Karnataka Board PUCPUC Science 2nd PUC Class 12

In Producing a Pure Spectrum, the Incident Light is Passed Through a Narrow Slit Placed in the Focal Plane of an Achromatic Lens Because a Narrow Slit - Physics

Fill in the Blanks

In producing a pure spectrum, the incident light is passed through a narrow slit placed in the focal plane of an achromatic lens because a narrow slit ___________ .


  • produces less diffraction

  • increases intensity

  • allows only one colour at a time

  • allows a more parallel beam when it passes through the lens



allows a more parallel beam when it passes through the lens


To produce a pure spectrum, a parallel light beam is required to be incident on the dispersing element. So, the incident light is passed through a narrow slit placed in the focal plane of an achromatic lens.

Concept: Lenses
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Chapter 20: Dispersion and Spectra - MCQ [Page 442]


HC Verma Class 11, Class 12 Concepts of Physics Vol. 1
Chapter 20 Dispersion and Spectra
MCQ | Q 3 | Page 442

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The focal length of an equiconvex lens is equal to the radius of curvature of either face. What is the refractive index of the material of the lens?

The line AB in the ray diagram represents a lens State whether the lens is convex or concave.

A symmetric biconvex lens of the radius of curvature R and made of glass of refractive index 1.5, is placed on a layer of liquid placed on top of a plane mirror as shown in the figure. An optical needle with its tip on the principal axis of the lens is moved along the axis until its real, inverted image coincides with the needle itself. The distance of the needle from the lens is measured to be x. On removing the liquid layer and repeating the experiment, the distance is found to be y. Obtain the expression for the refractive index of the liquid in terms of x and y

A biconvex lens made of glass (refractive index 1.5) has two spherical surfaces having radii 20 cm and 30 cm. Calculate its focal length.

What type of lens is used to correct long-sightedness?

You are given three lenses L1, L2 and L3 each of focal length 20 cm. A object is kept at 40 cm in front of L1, as shown. The final real image is formed at the focus ‘I’ of L3. Find the separation between L1, L2 and L3.

A convex lens made up of glass of refractive index 1.5 is dipped, in turn, in (i) a medium of refractive index 1.6, (ii) a medium of refractive index 1.3.

(a) Will it behave as a converging or a diverging lens in the two cases?

(b) How will its focal length change in the two media?

A convex lens of refractive index 1.5 has a focal length of 18 cm in air .Calculate the change in its focal length when it is immersed in water of refractive index `4/3`.

The maximum focal length of the eye-lens of a person is greater than its distance from the retina. The eye is

Does focal length of a lens depend on the colour of the light used? Does focal length of a mirror depend on the colour?

Which of the following quantities related to a lens depend on the wavelength or wavelengths of the incident light?

(a) Power

(b) Focal length

(c) Chromatic aberration

(d) Radii of curvature

Which of the following quantities increase when wavelength is increased? Consider only the magnitudes.

(a) The power of a converging lens

(b) The focal length of a converging lens

(c) The power of a diverging lens

(d) The focal length of a diverging lens

A person is viewing an extended object. If a converging lens is placed in front of his eyes, will he feel that the size has increased?

By mistake, an eye surgeon puts a concave lens in place of the lens in the eye after a cataract operation. Will the patient be able to see clearly any object placed at any distance?

The focal lengths of a convex lens for red, yellow and violet rays are 100 cm, 98 cm and 96 cm respectively. Find the dispersive power of the material of the lens.

An object is placed at a distance of 30 cm from a converging lens of focal length 15 cm. A normal eye (near point 25 cm, far point infinity) is placed close to the lens on the other side. (a) Can the eye see the object clearly? (b) What should be the minimum separation between the lens and the eye so that the eye can clearly see the object? (c) Can a diverging lens, placed in contact with the converging lens, help in seeing the object clearly when the eye is close to the lens?

A professor reads a greeting card received on his 50th birthday with + 2.5 D glasses keeping the card 25 cm away. Ten years later, he reads his farewell letter with the same glasses but he has to keep the letter 50 cm away. What power of lens should he now use?

Lenses are constructed by a material of refractive index 1.50. The magnitude of the radii of curvature are 20 cm and 30 cm. Find the focal lengths of the possible lenses with the above specifications.

Calculate the focal length of a convex lens whose radii of curvature of two surfaces is 10 cm and 15 cm respectively and its refractive index is 1.5. 

What is meant by shortsightedness?

What is meant by short-sightedness ?

When two thin lenses of focal lengths f1 and f2 are kept coaxially and in contact, prove that their combined focal length ‘f’ is given by: `1/f = 1/f_1 + 1/f_2`

 Show that, two thin lenses kept in contact, form an achromatic doublet if they satisfy the condition: `ω/f + (w')/(f') = 0`
where the terms have their usual meaning.

Choose the correct answer from given options
A biconvex lens of focal length f is cut into two identical plano-convex lenses. The focal length of each part will be

A thin convex lens of focal length 10 cm is .placed in contact of a thin concave lens of focal length 15 cm. Find:

  1. The focal length
  2. Power, and
  3. The nature of the combined lens system.

Can the optical centre of a lens be situated outside the lens?

Read the passage given below and answer the questions that follow.

There are two types of lenses: Converging lenses and diverging lenses, depending on whether they converge or diverge an incident beam of light. They are also called convex or concave lenses. Lenses are usually made of glass. Convex lenses are more popular as they form a real image of an object. They are widely used in our daily life, for instance, in microscopes, telescopes, projectors, cameras, spectacles, etc. Microscopes are used to view small and nearby objects, whereas telescopes are used to see distant objects.
  1. State any one factor on which the focal length of a lens depends.
  2. Give an example where a convex lens behaves like a diverging lens.
  3. What type of lens is used in a camera?
  4. Write an expression for magnifying power of a compound microscope when its final image lies at the least distance of distinct vision (D).
  5. State any one difference between a reflecting telescope and a refracting telescope.


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