Question

an object is kept in front of convex lens of focal length 20 cm the image is 3 times the size is object calculate two possible distance of object​

Answer 1

Answer:

It is given that,

Focal length

f=−20cm

h  

i

​  

=3h  

o

​  

 

The image is three times the size of the object. So there are two cases.

Case 1. For real image

m=−  

u

v

​  

=−3

v=3u

Using mirror’s formula

f

1

​  

=  

v

1

​  

+  

u

1

​  

 

−20

1

​  

=  

3u

1

​  

+  

u

1

​  

 

u=  

3

−80

​  

cm

Case 2. For virtual image

m=  

u

−v

​  

=+3

v=−3u

Again using mirror’s formula

−20

1

​  

=  

−3u

1

​  

+  

u

1

​  

 

u=  

3

−40

​  

cm

The possible values of u are  

3

−80

​  

cmand  

3 −40  cm.