a parallel beam of monochromatic light falls on a combination of a convex lens and a concave lens if focal length 15 cm and 5cm what is the distance between the two lenses to obtain a parallel beam of light from the concave lens?
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Answered by
118
Given
f1=+15cm
f2= -5cm[sign conventions ]
The effective focal length of system is :1/f=1/f1+1/f2 - d/f1f2
we need the final rays to be parallel hence f effective will be infinity .
1/f1+1/f2-d/f1f2=0
d/f1f2=1/f1+1/f2
d=(1/f1+1/f2)f1f2
=(1/15-1/5)[15x-5]
=-75[1-3/15]
=-75x-2/15
=-5x-2
=10cm
∴10cm is the distance between the two lenses to obtain a parallel beam of light from the concave lens
f1=+15cm
f2= -5cm[sign conventions ]
The effective focal length of system is :1/f=1/f1+1/f2 - d/f1f2
we need the final rays to be parallel hence f effective will be infinity .
1/f1+1/f2-d/f1f2=0
d/f1f2=1/f1+1/f2
d=(1/f1+1/f2)f1f2
=(1/15-1/5)[15x-5]
=-75[1-3/15]
=-75x-2/15
=-5x-2
=10cm
∴10cm is the distance between the two lenses to obtain a parallel beam of light from the concave lens
Answered by
13
You can simply use P=P1+P2-dP1.P2 here d is distance between two thin lenses. As P=1/f so you can substitute and find the value I.e. d=10cm.
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