Ch. No. 7. Wonders of light – part II
Q1. Fill in the blanks
1. The phenomenon of splitting of light into its component colours is ____________ (dipersion)
2. Very fine particles mainly scatter ___________ light. (blue)
3. The phenomenon of change in the ___________ of light when it passes from one transparent medium to another is refraction. (direction)
4. The refractive index depends upon the ___________ of propagation of light in different media. (relative speed)
Q2. Rewrite the following table in such a way that column 2 and column 3 matches column 1.
| Column 1 | Column 2 | Column 3 |
| Dispersion | Change in the direction of ray of light due to change in medium. | Blue colour of the sky. |
| Scattering | Splitting of white light into component colours | Twinkling of stars. |
| Refraction | Deflection of light by small particles | Spectrum of seven colours. |
Ans.
| Column 1 | Column 2 | Column 3 |
| Dispersion | Splitting of white light into component colours | Spectrum of seven colours. |
| Scattering | Deflection of light by small particles | Blue colour of the sky. |
| Refraction | Change in the direction of ray of light due to change in medium. | Twinkling of stars. |
Q3. Answer the followings.
1. What do you mean by dispersion? Name the different colours of light in the proper sequence in the spectrum of light.
Ans.
i. The phenomenon of splitting of white light into its component colours is dispersion.
ii. The various colours in the spectrum of light are in the sequence: Violet, indigo, Blue, Green, Yellow, Orange and Red.
2. Explain how the formation of rainbow occurs.
Ans.
1. The beautiful phenomenon of the rainbow is a combination of different phenomena – dispersion, refraction and reflection of light.
2. The rainbow appears in the sky after a rain shower.
3. The water droplets act as a small prisms.
4. When sunlight enters the water droplets present in the atmosphere, they refract and disperse the incident sunlight.
5. Then they reflect it internally inside the droplet and finally again refract it.
6. As a collective effect of all the phenomena, the seven coloured rainbow is observed.
3. What is refraction of light? How is it related to refractive index?
Ans.
i. The phenomenon of change in the direction of light when it passes from transparent medium to another is called dispersion of light.
ii. The extent of change in the direction of the light ray is different for different media and it is related to the refractive index of the medium.
Q4. Give reasons.
1. The sky appears dark instead of blue to a person located in space.
Ans.
i. At very high altitudes, there is no atmosphere.
ii. Since there is no atmosphere, the scattering of light does not take place at all.
iii. Hence, the sky appears dark instead of blue to a person located in space.
2. Stars twinkle at night.
Ans.
i. In the atmosphere, there are different layers of air with different refractive indices which keep on changing.
ii. When we observe the stars through this air the light coming from them refract randomly so that the intensity of light varies due to which the apparent position of the star fluctuates.
iii. When more light reaches our eyes the star is seen bright and when less light reaches our eyes, the star is seen dim.
iv. Thus, due to change in refractive index of atmosphere, stars appear twinkling at night.
3. The sun appears reddish early in the morning.
Ans.
i. At the time of sunrise, the blue and violet colours are scattered away from the path of sunlight as thickness of the atmosphere is more between the horizon and the observer.
ii. The light that reaches to the observer is mostly red.
iii. Hence the sun appears reddish early in the morning.
4. It is possible to enjoy a rainbow at fountains in any season.
Ans.
i. One can observe and enjoy the rainbow by standing in front of a water fountain in the evening facing the east.
ii. One can also enjoy it in the morning under the same conditions just by facing the west.
iii. Therefore, it is possible to enjoy a rainbow at fountains in any season.
Q5. Write short notes on
1. Refraction observed in the atmosphere.
Ans.
i. In the atmosphere, there are different layers of air with different refractive indices which keep on changing as the physical conditions of air are not stationary (still).
ii. When we observe any object through this air, the light coming from them refract randomly due to which the apparent position of the object fluctuates.
iii. The large scale effect of this phenomenon is the twinkling of stars, advanced sunrise and delayed sunset.
iv. Due to change in the refractive index of atmosphere, the intensity of light that reaches our eyes from the stars varies and hence the stars appear twinkling at night.
v. Advanced sunrise occurs as a ray of light from the sun enters the earth’s atmosphere; it follows a curved path due to refraction before reaching to the observer.
vi. It appears to the observer as if the rays are coming from the position where the sun is seen by the observer, hence, the sun is seen earlier before it reaches the horizon.
2. Dispersion of light.
Ans.
i. The phenomenon of splitting of light into its component colours is dispersion of light.
ii. The band of coloured components of a light beam is called it’s spectrum.
iii. When White light is dispersed into seven colours by a prism, different colours of light bend through different angles with respect to incident ray.
iv. Out of these seven colours, red light bend the least while violet light bends the most.
v. So the rays of each colour emerge along different paths and become distinct.
Hence we get a spectrum of seven different colours in the sequence, Violet, Indigo, Blue, Green, Yellow, Orange and Red.
