Question 307: Ray Optics

Question

Two similar thin equi-convex lenses, of focal length f each, are kept coaxially in contact with each other such that the focal length of the combination is F
1
. When the space between the two lenses is filled with glycerin (which has the same refractive index (m = 1.5) as that of glass) then the equivalent focal length is F
2
. The ratio F
1
: F
2
will be :

Accepted Answer

Equivalent focal length (in air) $${1 \over {{F_1}}} = {1 \over f} + {1 \over f} = {2 \over f}$$
When glycerin is filled inside, glycerin lens behaves
like a diverging lens of focal length (Ã¢ÂÂf)
$${1 \over {{F_2}}} = {1 \over f} + {1 \over f} - {1 \over f} = {1 \over f}$$
$$ 	herefore $$ $${{{F_1}} \over {{F_2}}} = {1 \over 2}$$

Answer

1 : 2

Answer

2 : 3

Answer

3 : 4

Answer

2 : 1

Ray Optics

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