A person is unable to see objects nearer than 50cm. HE wants to read a book placed at a distance of25 cm. Find the nature, focal length and power of the lens he requires for his spectacles.

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Solution

$N o w p o w e r i s g i v e n b y : P = \frac{100}{f} w h e n f i s i n c m P = \frac{100}{50} = 2 D S i n c e f o c a l l e n g t h i s p o s i t i v e h e n c e n a t u r e o f t h e l e n s i s c o n v e x .$

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A person is unable to see objects nearer than 50cm. HE wants to read a book placed at a distance of25 cm. Find the nature, focal length and power of the lens he requires for his spectacles.

Now power is given by :P = 100f when f is in cmP = 10050 = 2 DSince focal length is positive hence nature of the lens is convex.

$N o w p o w e r i s g i v e n b y : P = \frac{100}{f} w h e n f i s i n c m P = \frac{100}{50} = 2 D S i n c e f o c a l l e n g t h i s p o s i t i v e h e n c e n a t u r e o f t h e l e n s i s c o n v e x .$