayleigh has shown theoretically, the amount of scattered light depends on the wavelength of light as
i.e light has a smaller wavelength, scattered more and vice versa.
PHENOMENA BASED ON SCATTERING OF LIGHT
a. Blue color of the sky:-
The blue color of the sky is due to the scattering of sunlight by the molecules presents of the atmosphere. Since the wavelength of blue color is smaller than the wavelength of red color (), the scattering of blue light by molecules is very large. For this reason, sky appears blue.
In fact, violet gets scattered even more than blue, having a shorter wavelength. But since our eyes are more sensitive to blue color than violet, we see the sky blue.
b. Clouds are white:-
A cloud is composed of large-sized dust particles and water molecules. These large-sized particles do not obey of scattering. Therefore all colors are scattering nearly equally. Hence clouds appear white (or gray if shadowed)
c. Sun looks reddish at the sunset or sunrise:-
At the sunset or sunrise, the sun is near the horizon. The rays from the sun must travel much more distance through the atmosphere than at noon. Most of the blue and other shorter wavelengths are removed by scattering. Only the red color, which is least scattered, reaches our eyes. For this reason, the sun/ moon looks reddish at the sunset and sunrise.
d. danger signals are red:-
We know that red color has the longest wavelength so that it is least scattered by the particles of the atmosphere. Even in foggy conditions, the red colors larger distances without appreciable loss of intensity.
For this reason, danger signals are of red color.