Introduction
Exercise 6(C) of Spectrum (Selina ICSE Physics Class 10) is dedicated to numerical problems based on light, wavelength, frequency, and velocity relations. This section is crucial for scoring in the Physics board exam numericals.
ICSE Physics Spectrum Exercise 6(C) – Key Topics
- Numerical problems on refractive index
- Calculations using wavelength, frequency, and speed of light
- Relation between velocity of light in different media
- Problem-solving based on electromagnetic spectrum
(A) Multiple Choice Type
(Choose the correct answer from the options given below).
Question 1
For scattering of light, the necessary condition is that the size of air molecules should be …………… the wavelength of incident light.
(a) smaller than
(b) greater than
(c) equal to
(d) both (b) and (c)
Answer:
(a) smaller than
Explanation:
Scattering of light occurs when light interacts with particles or molecules in a medium, such as air, causing the light to deviate from its original path. For scattering to take place effectively, the size of the scattering particles (e.g., air molecules) must be smaller than the wavelength of the incident light.
Question 2
The intensity of scattered light is related to its wavelength as :
(a) \(I\ \propto\ \lambda^4\)
(b) \(I\ \propto\ \lambda^2\)
(c) \(I\ \propto\ 1/\lambda^4\)
(d) \(I\ \propto\ 1/\lambda^2\)
Answer:
(c) \(I\ \propto\ 1/\lambda^4\)
Explanation:
Intensity of scattered light is found to be inversely proportional to the fourth power of the wavelength of light.
(i.e., \(I\ \propto\ 1/\lambda^4\)).
Question 3
In the white light of Sun, maximum scattering by the air molecules present in the earth’s atmosphere is for :
(a) red colour
(b) yellow colour
(c) green colour
(d) blue colour
Answer:
(d) blue colour
Explanation:
Among the options, blue has the shortest wavelength, so it scatters the most in air.
Question 4
Violet light is scattered nearly …………… times more than the red light.
(a) 4
(b) 8
(c) 16
(d) 9
Answer:
(c) 16
Explanation:
According to Rayleigh scattering, the amount of scattering is inversely proportional to the fourth power of the wavelength of the light.
We know,
\(\lambda_{violet}=4000\ Å\) and \(\lambda_{red}=8000\ Å\)
According to Rayleigh scattering,
\(\frac{Intensity\ of\ scattering\ of\ violet}{Intensity\ of\ scattering\ of\ red}=\left(\frac{\lambda_{red}}{\lambda_{violet}}\right)^4\)
So,
\(\frac{Intensity\ of\ scattering\ of\ violet}{Intensity\ of\ scattering\ of\ red}=\left(\frac{8000}{4000}\right)^4=2^4=16\)
Hence, Intensity of scattering of violet = 16 time Intensity of scattering of red.
Question 5
To an astronaut in a space-ship, the earth appears :
(a) White
(b) Red
(c) Blue
(d) Black
Answer:
(c) blue
Explanation:
When an astronaut travels beyond Earth’s atmosphere in a rocket, the sky appears black, but the Earth looks blue due to the scattering of sunlight’s blue wavelengths by the atmosphere reaching the astronaut.
Question 6
The red colour of the sun at sunrise and sunset is due to which of the following phenomenon?
(a) deviation
(b) dispersion
(c) both deviation and dispersion
(d) scattering
Answer:
(d) scattering.
Explanation:
The red colour of the sun at sunrise and sunset is due to the scattering of sunlight by the Earth’s atmosphere, specifically through a process dominated by Rayleigh scattering.
Question 7
The sky in a direction other than that of the sun appears …………… to an observer on the moon.
(a) Red
(b) Blue
(c) Orange
(d) Black
Answer:
(d) black
Explanation:
To an observer on the Moon, the sky in a direction other than that of the Sun appears black. This is due to the absence of a significant atmosphere on the Moon, which prevents the scattering of sunlight that causes the sky to appear blue on Earth.
