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AP EAMCET2013PhysicsRay Optics

The two surfaces of a concave lens, made of glass of refractive index 1.5 have the same radii of curvature R . It is now immersed in a medium of refractive index 1.75 , then the lens

Options

  1. Abecomes a convergent lens of focal length 3.5 R
  2. Bbecomes a convergent lens of focal length 3.0 R
  3. Cchanges as a divergent lens of focal length 3.5 R
  4. Dchanges as a divergent lens of focal length 3.0 R

Correct answer

A. becomes a convergent lens of focal length 3.5 R

Step-by-step solution

From lens maker's formula 1 f = ( _g^m -1 ) ( 1 R₁ - 1 R₂ ) Now, _g^m = g _m = 1.5 1.75 For concave lens as shown in the figure, in this case R₁=-R and R₂=+R gathered 1 f = ( 1.5 1.75 -1 ) (- 1 R - 1 R )=+ 0.25 2 1.75 R f=+3.5 R gathered The positive sign shows that the lens behaves as a convergent lens.

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