Physics Practical Book XII: 8

EXPT NO: 08. FOCAL LENGTH OF CONVEX LENS

Aim : To find the focal length of a convex lens by plotting graphs between u and v or

between 1/u and 1/v.

Apparatus : Convex lens, screen, index pins, meter scale.

Ray Diagrams :

Formula :

f = focal length of concave lens

u = object distance for a concave lens,

v = image distance for a concave lens.

Procedure :

Using convex lens, focus the image of distant object on the screen. Note the distance between screen and lens, which is approximate focal length '

Keep the object pin at distance '2' and note the object distance

. Adjust the image pin on the other side of the lens so that there is no parallax between image of object pin seen through the lens and image pin. Measure and note the image distance '

In the same way take two readings for object distance

greater than 2 and two readings for

between

and 2.

Keep the object within focal length

. In this case virtual image is obtained on the same side as object. Adjust the image pin behind the object pin so that there is no parallax between the image of object pin and upper part of image pin. Take one more reading for

less than

Calculate the focal length of the convex lens f.

Plot a graph of '1/v' against '1/u'. Measure the intercepts on both the axes. Hence, calculate f.

Plot a graph of 'v' against

. Draw the line making an angle of 45° with the axes starting from origin and meeting the curve at point P. Draw perpendiculars from point P on both the axes and note down values of 'u' and 'v'. Hence, calculate f

Observations and Calculations :

Approximate focal length of convex lens = f = …………………. cm.