Example 10.3 - Chapter 10 Class 10 - Light - Reflection and Refraction

Last updated at Sept. 10, 2019 by Teachoo

A concave lens has focal length of 15 cm. At what distance should the object from the lens be placed so that it forms an image at 10 cm from the lens? Also, find the magnification produced by the lens.

Example 10.3
A concave lens has focal length of 15 cm. At what distance should
the object from the lens be placed so that it forms an image at 10 cm
from the lens? Also, find the magnification produced by the lens.
Since focal length of concave lens is negative
Focal length = f = − 15cm
Image formed by a concave lens is virtual, erect and infront of the lens.
Hence, the image distance will be negative
Image distance = v = − 10 cm
Finding object distance
Let the object distance = u
Using lens formula,
1/𝑓 = 1/𝑣 − 1/𝑢
1/𝑓 − 1/𝑣 = (−1)/𝑢
(−1)/𝑢 = 1/𝑓 − 1/𝑣
1/𝑢 = 1/𝑣 − 1/𝑓
1/𝑢 = 1/((−10)) − 1/((−15))
1/𝑢 = (−1)/10 + 1/15
1/𝑢 = (−3 + 2)/30
1/𝑢 = (−1)/30
u = −30 cm
Object is placed at a distance of 30 cm
Negative sign indicates that object is infront of the lens.
Finding Magnification
Magnification of a lens = 𝑣/𝑢
= (−10)/(−30)
= 10/30
= 1/3
= + 0.33
Since magnification is positive,
Image is erect.
And Size of image is 1/3 (or 0.33) times the size of object
And since it is a concave lens, image formed will be virtual
Thus,
Image formed is virtual, erect and size is 𝟏/𝟑 times the size of object

Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 9 years. He provides courses for Maths and Science at Teachoo.