Q154 / 180 · KCET 2008 · Physics

A convex and a concave lens separated by distance are then put in contact. The focal length of the combination

  • Correct answer
    A
    decreases
  • B
    increases
  • C
    becomes 0
  • D
    remains the same

Solution

When two lenses are placed coaxially at a distance from each other, then equivalent focal length ( ) is given by $$ \frac{1}{F}=\frac{1}{f_{1}}+\frac{1}{f_{2}}-\frac{d}{f_{1} f_{2}} $$ When the lenses are put together. $$ \mathrm{d}=0 $$ Hence, the focal length of the combination will decrease.

Hence, the correct answer is option: 'A'.

More Ray Optics Questions