an image 2/3 of the size of the object is formed by a convex lens at a distance of 12cm from it. Find the focal length of the lens.
Share
an image 2/3 of the size of the object is formed by a convex lens at a distance of 12cm from it. Find the focal length of the lens.
Sign Up to our social questions and Answers Engine to ask questions, answer people’s questions, and connect with other people.
Login to our social questions & Answers Engine to ask questions answer people’s questions & connect with other people.
or, magnification = size of image/ size of object = v/u
or, -2/3 × size of object/size of object = v/u
or, -2/3 = v/u
or, u = (3/2)v = -1.5v ..........(1)
image distance , v = 12cm
object distance , u = -1.5v = -1.5 × 12 = -18cm
use formula, 1/v - 1/u = 1/f
or, 1/12 - 1/-18 = 1/f
or, 1/12 + 1/18 = 1/f
or, 1/f = 5/36
hence, f = 7.2 cm
Verified answer
The image formed is real (convex lens forms diminished real image)
m= -2/3
v= + 12cm
m=v/u
-2/3=12/u
-2u=36
u= -18 cm
now
focal length formula
1/f= 1/v - 1/u
1/f= 1/12 - 1/(-18) = 5/36
f=36/5
focal length of the lens= 7.2cm