Question

A converging lens forms a three times magnified image of an object, which can be take on a screen. If the focal length of the lens is 30 cm, then the distance of the object from the lens is?​

Answer 1

Answer:

Here, filament of the lamp acts as the object.

v = image distance from lens and u = object distance

According to the question, v + u = 30 ( taking only magnitude ) ---(i)

and magnification m = v/u = 3

⇒

v = 3u ---(ii)

Solving (i) and (ii),we get u = 15 cm.

For finding the focal length, we have to apply the sign convention.

So, u = -15 cm

v = 3u = 3

×

15= 45cm (+ve as it is a real image)

Using lens formula 1/v - 1/u = 1/f

(1/60) - (1/-20) = 1/f

⇒

f = 15 cm

Answer 2

Answer:

it is -45 cm

Explanation:

when object is placed between C and F2

it gives magnified image than object.

as you can see in diagram

so -45 cm