Question

State the lens formula for a thin lens with small aperture​

Answer 1

Explanation:

Lens formula gives relation between object distance, image distance and focal length of lens

1/f=1/v-1/u

New Cartesian Sign conventions:

1. All distances are measured from the optical centre of the lens.

2. The distances measured in the direction of incident light are taken as positive and those measured in opposite direction to the incident light are taken as negative.

3. The focal length of a convex lens is positive and concave is negative.

The following assumptions are made in the derivation of the formula:

1. The lens is thin

2. The lens has small aperture

3. The point lie on to the principal axis and placed perpendicular.

4. The incident ray makes small angles with the lens surface or the principal axis.

Answer 2

Lens formula, $\dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u}$

Explanation:

The lens formula is applicable the lenses having small aperture. It is given by :

$\dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u}$

Where

f is the focal length of the lens

v i the image distance

u is the object distance

The focal length of concave lens is negative while it is positive for convex lens. Object distance is taken to be negative always. By inserting the value of f and u we can find out the value of v i.e. image distance.

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Lens formula

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