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? - Physics


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 focal length and the radius of curvature of a lens are equal if the lens is made of glass having a refractive index of 1.5.

Concept: Lenses
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2014-2015 (March) Panchkula Set 3

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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 concave lens of refractive index 1.5 is immersed in a medium of refractive index 1.65. What is the nature of the lens?

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?

Under what condition does a biconvex lens of glass having a certain refractive index act as a plane glass sheet when immersed in a liquid?

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 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`.

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

The focal length of a converging lens are fv and fr for violet and red light respectively.

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 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

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

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?

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.

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.

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?

A magnifying glass is used, as the object to be viewed can be brought closer to the eye than the normal near point. This results in ______.

  1. a larger angle to be subtended by the object at the eye and hence viewed in greater detail.
  2. the formation of a virtual erect image.
  3. increase in the field of view.
  4. infinite magnification at the near point.


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