Question

Analyse the following observation table showing variation of image-distance (v) with object-distance (u) 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
–60
+12
2
–30
+15
3
–20
+20
4
–15
+30
5
–12
+60
6
–9
+90
(a) What is the focal length of the convex lens? State reason for your answer.
(b) For what object-distance (u) is the corresponding image-distance (v) not correct? How did you arrive at this conclusion?
(c) Choose an appropriate scale to draw a ray diagram for the observation at S. No. 4 and find the approximate value of magnification.

Answer 1

Convex lens

Explanation:

From the given information in serial number 3, we can infer that the radius of curvature of the lens is 30cm. When object is at centre of curvature, image is at the same distance at the other side of the lens.  

Focal length f = 1/2 radius of curvature = 15cm.

In serial number 6, object is between focus and pole, so a virtual image will be formed. But it is given that a real image is formed as the image distance is positive. So the entry is incorrect.

Approx. magnification = object distance / image distance = 20 / +60 = 3