A professor read a greeting card received on his 50th birthday with + 2 5 D glasses keeping the card 25 cm away. Ten years later, he reads his farewell letter with the same glasses but he has to keep the letter 50 cm away. What power of lens should he now use?

Question

Philoid · Accepted Answer

Given that-

The professor is able to read greeting card at a distance of 25 cm using a glass of magnifying

power +2.5 D.

10 years later, he can read the farewell card using same glass of magnifying power +2.5D at a

distance (u) of 50 cm.

Using lens formula –

Substituting the values-

Therefore, his near point of vision, v = 200 cm

Now, after 10 years, to read the farewell letter at a distance of 25 cm,

u = – 25 cm

v= 200cm

Using lens formula –

Substituting the values

Hence, the required power of the lens -

So, the professor should use a lens of power + 4.5 D to read greeting card from 25cm.

A professor read a greeting card received on his 50th birthday with + 25 D glasses keeping the card 25 cm away. Ten years later, he reads his farewell letter with the same glasses but he has to keep the letter 50 cm away. What power of lens should he now use?