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Home » An illuminated object lies at a distance 1.0 m from a screen. A convex lens is used to form the image of object on a screen placed at distance 75 cm from the lens. Find: (i) the focal length of lens, and (ii) the magnification.
An illuminated object lies at a distance 1.0 m from a screen. A convex lens is used to form the image of object on a screen placed at distance 75 cm from the lens. Find: (i) the focal length of lens, and (ii) the magnification.
Class 10 | ICSE | Physics | Refraction through a Lens | Selina
An illuminated object lies at a distance 1.0 m from a screen. A convex lens is used to form the image of object on a screen placed at distance 75 cm from the lens. Find: (i) the focal length of lens, and (ii) the magnification.

Solution:

According to the question,

Image distance is v = 75 cm

Object distance is u = -25 cm

The expression for Lens formula is –

1 / v – 1 / u = 1 / f

Upon substituting values, we get

1 / f = 1 / 75 – 1 / -25 = 1 / 75 + 1 / 25

Therefore, 1 / f = 4 / 75

Or, f = 75 / 4

thus, we get focal length (f) = 18.75 cm

For a lens, the expression for magnification is

m = v / u

Upon substituting values, we have => m = 75 / -25

Thus, m = -3

i

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