Question

a convex lens has a focal length of 0.5m. It has to be combined with another lens such that it's power is 1.5 D. what could be the other lens and focal length​

Answer 1

Answer:

Power of combination P=1.5 dioptre

Power of convex lens having focal length of 0.5m,  P1=0.51 dioptre

Using P=P1+P2

∴ 1.5=0.51+P2

We get P2=−0.5 dioptre

Thus focal length of second lens f2=P21 

⟹ f2=−0.51=−2m (negative sign implies the lens must be a concave lens)

Thus second lens could be concave lens of focal length 2m.

Answer 2

Answer:

Explanation:Power of combination P=1.5 dioptre

Power of convex lens having focal length of 0.5m,  P  

1

​  

=  

0.5

1

​  

 dioptre

Using P=P  

1

​  

+P  

2

​  

 

∴ 1.5=  

0.5

1

​  

+P  

2

​  

 

We get P  

2

​  

=−0.5 dioptre

Thus focal length of second lens f  

2

​  

=  

P  

2

​  

 

1

​  

 

⟹ f  

2

​  

=−  

0.5

1

​  

=−2m (negative sign implies the lens must be a concave lens)

Thus second lens could be concave lens of focal length 2m.