Question

The power of a convex lens is 20D. An object is placed 15cm in front of the convex lens. Calculate the distance between the object and the image. Explain how to do please.

Answer 1

here, power = 20D
So, focal length(in metres) = 1/power = 1/20 = 0.05m = 5 cm
We know that, by lens formula,
(1/v) - (1/u) = 1/f
Where, u = object distance = -15cm, v = image distance, f =focal length= 5cm
So, (1/v) = (1/f) + (1/u)
= (1/5) - (1/15)
= (3-1)/15 = 2/15
Or, v = 15/2 = 7.5 cm
So, net distance between image and object = 15 + 7.5 = 22.5 cm

Answer 2

For ur kind information, in a convex lens, power and focal length r always positive. and object distance is always negative. The image will be formed on the other side of the lens , so the total distance is what I said above. Remember, pole is the origin in a lens.

Answer 3

Thanks. That is very helpful. Can you kindly tell me about concave lens as well?

Answer 4

For Concave lens, the same formula is valid.
Only thing, focal length and power is negative, also image distance is negative.