Question

A concave lens forms an image magnified ½. Calculate the focal length of the lens if the object is at 30 cm from the lens.

Answer 1

Answer:

10cm

Explanation:

u=30

v= ?

Magnification = 0.5

magnification = V/u

0.5 = v/30

v= 30 x 0.5

v = 15

to find focal length , we use

1/f = 1/v + 1/u

1/f = 1/15 +1/30

1/f = 3/30

f= 10cm

Answer 2

u=30cm

m=1/2

m=-v/u

1/2=-v/30

-2v=30

v=-15

mirror formula

1/f = 1/v + 1/u

= 1/-15 +1/30

=-2+1/30

1/f = -1/30

f = -30

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