Two thin lenses, one of focal length + 60 cm and the other of focal length -20 cm are put in contact, the combined focal length is:

Question

Two thin lenses, one of focal length + 60 cm and the other of focal length -20 cm are put in contact, the combined focal length is: (A) 15 cm (B) -15 cm (C) -30 cm (D) 30 cm

Tardigrade Admin · Accepted Answer

Using the relation for combined focal length (1/f)=(1/f1)+(1/f2)=(1/60)+(1/(-20)) (Here: f1=60 cm f2=-20 cm ) (1/f)=(1/60)-(1/20)=(1-3/60)=(-2/60) f=-30 cm

Q. Two thin lenses, one of focal length + 60 cm and the other of focal length -20 cm are put in contact, the combined focal length is:

15 cm

-15 cm

-30 cm

30 cm

Using the relation for combined focal length $\frac{1}{f} = \frac{1}{f _{1}} + \frac{1}{f _{2}} = \frac{1}{60} + \frac{1}{( - 20 )}$ (Here: $f_{1} = 60 c m f_{2} = - 20 c m$ ) $\frac{1}{f} = \frac{1}{60} - \frac{1}{20} = \frac{1 - 3}{60} = \frac{- 2}{60}$ $f = - 30 c m$

Q. Two thin lenses, one of focal length + 60 cm and the other of focal length -20 cm are put in contact, the combined focal length is:

15 cm

-15 cm

-30 cm

30 cm

Using the relation for combined focal length f1​=f1​1​+f2​1​=601​+(−20)1​ (Here: f1​=60cmf2​=−20cm ) f1​=601​−201​=601−3​=60−2​ f=−30cm

Using the relation for combined focal length $\frac{1}{f} = \frac{1}{f _{1}} + \frac{1}{f _{2}} = \frac{1}{60} + \frac{1}{( - 20 )}$ (Here: $f_{1} = 60 c m f_{2} = - 20 c m$ ) $\frac{1}{f} = \frac{1}{60} - \frac{1}{20} = \frac{1 - 3}{60} = \frac{- 2}{60}$ $f = - 30 c m$