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Question
CBSEENSC10014456

"A convex lens can form a magnified erect as well as a magnified inverted image of an object placed in from of it." Draw ray diagram to justify this statement stating the position of the object with respect to the lens in each case.

An object of height 4 cm is placed at a distance of 20 cm from a concave lens of focal length 10 cm. Use lens formula to determine the position of the image formed.

Solution

a) When the object is placed between O and F1, magnified erect image is formed.

                               

b) When the object is placed between F1 and 2F1 , magnified inverted image is formed.

                    

Object distance, u = 20 cm

Image distance, v=?

Focal length, f = 10 cm

According to the sign convention, f = -10 cm and u=-20 cm

Now, using the lens formula,

 1 over straight f space equals 1 over v minus 1 over u

1 over v space equals 1 over f plus 1 over u

1 over v space equals fraction numerator 1 over denominator negative 10 end fraction plus fraction numerator 1 over denominator negative 20 end fraction

equals fraction numerator negative 3 over denominator 20 end fraction
equals space v equals fraction numerator negative 20 over denominator 3 end fraction space equals negative 6.6 space c m

The image is formed at a distance of 6.6 cm from the lens at the same side where the object is placed.