Page 68 - Physics - XII

Table for the focal length of convex mirror

S. Position (in cm) of Observed Corrected
No. PI ′ PI′
Tip of the Optical centre Pole of Image needle (cm) (cm)
object needle of the lens mirror I′
O L P

1.

2.
3.

4.

Mean PI' = _____ cm
Calculations

Corrected distance PI' = _____ cm
PI'
Focal length of convex mirror = f = = _____ cm
2

Result

The focal length of the given convex mirror by using convex lens is _____ cm.

Precautions

1. All the uprights should be vertical.
2. The tip of the needle, centre of the mirror, and centre of the lens should be at same height.
3. Tip to tip parallax should be removed and while removing the parallax, the eye should be kept at least
30 cm from the needle.

4. The lens L must have suffi ciently large focal length.
5. Convex mirror should be placed close to the convex lens.
6. Index correction should be applied between the image needle I' and back surface of the convex mirror.

Sources of Error
1. The uprights may not be vertical.
2. The parallax removal may not be perfect.

EXPERIMENT - 3

Aim
To fi nd the focal length of a convex lens by plotting graphs between u and v or between 1 and 1 .
u v
Apparatus and Materials Required
An optical bench with three uprights, a convex lens with lens holder, two needles, an index needle, and

a half metre scale

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