Education Journalist | Study Abroad Strategy Lead
The scattering of light and its interaction with it surrounding environment allows us to experience a number of beautiful phenomena. Sunlight allows us to experience the entire plethora of colors the world has to offer. Let us discuss some of the phenomena that occur in nature due to sunlight.
| Table of Content |
Key Takeaways: Scattering of Light, Rainbow, Blue colour of the Sky, White Clouds
The Rainbow
[Click Here for Sample Questions]
The rainbow with its spectacular 7 colours spread across the sky, is an amazing and most beautiful optical phenomenon. It can be seen when a person has his/her back towards the sun. When the water droplets in the atmosphere disperse in the presence of sunlight, the rainbow is visible to us. For a rainbow to be visible, there are two conditions that should be met-
- The sun should be shining in a part of the sky.
- In the opposite part of the sky to the sun, it should be raining
Process of Rainbow Formation
As the light from the sun enters a water droplet present in the atmosphere, it is refracted. The white light then splits into 7 component colours of the rainbow of different wavelengths. The colour having a longer wavelength of light i.e. red is bent the least. The colour having a shorter wavelength of light i.e. violet is bent the most. These rays of light then strike the inner surface of that water droplet leading to reflection if the angle between the refracted ray and normal to the drop surface is greater than the critical angle i.e. 48 degrees in this case. As this light moves forward to come out of the droplet, it undergoes refraction again. The red light comes out at an angle of 42 degrees to the sunlight while the violet light comes out at an angle of 40 degrees. All the remaining colours exist between these two.
Process of Rainbow Formation
This is how a primary rainbow is formed as a result of 3 steps of processes - refraction, reflection and refraction.
But why is it that when you observe a rainbow, you see a red colour on the top and violet on the bottom?
This is because the violet light from drop 1 and the red one from drop 2 are directed at various levels that in practice lies above or below the observer. So, the red light coming from drop 1 and the violet light coming from drop 2 reaches the eyes of the observer and we see a rainbow with red colour on top and violet on bottom.
Sometimes two rainbows can be seen in the sky. The other one is the secondary rainbow which is formed by undergoing 2 internal reflections inside the raindrop. The intensity of the light gets reduced during the second reflection and hence the secondary rainbow appears fainter than the primary one.
Also Read:
The Blue Colour of the Sky
[Click Here for Sample Questions]
When sunlight enters the earth’s atmosphere and travels, it gets scattered by the particles present in the atmosphere. The light changes its direction hence. The light having shorter wavelengths is scattered much more than the light having longer wavelengths. It is due to this reason that blue colour dominates the sky above as it has a shorter wavelength, it scatters more. Now, violet has a much shorter wavelength than blue and so it scatters the most but why are we not able to see it? Because our eyes are more sensitive to blue colour than violet and hence the sky appears blue to us.
White Clouds
[Click Here for Sample Questions]
There are various large particles present in the atmosphere like water and dust. λ is the relative size of the wavelength and the scatterer is of size a. For a << λ, the scattering occurs in accordance with Rayleigh scattering (The amount of scattering is inversely proportional to the fourth power of the wavelength). While for large scattering objects like a raindrop, dust or ice particles, a >> λ, it is not true that all wavelengths are nearly scattered equally. Hence the clouds having a >> λ are generally white.
The Reddish Appearance of the Sun and Full Moon near the Horizon
During the times of sunrise or sunset, the light rays have to travel more in the atmospheric air. Most of the blue and colours of other such wavelengths get scattered thus leading to the least scattered light i.e. red reaching our eyes. Hence, the sun looks reddish to us during sunrise and sunset and the full moon has a reddish appearance near the horizon.
Things to Remember
- The scattering of light and its interaction with it surrounding environment allows us to experience a number of beautiful phenomena.
- Sunlight allows us to experience the entire plethora of colors the world has to offer.
- The rainbow with its spectacular 7 colours spread across the sky, is an amazing and most beautiful optical phenomenon.
