Even 7. Determination of Focal Length of a given 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
An optical bench , two convex lens with lens holder,  a half metre scale.

Formulae: The relation between u, v and f for a convex lens is

1/f = 1/v - 1/u 

where,
f = focal length of convex lens
u = distance of object needle from optical centre of the lens
v = distance of image needle from optical centre of the lens.

Diagram: 

Procedure: 

To determine focal length of given lens

1. Fix the convex lens in lens holder.
2. Go out in the open and face the lens towards distant tree or building.
3. Obtain the image of the tree or the building on (a white painted wall) screen and move the lens forward and backward to get a sharp image on the wall.
4. Measure the distance between the lens and the wall (screen). This will be equal to the focal length of the lens.
5. Repeat the above steps for more accurate measurement of focal length.
6.  To calculate the focal length  of convex lens.Record the observations for different values of u and v.

Observations:

focal length of the given convex lens is calculated using the formula given in the experiment

Distance of object from lens recorded as 'u'

And image from lens as 'v'

Result
The focal length of the given convex lens as determined from 
f = uv/ (u-v), 

when u = 6 cm, v = 4 cm

Using sign convention u = -6 cm ,v = 4 cm

f = -24/(-6-4) = -24/(-10)  = 2.4 cm

Precautions:

Distances u,v should be taken carefully and minutely.

According to sign-convention, u has negative value and v has positive value. Hence, f comes positive.