Analyze 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
−90
+18
2
−60
+20
3
−30
+30
4
−20
+60
5
−18
+90
6
−10
+100
(a) What is the focal length of the convex lens? Give reason in support of your answer.
(b) Write the serial number of that observation which is not correct. How did you arrive at this conclusion?
(c) Take an appropriate scale to draw a ray diagram for the observation at S. No. 4 and find the approximate value of magnification.
Answers
The tabular column given in the question given below:
(a) Focal length of the convex lens:
Focal length is distance between centre of curvature and focal point of lens or mirror.
From the given table, the row in the serial number 3 has 30 as v and -30 as u. From this we can understand the the distance between lens and object and distance between lens and image is same. So, the radius of the curvature of lens is 30 cm. The focal length is half of radius of curvature. Now, the focal length is 15 cm.
(b)
The observation in the serial number 6 is incorrect. Since, it is a converging lens and the object distance is negative then the image distance should also be negative. But it is given as positive.
(c)
Using ray diagram to find magnification takes longer time. So, we can simply use image and object distance to find the magnification. The magnification at serial number 4 is 3.
