Question

if the power of lens is - 5.5 d then which lens is used​

Answer 1

Answer:

(i) Correcting distant vision f = 1P = 1/( - 5.5) = - 0.18 m (ii) Correcting near vision f = 1P = 11.5 = 0.67 m.

Explanation:

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Answer 2

Since the power of the lens given here is -5.5 D

D stands for dioptre and it is used for measuring the power of the lens .

I D = 1m to the power of -1 .

m here is metre

1 D = 1/1m

Power is defined as the reciprocal of the focal length of the given lens .

Since the power given here is negative , the reciprocal of it should also be negative .That means the focal length is also negative .

And only a CONCAVE lens can have a negative focal length .

So this is a concave lens .

Using all this information , we can find out the focal length

-5.5 D is written as - 55/10 D

-55/10 D = 1/f

f is the focal length

Rearranging the above terms we get ;

f = -10 /55 = -2/11 = approx. -0.18

Again we find that the focal length is negative 0.18

Which has to be a concave lens .

We can naturally write that the focal length of a concave lens is negative as the convention of lenses already state it .

Hope this helps .