- When sunlight enters the earth’s atmosphere and travels, it gets scattered by the particles present in the atmosphere.
- The light changes its direction hence. The light having shorter wavelengths is scattered much more than the light having longer wavelengths.
- It is due to this reason that blue colour dominates the sky above as it has a shorter wavelength, it scatters more.
- There are various large particles present in the atmosphere like water and dust.
Also Read:
Sample Questions
Ques 1. The angle of minimum deviation a glass prism will be the smallest for the light of
a) blue colour
b) red colour
c) green colour
d) yellow colour (CBSE ’20)
Ans. Red colour
Ques 2. A ray of light on passing through an equilateral glass prism, suffers a minimum deviation equal to the angle of the prism. The value of the refractive index of the material of the prism is _______. (CBSE ’20)
Ans. 
Ques 3. If the incident violet light is replaced by red light, how does the angle of minimum deviation of glass prism vary? (CBSE ’17)
Ans. The wavelength of red light is greater than that of the red light. Again, the refractive index of any light is in inverse proportion to the wavelength for which the refractive index of red light is less. In turn, the angle of minimum deviation possesses linear dependence with refractive index.
δ = (μ – 1) A
This is the reason why when the violet light is replaced by red light, the angle of minimum deviation decreases.
Ques 4. If the incident violet light is replaced with red light, How does the angle of minimum deviation of a glass prism vary? (Delhi ’17, All India ’8)
Ans. As we know that λ red is greater than λ violet which is why μ red is lesser than μ violet and therefore, δ red is lesser than δ violet. The angle of minimum deviation of a glass decreases if incident violet light is replaced with the red light.
Ques 5. What is the reason that bluish colour is predominant in the clear sky? (All India ’8, ’15)
Ans. The scattering of the light by the atmosphere is colour dependent. When sunlight enters the earth’s atmosphere and travels, it gets scattered by the particles present in the atmosphere. Based on the Rayleigh’s law, the intensity of scattered light I ∝ 1/ λ4, the blue light gets scattered more strongly than the red light which is why the colour of the sky becomes blue. Violet has a much shorter wavelength than blue and so it scatters the most but our eyes are more sensitive to blue colour than violet and hence the sky appears blue to us.
Ques 6. A ray of light incident normally on one of the faces of a right isosceles prism is totally reflected as shown in figure. Give relevant calculations for what must be the minimum value of the refractive index of glass. (CBSE ’16)
Ans. ABC is the section of the prism where, B is the right angle, A and C are equal angles i.e, A=C=45°
The ray PQ is normally incident on the face AB which is why it is normally refracted and the ray and the ray QR strikes the face AC at an angle of incidence 45° and as it is given that the ray does not undergo refraction, so it is totally reflected at the face AC. this gives the maximum value for the critical angle as 45°
Since, Sin C = Sin 45° = 1/√2 in the limit
μ = 1/Sin C = 1/√2
Or, μ = 1/ Sin 45°, μmin= √2
Therefore, the minimum value of refractive index is √2
Ques 7. An equilateral glass prism has a refractive index 1.6 in air. When kept in a medium of refractive index 4√2/5, calculate the angle of minimum deviation of the prism. (CBSE ’19)
Ans. A prism PQR given below is a wedge-shaped body bounded by two plane faces inclined to each other at an angle made from refracting medium.
When a light ray travels through a prism, it bends towards the base of a prism. And the angle made by the emergent ray with the incident ray is known as angle of deviation. As the angle of incidence increases, the angle of deviation decreases to minimum δm.
The angle of minimum deviation is, 
A= angle of the prism
μ= refractive index and for a equilateral prism, A= 60°
The refractive index of the prism when kept in air is 1.6
Hence, by using equation 1, the angle of minimum duration in air is,
And when the prism is immersed in a medium of refractive index 4√2/5, then by applying equation 1 we get,
Therefore, the angle of minimum deviation of the prism is 30°.
For Latest Updates on Upcoming Board Exams, Click Here: https://t.me/class_10_12_board_updates
Check-Out:
Comments