Aim :
To find focal length of given lens.
Apparatus :
Object (candle), convex lens, v – stand, screen.
Procedure :
- Take a v-stand and place it on a long table at the middle. Place a convex lens on the v-stand. Imagine the principal axis of the lens.
- Light a candle and ask your friend to take the candle far away from the lens along the principal axis.
- Adjust a screen (a sheet of white paper placed perpendicular to the axis) which is on other side of the lens until you get an image on it.
- Measure the distance of the image from the v-stand of lens (image distance V) and also measure the distance between the candle and stand of lens (object distance ‘u’). Record the values in the table.
| Object distance 'u' |
Image distance 'v' |
Focal length 'f' |
|
|
|
- Now place the candle at a distance of 60 cm from the lens, try to get an image of the candle flame on the other side on a screen. Adjust the screen till you get a clear image.
- Measure the image distance V and object distance ‘u’ and record the values in table.
- Repeat the experiment various object distances like 50 cm, 40 cm, 30 cm, etc. Measure the image distances in all cases and note them in table.
- Using the formula 1/f = 1/v - 1/u, find f in all the cases. We will observe the value ‘f is equal in all cases. This value off is the focal length of the given lens.
