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Physics Question on Ray optics and optical instruments

A convex lens forms a real image of the object when the lens is at a distance of 1 m form the object. The size of the image is 3 times that of the object. The position of the object and the screen are kept fixed but the distance of lens from the object is doubled. The image is now one half of the size of the object, the focal length of the lens is

A

1.6 m

B

0.80 m

C

0.40 m

D

0.20 m.

Answer

0.40 m

Explanation

Solution

f = d/(m1−m2) d/ (m_1 - m_2). Here d = 1 m, m, = 3 and m2m_2 = 1 / 2. Hence f = 40 cm