Question

An object 4 cm tall is placed on the principal axis of a concave mirror of focal length 20 cm at a distance of 30 cm from it. find position, nature and size of the image.

Answer 1

u = -30cm

f = -20 cm ( sign is negatie cuz the focal length of a concave mirror is always negative )

h = 4cm

now...

mirror formula states :

1/v + 1/u = 1/f

or 1/v - 1/30 = -1/20

or 1/v = (-3 + 2)/ 60 = -1/60

or v = - 60 cm

therefore position of image = - 60 cm

now...

we know that..

m = hi/ h = -v/u for spherical mirrors...

therefore m = 60 / -30 = -2

therefore size of image = 4 * (-2) = - 8 cm

therefore the image is real, inverted, magnified and its size is -8cm

also...the ques should be :

an object 4cm tall is placed on a principal axis on a concave mirror of focal length 20cm at a distance of 30cm from i

Answer 2

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