Shown in the figure is a bi-convex lens made of different transparent strips of two different refqractive indexes. If the parallel rays of light is incident on the lens;
Answer & explanation
Correct answer: option 4
Shown in the ray diagram, is an incident beam of parallel rays 1,2,3 & 4. Rays 1 & 3 pass through the medium 1
⇒ It has a focal length $f_1$ given as
$f_1 =\frac{R_1}{2(n_1-1)}$
Similarly rays 2 and 4 pass through the medium 2.
⇒ It has a focal length
$f_2 =\frac{R_1}{2(n_2-1)}$
when $n_2 >n_1, f_2 < f_1$ and vice versa.
Therefore there are two focal points at right hand sides; for the other sides $f'_1=\frac{R_2}{2(n_1-1)}$ and $f'_2=\frac{R_1}{2(n_2-1)}$. Therefore another two focal points are for left hand side. We see that the focal points (focal lengths) need not to be same if $R_1 ≠ R_2$ we obtain four focal points but they are different from each other
∴ (D)