Question

2. A real image, 1/5 th size of object is formed at a distance of 18 cm from
a mirror. What is the nature of the mirror ? Calculate its focal length.
​

Answer 1

Answer: Concave mirror

Explanation: Given: a real image is  

5

1

​  

 

the

 the size of the object, and it is formed at a distance of 18cm from the mirror.

To find the focal length of the mirror and the nature of the mirror.

Solution:

According to the given criteria,

image distance, v=−18cm

magnification, m=−  

5

1

​  

 as the image formed is real hence it will be inverted.

We know,  

m=−  

u

v

​  

 

⟹−  

5

1

​  

=−(  

u

−18

​  

)

⟹u=−(5×18)=−90cm

Applying the lens formula,

f

1

​  

=  

v

1

​  

+  

u

1

​  

 

⟹  

f

1

​  

=  

−18

1

​  

+  

−90

1

​  

 

⟹  

f

1

​  

=  

90

−5−1

​  

 

⟹f=−15cm

Since focal length is negative, therefore it is a concave mirror.