Page 65 - Physics - XII

5. Now, move the thick image needle at the position I of the image of the object needle and adjust the
height of the image needle so that its tip is seen in the line with the tip of the image when seen right
through open eye. Now, move the eye towards right, the tips will get separated. The image tip and

image needle tip have parallax. Remove the parallax between the image needle and the image I of the
object needle. In the position of no parallax, the image needle locates the position of the image of the
object needle.
6. Note the positions of the mirror, the thin object needle, and the image needle on the bench scale.
7. Repeat the above steps for fi ve diff erent positions of object needle and note the observations in the
observation table.

Observations and Calculations
1. Rough focal length of the given concave mirror = _____ cm
2. Actual length of the index needle = x = _____ cm
3. Observed distance between the mirror and the object needle when the index needle is placed in between

them = y = _____ cm
4. Observed distance between the mirror and the image needle when index needle is placed in between
them = z = _____ cm
Index correction for the object distance u = (x – y) = _____ cm

Index correction for the image distance v = (x – z) = _____ cm

Table for u, v, 1 , and 1
u v
S. Position of Observed distance Corrected distance 1 1 uv

No. Concave Object Image u = PO v = PI u v u v f == u+ v
mirror needle needle (cm) (cm) (cm) (cm) (cm ) (cm ) (cm)
–1
–1
P O I
(cm) (cm) (cm)
1.
2.
3.

4.
5.

Mean f = _____ cm.

Error
We know that 1 1 1
f u v

f u v
⇒
f 2 u 2 v 2

u v
So, error in focal length, f f 2 u 2 v

2

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