Question

A convex lens of focal length 20cm is mounted on an upright of an

optical bench. The image of a distant object is formed on a screen.
A concave lens of focal length 25cm is placed in contact with the given lens. The screen is moved suitably to focus the image of the same distant object.

a. What is the distant of screen from convex lens, where image of distant object is obtained?

b. How does the combination of lenses behave?

c. Where will the image of distant object be formed?

d. Will the image formed by combination be real or virtual?

e. What is the formula for magnification obtained with a lens?

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Answer 1

Answer:

Image formed by lens

v

1

−

u

1

=

f

1

v

1

+

30

1

=

20

1

v=+60cm

If image position does not change even when mirror is removed it means image formed by lens is formed at centre of curvature of spherical mirror.

Radius of curvature of mirror = 80−60=20cm

⇒focal length of mirror f=10cm for virtual image, object is to be kept between focus and pole.

⇒ maximum distance of object from spherical mirror for which virtual image is formed, is 10cm.

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Answer 2

Answer:

a - image of distant object is formed at focus of combex lens I. e. at a distance of 20cm

B-it will be virtual formed on screen

C-

D- virtual

E-m=h'=v

-- --

h u