NEET2006PhysicsRay OpticsActual
A lens is made of flint glass (refractive index =1.5 ). When the lens is immersed in a liquid of refractive index 1.25 , the focal length
Options
- Aincreases by a factor of 1.25
- Bincreases by a factor of 2.5
- Cincreases by a factor of 1.2
- Ddecreases by a factor of 1.2
Correct answer
B. increases by a factor of 2.5
Step-by-step solution
Let f be focal length of lens. aligned & 1 f = ( ^a _g-1 ) ( 1 R₁ - 1 R₂ )=(1.5-1) ( 1 R₁ - 1 R₂ ) & or, 1 f =0.5 ( 1 R₁ - 1 R₂ ) or, 1 R₁ - 1 R₂ = 1 0.5 f . . aligned Let f^ be focal length of the lens when immersed in a liquid. aligned & 1 f^ = ( ^L _g-1 ) ( 1 R₁ - 1 R₂ )= ( 1.5 1.25 -1 ) ( 1 R₁ - 1 R₂ ) & = ( 0.25 1.25 ) ( 1 R₁ - 1 R₂ )= 0.25 1.25 1 0.5 f (using (i)) & 1 f^ = 1 2.5 f or, f^ =2.5 f . aligned i.e. focal length of lens when immersed in a liquid is increased by a factor of 2.5 .