COMEDK2013PhysicsRay Optics
A beam of parallel rays is brought to focus by a plano-convex lens. A thin concave lens of the same focal length is joined to the first lens. The effect of this is
Options
- Athe focus shifts to infinity
- Bthe focal point shifts towards the lens by a small distance
- Cthe focal point shifts away from the lens by a small distance
- Dthe focus remains undisturbed
Correct answer
A. the focus shifts to infinity
Step-by-step solution
The focal length of the combination is given by 1 F = 1 f₁ + 1 f₂ aligned f₁ &= focal length of plano-convex lens =f f₂ &= focal length of concave lens =-f 1 F &= 1 f + 1 -f =0 F &= aligned Hence, focus shifts to infinity.