Karnataka Board PUCPUC Science 2nd PUC Class 12

A Nearsighted Person Cannot Clearly See Beyond 200 Cm. Find the Power of the Lens Needed to See Objects at Large Distances. - Physics

Answer in Brief

A nearsighted person cannot clearly see beyond 200 cm. Find the power of the lens needed to see objects at large distances.



For a near-sighted person, u = ∞.
Now, we have:

v = – 200 cm = – 2 m
The lens formula is given by

`1/v -1/u =1/f`

On putting the respective values, we get:

`1/f = 1/((-2)) -1/∞ =-1/2`

∴ The power of the lens = `1/f`= -0.5 D

Concept: Lenses
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Chapter 19: Optical Instruments - Exercise [Page 432]


HC Verma Class 11, Class 12 Concepts of Physics Vol. 1
Chapter 19 Optical Instruments
Exercise | Q 16 | Page 432

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A screen is placed at a distance of 100 cm from an object. The image of the object is formed on the screen by a convex lens for two different locations of the lens separated by 20 cm. Calculate the focal length of the lens used.

A convex lens of focal length 20 cm is placed coaxially with a convex mirror of radius of curvature 20 cm. The two are kept at 15 cm apart. A point object lies 60 cm in front of the convex lens. Draw a ray diagram to show the formation of the image by the combination. Determine the nature and position of the image formed

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?

What is the chromatic aberration? How can it be minimised or eliminated?

You are given three lenses L1, L2 and L3 each of focal length 15 cm. A object is kept at 20 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`.

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 ___________ .

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.

A farsighted person cannot see objects placed closer to 50 cm. Find the power of the lens needed to see the objects at 20 cm.

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?

The near point and the far point of a child are at 10 cm and 100 cm. If the retina is 2.0 cm behind the eye-lens, what is the range of the power of the eye-lens?

A person has near point at 100 cm. What power of lens is needed to read at 20 cm if he/she uses (a) contact lens, (b) spectacles having glasses 2.0 cm separated from the eyes?

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.

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.

Calculate the radius of curvature of an equi-concave lens of refractive index 1.5, when it is kept in a medium of refractive index 1.4, to have a power of -5D? 

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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