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 0.1cm towards mirror,the image will shift by about...........

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 //