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Focus of a Concave Lens

why does a co...

Question

why does a concave lens have -ve dioptre?

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Solution

Using the convention of the real and virtual images formed, the real being positive and the virtual being negative. The concave lens is always virtual and hence negative focal length and hence negative power.

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Question 8
Which of the following statements is true?
(a) A convex lens has 4 dioptre power having a focal length 0.25 m.
(b) A convex lens has -4 dioptre power having a focal length -0.25 m.
(c) A concave lens has 4 dioptre power having a focal length 0.25 m.
(d) A concave lens has 4 dioptre power having a focal length -0.25 m.

Q. An eye specialist prescribes spectacles having a combination of a convex lens of focal length

40 cm

in contact with a concave lens of focal length

25 cm

. The power of this lens combination in Dioptre is:

Q. A convex lens of power

+ 6

dioptre is placed in contact with a concave lens of power

- 4

dioptres. What will be the nature and focal length of this combination?

Q. Two convex lens of focal length 0.2 m and one concave lens of focal length 0.1 m is kept, as shown below. What is the effective power of the combination of the lens?

Q. An equi-concave lens is to be made of crown glass. What should be radii of the surfaces of the lens if the power of the lens is

- 2.5

dioptre and refractive index of crown glass is

1.65

?

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