Question

A concave lens of power 4D is placed at a distance of 40 cm from a wall. At what Distance from the lens should a candle be placed so that its image is formed on the wall ?​

Answer 1

Given:-

• Power of Convex lens (P) = 4D

• Image Distance (v) = +40cm

To Find:-

• Object Distance (u)

Solution:-

Firstly we have to calculate the Focal Length.

So, By using this formula

⟹ f = 1/P

Putting all the values which is given above

⟹ f = 1/4D

⟹ f =1/4 m

⟹ f = 1/4m = 25cm (here we convert metre into cm)

Thus, Focal length is 25 cm.

And Now,

By Using Lens Formula

⟹ 1/v -1/u = 1/f

Putting all the values

⟹ 1/u = 1/v - 1/f

⟹ 1/u = 1/40 - 1/25

⟹ 1/u = -3/200 cm

⟹ u = -200/3 cm

Therefore, the candle should be placed 200/3 cm from the lens.