Question

a converging lens is to project image of a lamp 4 times the size of the lamp on a wall at a distance of 10m.from the lamp. find the focal length of the lens

Answer 1

The focal length of the lens is 1.6 m

Explanation:

• Magnification "m" = v / u

−4 = v / u or v = −4v

Now, v−u = 10

or − 5u = 10

u = −2 m

v = 8cm

• Now using lens mirror formula

1 / f = 1 / v − 1 /u

or 1 / f = 1 / 8 − 1 / −2

= 1 / 8 + 1 / 2

= 1 + 4 / 8 = 5 / 8

f= 8 / 5 m = 1.6 m

Thus the focal length of the lens is 1.6 m

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