Question
Three lenses 
are placed co-axially as shown in figure. Focal length's of lenses are given 30 cm, 10 cm and 5 cm respectively. If a parallel beam of light falling on lens 
, emerging 
as a convergent beam such that it converges at the focus of . Distance between and 
will be
- 40 cm
- 30 cm
- 20 cm
- 10 cm
The correct answer is: 20 cm
According to the problem, combination of and act a simple glass plate. Hence according to formula 
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