WBCHSE Class 12 Physics Dispersion Of Light Long Question and Answers

Unit 6 Optics Chapter 4 Dispersion Of Light Long Question

Question 1. A ray of white light is allowed to pass through a hollow prism made by parallel glass plates. Will any spectrum be produced? Explain with figure
Answer:

Given

A ray of white light is allowed to pass through a hollow prism made by parallel glass plates.

A spectrum will not be produced if a ray of white light is allowed to pass through a hollow prism. The path of a ray of white light through a hollow prism Is shown. The two refracting faces LM and LN of the hollow prism are two parallel glass plates.

A ray of light PQ Is refracted In the glass along QR after incidence on the face LM and moves along RS parallel to lQ. As the prism is hollow the ray RS

As the prism is hollow the ray RS is incident on the second refracting face LN. The ray is refracted along ST and emerges from the prism along TU Parallel to PQ.

As the rays of all colours trim the same white Is note merge direction, the light produced. from takes node place.dispersion prism rays. So of along spectrum is not produced

Dispersion And Scattering Of Light Prism MAde By Parallel Glass Plates

Question 2. Explain why dispersion of white light does not take place by using a rectangular slab of glass instead of a prism
Answer:

Dispersion of white light does not take place by using a rectangular slab of glass instead of a prism

Dispersion of white light does not take place when it is refracted through a rectangular glass slab instead of a prism. This type of glass slab may be considered as a combination of two similar prisms P1 and P2

The refracting faces of the two prisms are parallel to each other, but the bases of the prisms are situated opposite to each other. White light passing through the prism Pj produces spectrum but the different colours of the spectrum again recombine after passing through the prism P2. As a result, the emergent ray becomes white

Question 3. Why is red light always used as a danger signal?
Answer:

Red light always used as a danger signal

The wavelength of red light is maximum. The intensity of scattered light is inversely proportional to the fourth power of wavelength. So, red light is scattered much less than green or blue light while passing through the dust particles floating in air. The red light can be seen from long distances as a necessary caution

Question 4. A tree having green leaves and red flowers is observed by lighting it successively in green, red and blue light. What will be the colours of the leaves and the flowers in each case?
Answer:

A tree having green leaves and red flowers is observed by lighting it successively in green, red and blue light.

When the tree is illuminated with green light the leaves will reflect green light. So the leaves look green. The red flowers absorb all colours except red. So in this case the green light will be absorbed by the flowers. No light will come to the eyes of the observer. Hence, the flowers will look dark.

When the tree is illuminated with red light the green leaves will absorb the red light. So the leaves will look black. The flowers will reflect the incident red light. So they will look red. If the tree is illuminated with blue light, it will be absorbed by both the leaves and the flowers. So in this case both the leaves and the flowers will look dark.

Question 5. Which is the fundamental factor frequency or wavelength—in the analysis of a spectrum?
Answer:

The fundamental factor frequency or wavelength—in the analysis of a spectrum

Frequency is fundamental compared to wavelength for analysis of a spectrum because the frequency of a monochromatic light remains the same in different media

Question 6. A piece of cloth which looks red in daylight may look black at night if properly illuminated. But a piece of cloth which looks black in daylight may not look red if illuminated by any colour of light. Explain the reason.
Answer:

A piece of cloth which looks red in daylight may look black at night if properly illuminated. But a piece of cloth which looks black in daylight may not look red if illuminated by any colour of light.

The piece of cloth which looks red in daylight absorbs all the colours except red. The cloth looks red as it reflects only the red light. So at night if the cloth is illuminated by light of any colour other than red, it will absorb the light. Hence, the cloth will look black.

The piece of cloth which looks black in daylight absorbs all the colours. So at night when the cloth is illuminated by any colour of light, it will absorb that colour. No light will be reflected from it. So the clothes will look black. Hence, the piece of cloth which looks black in daylight will look black if illuminated by any colour of light at night

Question 7. How will a blue object look in sodium vapour flame?
Answer:

A blue object look in sodium vapour flame

A blue object absorbs all the colours of white light except blue. In sodium vapour flame only yellow colour exists. This yellow colour is completely absorbed by the blue object. So no other colours come to our eyes after reflection from the object. Hence, the object will look black

Question 8. When the light of an electric bulb is reflected from a thick red glass slab two images are formed—one white and another red. Why?
Answer:

The light of an electric bulb is reflected from a thick red glass slab two images are formed—one white and another red.

The light rays from the electric bulb are reflected from the front face of the thick red glass slab and form an image. This image is white as light rays of all colours are reflected from the front face of the slab. The other image is formed due to the reflection from the back face.

When light from the electric bulb enters in the red glass slab all the colours except red are absorbed. So the image formed by the reflection from the back face of the red glass slab is red in colour.

Question 9. Does dispersion of light take place In a vacuum
Answer:

No, dispersion of light does not take place in a vacuum because light of all colours has the same velocity in a vacuum.

Question 10. What will be the nature of the spectrum formed from the following Source the sun, an electric but, a nemo bulb and sodium salt in a Bunsen flame?
Answer:

  1. Sun: Line absorption spectrum.
  2. Electric bulbs: Continuous emission spectrum.
  3. Neon bulb: Band emission spectrum
  4. Sodium salt in Bunsen flame: Line emission spectrum.

Question 11. Why do the clouds generally look white?
Answer:

The clouds generally look white

If the particles causing scattering of light have dimensions greater than the wavelength of light the intensity of scattered light is not inversely proportional to the fourth power of its wavelength. All the colours are almost equally scattered by the dust particles, water droplets etc present in the clouds. So the clouds generally look white.

Question 12. Red and blue rays of light are incident on a then prism. Explain which will have the larger angle of deviation
Answer:

Red and blue rays of light are incident on a then prism.

The blue light has a smaller velocity in glass than the j dry blue light, moon looks yellowish red light. So, the refractive index of the glass prism is greater for blue j light. Since deviation is given by δ = ( μ- l)A , so the blue light well is (just the dispersive power of a material medium be negative? be deviated through a larger angle.

Question 13. Is it convenient to use red light, while driving through fog?
Answer:

The wavelength of red light is greatest in the visible spectrum. We know that the intensity of scattered light is inversely proportional to the fourth power of its wavelength. So red light is scattered less than other lights.

Fog consists of water molecules which help to scatter other lights more than red light. As red light is less scattered than other lights, it can traverse a greater path. Hence, while driving through fog light can be used.

It should be mentioned that most cars use yellow light as fog light because our eyes are more sensitive to yellow light than any other light. But red light is used in taillight of cars to make cars visible from a large distance at night to other drivers

Question 14. Why does the moon look completely white during the daytime and after sunset look yellowish?
Answer:

White light Earth’s entering the atmosphere from outer space is scattered by particles present in the atmosphere. Blue and violet light have less wavelength and due to scattering, these ‘ lights are spread everywhere. Hence, a white source in space looks yellowish.

During the daytime, sunlight is scattered by the particles present in the atmosphere huge amount of blue| light is spread everywhere. This blue light mixes with the yellowish colour of the moon and the moon looks white. After sunset,  due to the scarcity of blue light, the moon looks yellowish.

Question 15. Can the dispersive power of a material medium be negative?
Answer: 

The dispersive power of a material medium be negative

Dispersive power of a medium \(\)

For any return. medium (μv – μr)pf and μ > 1

Hence, dispersive power w is always positive. So dispersive power of material medium cannot be negative

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