FA LAB ACTIVITY CLASS X

FIND THE FOCAL LENGTH OF A CONVEX LENS

Aim: To know the focal length of a convex lens by u,v method.

Apparatus: Illuminated wire gauze, Lens stand ,Screen, Metre scale, A convex lens of short focal length.

Theory: The relation between u,v and f of a 

convex lens is 

f= focal length,

u= object distance,

v= image distance.

Ray diagram:

Procedure:

1. Take a convex lens and placed on V stand.

2. Arranged the convex lens between wire 

gauze and the screen.

3. Allow the light passes through the wire 

gauze.

4. Place the wire gauze 60 cm from the 

convex lens. It is taken as “u” cm.

5. The position of the screen is adjusted to 

get a clear image of the wire gauze on the 

screen.

6. Measure the distance between the lens and the screen. It is taken as v cm

7. Calculate the focal length of the convex

lens using the formula.

 1/f = 1/v - 1/u

8. Repeat the experiment by placing the lens

at different distances away from the wire

gauze.

9. Note the values of u and v in the table.

10.Then calculate the mean value of ‘f’ in each case.

11.Find the average focal length from the

values of f.

Observations:

Average f = 20.05 cm

Precautions:

1. Take the distances without parallax error.

2. Measure the distances from the center of the lens.

3. Place the wire gauge, V stand and screen in straight line.