Question

Assertion :A light ray is incident on a glass slab. Some portion of it is reflected and some is refracted. Refracted and reflected rays are always perpendicular to each other. Reason: Angle of incidence is equal to angle of reflection.

• A. Both Assertion and Reason are correct and Reason is the correct explanation for Assertion
• B. Both Assertion and Reason are correct but Reason is not the correct explanation for Assertion
• C. Assertion is correct but Reason is incorrect
• D. Assertion is incorrect but Reason is correct

Answer 1

The correct option is D Assertion is incorrect but Reason is correct

Solution:

Let angle of incidence be $i$ and angle of refraction be $r$.

As per laws of reflection, angle of incidence is equal to angle of reflection. So, $r_{reflection}=i$.

Applying snell's law on the glass slab, we can write,

$\frac{\sin i}{\sin r}=\frac{\mu }{1}$

where $\mu$ is the refractive index of the glass.

Hence,

$1\times \sin i$ =$\mu \sin r$

Let reflected and refracted ray be perpendicular.

So, $r_{reflection}+r+90=180$

$i+r+90=180$

$\Rightarrow i+r=90$

$\Rightarrow r=90-i$

So,$\sin r=\sin (90-i)$

So, $\sin r=\cos i$

Putting it in snell's law we get $tani=\mu$

So assertion is only true when $tani=\mu$

Hence option D is correct.