Question

An object placed 45cm from a lens forms an image on a screen placed 90cm on the other side of the lens. Identify the type of lens and it's focal length.

Answer 1

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Given,
Object distance, u=-45 cm
Image distance, v= 90 cm
Using the lens formula,
1/f=1/v - 1/u
=1/90 - (-1/45)
f=+30 cm
Its focal length is positive ,so it is a convex lens.

Answer 2

Given,

Object distance, u=-45 cm

Image distance, v= 90 cm

Using the lens formula,

1/f=1/v - 1/u

=1/90 - (-1/45)

f=+30 cm

Its focal length is positive ,so it is a convex lens.