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A student has obtained a point image of a distant object using the given convex lens. To find the focal length of the lens he should measure the distance between the :

(A) lens and the object only

(B) lens and the screen only

(C) object and the image only

(D) lens and the object and also between the object and the image

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#### Solution

In the given case, the student has obtained the point image of a distant object using a convex lens.

According to the lens formula,

`1/f=1/v-1/u`

Here,

*v* = Image distance

*u* = Object distance

*f *= Focal length of the lens

Because the object is placed very far, the object distance is taken to be infinity. As u = ∞

`"Thus, "1/u=0" and "f="v"`

To find the focal length of the lens, the student should know the image distance, which is the distance between the lens and the screen.

Hence, the correct option is B.

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