Question

a point object is placed at a distance of 10cm and its real image is formed at a distance of 20cm from a concave mirror. If the object is moved by 0.1com towards the mirror.The image will shift by about?

Answer 1

1/u + 1/v = 1/f

Here 1/10 + 1/20 = 3/20 = 1/f

f = 20/3

1/v = 1/f – 1/u = (u – f)/uf

v = uf/(u-f)

dv/du = [(u – f)*f – uf]/(u-f)2     -quotient rule

          = -f2/(u-f)2

[dv/du]f=20/3,u=10 = 4

Δv = [dv/du]f=20/3,u=10 * Δu

     =4*0.1 = 0.4

 therefore the image will shift by 0.4 cm //