You Are Given Prisms Made of Crown Glass and Flint Glass with a Wide Variety of Angles. Suggest a Combination of Prisms Which Will Deviate a Pencil of White Light Without Much Dispersion, Disperse (And Displace) a Pencil of White Light Without Much Deviation. - Physics


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)Place the two prisms beside each other. Make sure that their bases are on the opposite sides of the incident white light, with their faces touching each other. When the white light is incident on the first prism, it will get dispersed. When this dispersed light is incident on the second prism, it will recombine and white light will emerge from the combination of the two prisms.

(b)Take the system of the two prisms as suggested in answer (a). Adjust (increase) the angle of the flint-glass-prism so that the deviations due to the combination of the prisms become equal. This combination will disperse the pencil of white light without much deviation.

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Chapter 9: Ray Optics and Optical Instruments - Exercise [Page 348]


NCERT Physics Class 12
Chapter 9 Ray Optics and Optical Instruments
Exercise | Q 23 | Page 348


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.

A ray of light X Y passes through a right-angled isosceles prism as shown below

1) What is the angle through which the incident ray deviates and emerges out of the prism?

2) Name the instrument Whore this action of the prism is put into use.

3) Which prism surface will behave as a mirror?

The given figure shows a point source P inside a water container three rays A, B, and C starting from the source P are shown up to the water surface. (a) show in the diagram the path of these rays after striking the water. The critical angle for water-air surface is 48°. (b) Name the phenomenon which the rays A, B, and C exhibit. 

Write the necessary conditions for the phenomenon of total internal reflection to

occur ?

One day Chetan’s mother developed a severe stomach ache all of a sudden. She was rushed to the doctor who suggested for an immediate endoscopy test and gave an estimate of expenditure for the same. Chetan immediately contacted his class teacher and shared the information with her. The class teacher arranged for the money and rushed to the hospital. On realizing that Chetan belonged to a below average income group family, even the doctor offered concession for the test fee. The test was conducted successfully.

Answer the following questions based on the above information:

(a) Which principle in optics is made use of in endoscopy?

(b) Briefly explain the values reflected in the action taken by the teacher.

(c) In what way do you appreciate the response of the doctor on the given situation?

Calculate the speed of light in a medium whose critical angle is 30° ?

Prove the statement.

A rainbow is the combined effect of the refraction, dispersion, and total internal reflection of light.

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.

Total internal reflection can take place only if ______.

Solve the previous problem if the paperweight is inverted at its place so that the spherical surface touches the paper.

The observation made by Swarali while doing the experiment is given below. Based on these write answers to the questions:

Swarali found that the light ray travelling from the denser medium to rarer medium goes away from the normal. If the angle of incidence (i) is raised by Swarali, the angle of refraction (r) went on increasing. However, after certain value of the angle of incidence the light ray is seen to return back into the denser medium.


  1. What is the specific value of∠i called?
  2. What is the process of reflection of incident rays into denser medium called?
  3. Draw the diagrams of three observations made by Swarali.

 Define critical angle for a given medium. 

Plot a graph between
 Angle of incidence versus angle of reflection,

Plot a graph between

Sine of angle of incidence versus sine of angle of refraction,

Plot a graph between

Angle of incidence versus angle of refraction.

Name the principle on the basis of which optical fibres work.

How does a ray of light bend when it travels from rarer to denser medium

If a ray of light passes from medium I to medium II without any change of direction, what can be said about the refractive indices of these media (angle I is not 0)?

State one factor on which a critical angle for a given pair of media depends. The critical angle for the glass-air interface is 45° for the yellow light. Will it be equal to, less than or greater than 45° for (i) red light, (ii) blue light?

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.

Choose the correct option.

Consider the following phenomena/ applications: P) Mirage, Q) rainbow, R) Optical fibre and S) glittering of a diamond. Total internal reflection is involved in

A spherical marble, of refractive index 1.5 and curvature 1.5 cm, contains a tiny air bubble at its centre. Where will it appear when seen from outside?

Choose the correct option.

Select the WRONG statement.

Choose the correct option.

Angles of deviation for extreme colours are given for different prisms. Select the one having maximum dispersive power of its material.

