Question

The radius of curvature of concave mirror is 20 cm. A candle is kept at (i) 5 cm apart, (ii) 15 cm apart, from mirror. Where will image be formed in each case ?​

Answer 1

radius of curvature of concave mirror, R = -20cm

so, focal length of concave mirror, f = R/2 = -10cm

(i) candle is kept at 5cm apart

so, object distance, u = -5cm

using mirror formula,

1/v + 1/u = 1/f

or, 1/v + 1/-5 = 1/-10

or, 1/v = -1/10 + 1/5

= 1/10

v = 10cm

means, image forms 10cm apart opposite side of object.

(ii) candle is kept at 15cm apart

so, object distance, u = -15cm

using mirror formula,

1/v + 1/u = 1/f

or, 1/v + 1/-15 = 1/-10

or, 1/v = 1/-10 + 1/15

or, 1/v = -1/30

or, v = -30cm

image forms 30cm apart same object same side of object.

Answer 2

Answer:

The radius of curvature of concave mirror=-20cm

focal length,

f=r/2

= -20/2

= -10

Where u=-5cm,by using formula,

1/f=1/v+1/u

-1/5 + 1/v = -1/10

or 1/v=1/5-1/10

So, 1/v=1/10

v=10cm

So, the virtual image will be formed behind the mirror at a distance (v) = 10cm