Question

three times enlarged (i)real (ii)virtual image of an object placed at a distance of 30 cm from a concave mirror is obtained find the focal length of the concave mirror in each case​

Answer 1

Answer:-

(a) The image formed in front of the concave mirror is real, so m is negative,

m = -3, u = -10 cm

As m = -v/u or -3 = -v/-10

v = -30 cm

By mirror formula,

1/f = 1/v + 1/u = 1/-30 + 1/-10 = -4/30

or f = -30/4 = -7.5 cm

Radius of curvature, R = 2f = 2 x (-7.5) = 15cm

(b) A lens is a piece of transparent glass bound by two spherical surfaces.

Answer 2

Answer:

Explanation:

In first case , the image is formed in front of concave mirror because it is real and hence, Magnification is -ve

m = -3

u = -30 { u is always negative }

v = ?

f = ?

m = - v / u

- v = mu

- v = (-3) (-30)

- v = 90 cm

v = -90 cm

By mirror formula ,

1/f = 1/v + 1/u

= 1/ (-90) + 1/ (-30)

= - 1/ 90 - 1/ 30

= - 1 - 3 / 90

1 / f = - 4 / 90

f = -90 / 4

f = - 22.5

Similarly in second case ,

m = 3

u = -30 cm

v = ?

f = ?

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