Question

A ray of light passes through four transparent media with refractive indices $\mu_1,\mu_2,\mu_3$ and $\mu_4$ as shown in the figure. The surfaces of all media are parallel. If the emergent ray CD is parallel to the incident ray AB, we must have

• A. $\mu_1=\mu_2$
• B. $\mu_2=\mu_3$
• C. $\mu_3=\mu_4$
• D. $\mu_4=\mu_1$

Answer 1

Answer: (D)

$\mu_4=\mu_1$

Answer 2

Applying Snell's law at B and C,
$\mu$sin i=costant or
$\mu_1 sin i_B=\mu_4 din i_c$
But \hspace15mm AB||CD
\therefore \hspace15mm i_B=i_C
or \hspace15mm \mu_1=mu_4
NOTE The Snell's law, which the
students read in their plus two syllabus$\mu=\frac{sin i}{sin r}$ is derived from
$\mu$ sin i= constant
or \hspace25mm \mu_1sin i_1=\mu_2 sin i_2
Here: $\mu_1=1,\mu_2=\mu,$
or sin i=$\mu$sin r or $\mu=\frac{sin i}{sin r}$