Question

Q20. Analyse the following observation table showing variation of image-distance with object-distance in case of a convex lens and answer the questions that follow without doing any calculations :

S.No. Object-Distance u (cm) Image-Distance v (cm)
1 -100 +25
2 -60 +30
3 -40 +40
4 -30 +60
5 -25 +100
6 -15 +120

a) What is the focal length of the convex lens? Give reason to justify your answer.
b) Write the serial number of the observation which is not correct. On what basis have you arrived at this conclusion? c) Select an appropriate scale and draw a ray diagram for the observation at S. No. 2. Also find the approximate value of magnification.

Answer 1

Answer:Given that the lens are convex lens.

So from the serial number 1

By lens equation,

1/f=1/u-1/v

1/f=1/-100-1/25

1/f=-1-4/100

1/f=-5/100

1/f=-1/20

f=-20cm

R=2f=2(-20)=-40cm

So when the object kept beyond the center of the curvature,then the image will get at between focus and center of the curvature... and the image is small compared to the object...it is a diminished image....

Explanation:

Hope it helps you.......