Which of the following is not involved in formation of a rainbow?

Answer the following question.

Explain ‘mirage’ as an illustration of refraction.

Answer the following question.

Under what conditions are total internal reflection possible? Explain it with a suitable example.

Answer the following question.

Define the critical angle of incidence and obtain an expression for it.

Answer the following question.

Describe the construction and working of an optical fibre.

Answer the following question.

What are the advantages of optical fibre communication over electronic communication?

Answer the following question.

Why is prism binoculars preferred over traditional binoculars? Describe its working in brief.

Solve Numerical example.

A monochromatic ray of light is incident at 37° on an equilateral prism of refractive index 3/2. Determine angle of emergence and angle of deviation. If angle of prism is adjustable, what should its value be for emergent rays to be just possible for the same angle of incidence?

Solve Numerical example.

From the given data set, determine angular dispersion by the prism and dispersive power of its material for extreme colours. nR = 1.62 nV = 1.66, δR = 3.1°

Observe the given figure and write appropriate phenomenon of light in the box.

Critical angle of light passing from glass to air is maximum for ____________.

The resultant `vec"R"` of `vec"P"` and `vec"Q"` is perpendicular to `vec"P"`. Also `|vec"P"|=|vec"R"|`. The angle between `vec"P"` and `vec"Q"` is ______.
[tan 45° = 1]

A green light is incident from the water to the air-water interface at the critical angle `(theta)`. Select the correct statement

For the same angle of incidence, the angle of refraction in four media A, B, C and D are 25°, 30°, 35° and 40° respectively. The speed of light is least in medium ______.

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)

For a ray of light, the critical angle is minimum when it travels from ______.

Total internal reflection can take place only if light is travelling from ______.

The twinkling effect of star light is due to ______.

A particle is projected such that the horizontal range and vertical height are equal. Then the angle of projection is :

The entire light is reflected back into the denser medium is called ______.

The outer concentric shell in optic fiber is called ______.

A bundle of glass threads, each of which is capable of transmitting messages using light waves is called ______.

The critical angle is defined as the angle of incidence at which the total internal reflection starts to occur.

Optical Fibres are based on the phenomenon of dispersion.

The angle of incidence at which the angle of refraction is Q£ is called the critical angle.

Define the critical angle.

What is a mirage? How does it occur?

What is the phenomenon used in optical fiber? Explain.

What are the examples of total internal reflection in nature?

A green light is incident from the water to the air-water interface at the critical angle (θ). Select the correct statement. 

A ray of light incident at an angle θ on a refracting face of a prism emerges from the other face normally. If the angle of the prism is 5° and the prism is made of a material of refractive index 1.5, the angle of incidence is ______.

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 ______.

  1. appear to be near A.
  2. appear to be near D.
  3. appear to be at the centre of AD.
  4. not be seen at all.

State the condition under which total internal reflection occurs. Give the mathematical expression for total internal reflection.

A Rainbow is a beautiful natural phenomenon formed because of the following:

Observe the given figure of the raindrop and answer the following questions:

  1. Label A and B of the given diagram. Why colour will deviate most.
  2. Name the phenomenon shown in label C.

A ray of light passes through a prism of refractive index `sqrt2` as shown in the figure. Find:

  1. The angle of incidence (∠r2) at face AC.
  2. The angle of minimum deviation for this prism.

A light is entering from one medium refractive index `("RI" =5/3)` to another medium at an angle 30°. The angle of refraction for other medium is sin-1 `(5/6)`. then the increase in angle of incidence is ______ such that the ray of light reflected back into the same medium.

Which of the following is used in optical fibres?

A ray of light passes from vacuum into a medium of refractive index µ, the angle of incidence is found to be twice the angle of refraction. Then the angle of incidence is ______.

A given ray of light suffers minimum deviation in an equilateral prism P. Additional prism Q and R of identical shape and of the same material as P are now added as shown in the figure. The ray will now suffer ______.

The angle made by incident ray of light with normal of the reflecting surface is called ______.

Find the value of θ in the given figure.

Name any two phenomena which take place in the formation of a rainbow.


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