AP EAMCET2005PhysicsRay Optics
The focal length of an equi-convex lens is greater than the radius of curvature of any of the surfaces. Then the refractive index of the material of the lens is
Options
- Agreater than zero but less than 1.5
- Bgreater than 1.5 but less than 2.0
- Cgreater than 2.0 but less than 2.5
- Dgreater than 2.5 but less than 2.0
Correct answer
A. greater than zero but less than 1.5
Step-by-step solution
Focal length of lens, 1 f =( -1) ( 1 R₁ - 1 R₂ ) For equi-convex lens, aligned R₁ & =+R, R₂=-R 1 f & =( -1) ( 1 R - 1 -R ) 1 f & =( -1) ( 2 R ) f & = R 2( -1) f < R, so, 2( -1) & < 1 ( -1) & < 1 2 i.e. ( -1) & < 0.5 & < 1.5 aligned Focal length of convex lens is positive. So, cannot be negative, hence should be greater than zero but less than 1.5.