Question

An image formed on a screen is three times the size of the object. The object and screen are 80 cm apart when the image is sharply focussed.
(a) State which type of lens is used.
(b) Calculate focal length of the lens.

Answer 1

Explanation:

magnification m is given by , m = v/ u, v is the distance between lense and image, u is the distance between object and lens.

magnification m = 3 = v/u, i.e., v = 3u

object to screen distance u+v = 80 cm;

solving for u and v, we get u = 20 cm, v = 60 cm.

applying to lens formula, (1/u) +(1/v) = (1/f), we get focal length =15 cm

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Answer 2

a) The type of lens is used is convex lens.

b) Focal length is 15cm

Explanation:

• From the given we can say that since the image is formed on the screen so that it must be the real image. Hence the lens used is the convex lens.
• Basically Concave lens causes light to spread out, which results in a smaller image also makes bigger size.
• Concave lens are used for the several uses including eyeglasses and contacts, flash lights, peepholes, binoculars.

                       $M=\frac{v}{u}$

                       $-3=80+\frac{u}{u}$

                       $U=-20 \mathrm{cm}$

                       $V=80+u$

                       $=60 \mathrm{cm}$

                       $\text { Lens formula } \frac{1}{v}-\frac{1}{u}=\frac{1}{f}$

• By substituting the values in the above formula.

                       $F=15 \mathrm{cm}.$