A convex lens has focal length of 30 cm. Calculate at what distance should the object be placed from the lens so that it forms an image at 60 cm on the other side of the lens? Also, find the magnification produced by the lens in this case.

Asked by Topperlearning User | 11th Feb, 2015, 10:40: AM

Expert Answer:

f = +30 cm (convex lens)

v = +60 cm (Since image forms on the other side of the lens)

Use lens formula,
begin mathsize 12px style 1 over straight f space equals space 1 over straight v space minus space 1 over straight u
rightwards double arrow 1 over straight u space equals space 1 over 60 space minus space 1 over 30 space equals space fraction numerator 1 minus 2 over denominator 60 end fraction space equals space fraction numerator negative 1 over denominator 60 end fraction
rightwards double arrow space straight u space equals space minus space 60 space cm end style

Hence, the object is at a distance of 60 cm on the left side of the lens.
 
begin mathsize 12px style straight m space equals space fraction numerator Image space distance space left parenthesis straight v right parenthesis over denominator Object space distance space left parenthesis straight u right parenthesis end fraction
rightwards double arrow straight m space equals space fraction numerator 60 space cm over denominator negative 60 space cm end fraction space equals space minus 1 space cm end style
 
The negative sign of m indicates that image is inverted.

Answered by  | 11th Feb, 2015, 12:40: PM