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प्रश्न
Explain ‘mirage’ as an illustration of refraction.
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उत्तर
- On a hot, clear, sunny day, along a level road, there appears a pond of water ahead of the road. However, if we physically reach the spot, there is nothing but the dry road, and the water pond again appears some distance ahead. This illusion is called a mirage.
- Mirages result from the refraction of light through a non-uniform medium.
- On a hot day the air in contact with the road is hottest, and as we go up, it gets gradually cooler. The refractive index of air thus decreases with height. Hot air tends to be less optically dense than cooler air, which results in a non-uniform medium.
- Light travels in a straight line through a uniform medium but refracts when travelling through a non-uniform medium.
- Thus, the ray of light coming from the top of an object gets refracted while travelling downwards into less optically dense air and becomes more and more horizontal, as shown in the figure.
- As it almost touches the road, it bends (refracts) upward. Then onwards, upward bending continues due to denser air.
- As a result, for an observer, it appears to be coming from below, thereby giving an illusion of reflection from an (imaginary) water surface.
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संबंधित प्रश्न
You are given prisms made of crown glass and flint glass with a wide variety of angles. Suggest a combination of prisms which will
(a) deviate a pencil of white light without much dispersion,
(b) disperse (and displace) a pencil of white light without much deviation.
A ray of light travels from water to air as shown in the diagram given below :
1) Copy the diagram and complete the path of the way. Given the critical angle for water is 48°.
2) State the condition so that internal reflection occurs in the above diagram.
Name the factors affecting the critical angle for the pair of media.
The figure shows a ray of light falling normally on the face AB of an equilateral glass prism having refractive index`3/2`, placed in water of refractive index `4/3`.Will this ray suffer total internal reflection on striking the face AC? Justify your answer.
State the relation between the critical angle and the absolute refractive index of a medium.
What do you understand by the deviation produced by a prism? Why is it caused?
Name two factors which affect the critical angle for a given pair of media. State how do the factors affect it.
Write the relation between the refractive index and critical angle for a given pair of optical media?
Swarali has got the following observations while doing an experiment. Answer her questions with the help of observations.
Swarali observed that the light bent away from the normal, while travelling from dense medium to rarer medium. When Swarali increased the values of angle of incidence (i), the values of angle of refraction (r) went on increasing. But at a certain angle of incidence, the light rays returned into the dense medium.
So, Swarali has some questions. Answer them –
a) Name this certain value of ‘i ’. What is the value of ‘r’ at that time ?
b) Name this process of returning of light in dense medium. Explain the process.
i) Observe the given figure and answer the following questions.
a) Identify and write the natural process shown in the figure.
b) List the phenomena which are observe in this process.
c) Redraw the diagram and show above phenomena in it.
Plot a graph between
Sine of angle of incidence versus sine of angle of refraction,
If a light ray does not undergo, refraction at the boundary between two media, the angle of incidence is:
How are critical angles related to the refractive index of the medium?
Images formed by totally reflecting prisms are brighter than the image formed by ordinary reflected light, why?
A ray of light incident at an angle of incidence ‘i’ passes through an equilateral glass prism such that the refracted ray inside the prism is parallel to its base and emerges from the prism at an angle of emergence ‘e’.
How is the angle of emergence ‘e’ related to the angle of incidence ‘i’?
A ray of light passes through a right-angled prism as shown in the figure. State the angles of incidence at the faces AC and BC.
The adjacent diagram shows two right-angled isosceles prisms A and B. Complete the diagram to show the path of rays P and Q emerging out of the prism B. What principles have you used to complete the diagram?
Complete the following diagram to show the path of the ray of a single colour in the diagram as if enters in and emerges out of the prism. Mark the angles wherever necessary. (Critical angle for glass = 42°).
PQ and PR are two light rays emerging from the object as shown in the figure below:
(i) What is the special name given to the angle of incidence (∠PQN) of ray PQ?
(ii) Copy the ray diagram and complete it to show the position of the image of the object P when seen obliquely from above.
(iii) Name the phenomenon that occurs if the angle of incidence ∠PQN is increased still further.
Answer the following question.
Under what conditions are total internal reflection possible? Explain it with a suitable example.
Describe the construction and working of an optical fibre.
The angle of minimum deviation produced by a thin glass prism in air is 'δ'. If that prism is immersed in water, the angle of minimum deviation will be ______.
( aμg = refractive index of glass w.r.t, air aμw = refractive index of water w.r.t. air)
The twinkling effect of star light is due to ______.
The outer concentric shell in optic fiber is called ______.
Write any two uses of total internal reflection.
A rectangular block of glass ABCD has a refractive index 1.6. A pin is placed midway on the face AB (Figure). When observed from the face AD, the pin shall ______.
- appear to be near A.
- appear to be near D.
- appear to be at the centre of AD.
- not be seen at all.
Find the value of θ in the given figure.
- Assertion (A): Propagation of light through an optical fibre is due to total internal reflection taking place at the core-cladding interface.
- Reason (R): Refractive index of the material of the cladding of the optical fibre is greater than that of the core.
State two differences between normal reflection and total internal reflection.
