Question

Focal length of converging combination of 2 lens is 20 cm the ratio of dispersive power is 2:3 , one of the focal length will be

Answer 1

Dear Student,

◆ Answer -

f1 = 20/3 cm

f2 = -10 cm

● Explanation -

Ratio of focal lengths is -

f1/f2 = -p1/p2

f1/f2 = -2/3

f1 = -2f2/3

Total focal length is -

1/F = 1/f1 + 1/f2

1/20 = 1/(-2f2/3) + 1/f2

1/20 = -3/2f2 + 1/f2

1/20 = -1/2f2

-2f2 = 20

f2 = -10 cm

Other focal length is -

f1 = -2f2/3

f1 = -2(-10)/3

f1 = 20/3 cm

Thanks dear. Hope this helps you...

Answer 2

Answer:

◆ Answer -

f1 = 20/3 cm

f2 = -10 cm

● Explanation -

Ratio of focal lengths is -

f1/f2 = -p1/p2

f1/f2 = -2/3

f1 = -2f2/3

Total focal length is -

1/F = 1/f1 + 1/f2

1/20 = 1/(-2f2/3) + 1/f2

1/20 = -3/2f2 + 1/f2

1/20 = -1/2f2

-2f2 = 20

f2 = -10 cm

Other focal length is -

f1 = -2f2/3

f1 = -2(-10)/3

f1 = 20/3 cm