Question

A convex lens has focal length 15 cm. At what distance should he object from the lens be placed so
that it forms an image at 10 cm from the lens? Also find the magnification produced by the lens
​

Answer 1

Given:-

• Focal length ,f = +15 cm

• Image distance ,u = 10 cm

To Find:-

• Object Distance ,u

• Magnification ,m

Solution:-

Using lens formula

• 1/v + 1/u = 1/f

where,

where,v is the Image Position

where,v is the Image Positionu is the object distance

where,v is the Image Positionu is the object distancef is the focal length

where,v is the Image Positionu is the object distancef is the focal lengthSubstitute the value we get

→ 1/10 + 1/u = 1/15

→ 1/u = 1/15 -1/10

→ 1/u = 2-3/30

→ 1/u = -1/30

→ u = -30/1

→ u = -30 cm

Here , negative sign show the object is placed as the left side of lens.

Therefore, the object distance is 30 cm.

Now, Calculating the Magnification

• m = -v/u

substitute the value we get

→ m = -10/-30

→ m = 10/30

→ m = 1/3 = 0.333

Therefore, the Magnification of the lens is 1/3 .