Question 8
The danger signal is red because its wavelength is the …………… and therefore it gets ………… the least.
(a) least, deviated
(b) longest, scattered
(c) longest, dispersed
(d) least, scattered
Answer:
(b) longest, scattered
Explanation:
Red light has the longest wavelength of all visible colours. According to Rayleigh’s law, scattering is inversely proportional to the fourth power of wavelength . So, longer wavelengths scatter the least. That is why red light can travel farther through fog, rain, or dust, making it the safest choice for danger and warning signals.
(B) Very Short Questions
Question 1
When sunlight enters the earth’s atmosphere, state which colour of light is scattered (i) the most, and (ii) the least.
Answer:
(i) Violet is scattered the most.
(ii) Red is scattered the least.
Question 2
A beam of blue, green and yellow light passes through the the earth’s atmosphere. Name the colour which is scattered (a) the least, (b) the most.
Answer:
(a) Yellow light is scattered the least as it has the highest wavelength.
(b) Blue light is scattered the most as it has the lowest wavelength.
(C) Short Questions
Question 1
What is meant by scattering of light?
Answer:
Scattering is the process of absorption and then re-emission of light energy by the dust particles and air molecules present in the atmosphere.
Question 2
How does the intensity of scattered light depend on the wavelength of incident light? State the condition when this dependence hold.
Answer:
The intensity of scattered light is inversely proportional to the fourth power of the wavelength of the incident light, i.e. \(I\ \propto\ 1/\lambda^4\).
This means shorter wavelengths (like blue) are scattered much more strongly than longer wavelengths (like red).
Condition: This relation holds when the scattering particles are much smaller than the wavelength of light (Rayleigh scattering), such as air molecules in the atmosphere.
Question 3
Which colour of white light is scattered the least? Give reason.
Answer:
Red light is scattered the least because it has the longest wavelength among the visible colours. Since scattering is inversely proportional to the fourth power of wavelength (\(I\ \propto\ 1/\lambda^4\)), longer wavelengths scatter much less than shorter ones.
Question 4
The danger signal is red. Why?
Answer:
Red is scattered the least, so it travels farther and remains visible through fog, dust and rain.
Question 5
How would the sky appear when seen from the space (or moon)? Give reason for your answer.
Answer:
Black, because there is no atmosphere there to scatter sunlight.
Question 6
What characteristic property of light is responsible for the blue colour of the sky?
Answer:
The characteristic property of light responsible for the blue colour of sky is scattering.
Question 7
The colour of the sky, in direction other than of the Sun is blue. Explain.
Answer:
Blue (or violet) light, having a shorter wavelength, is scattered much more than red light, which has a longer wavelength. As a result, the light that comes directly from the Sun is rich in red, while the scattered light coming from all other directions is mostly blue. That is why the sky, away from the Sun, appears blue.
Question 8
Why does the Sun appear red at sunrise and sunset?
Answer:
As sunlight travels a longer path through the atmosphere, most of the blue and green light gets scattered away. What finally reaches our eyes is mainly red and orange light.
As a result, the Sun and the region near by it, is seen red.
Question 9
The sky at noon appears white. Give reason.
Answer:
At noon, the Sun is directly above our head, so we get the light rays directly from the Sun after travelling the shortest distance, without much scattering of any particular colour. Hence, the sky is seen white.
Question 10
The clouds are seen white. Explain.
Answer:
Cloud droplets are much larger than the wavelength of light, so they scatter all colours nearly equally, which looks white.
Question 11
Give reason why the smoke from a fire looks white.
Answer:
The smoke from a fire looks white because its particles are larger than the wavelength of light. Such particles scatter all colours equally, and the combined effect of this scattering makes the smoke appear white.
Free PDF Download – Spectrum Exercise 6(C) Solutions
Download ICSE Class 10 Physics Selina Solutions Chapter 6 Spectrum Exercise 6(C) PDF and practice offline.