Question 7 - Assertion Reasoning Questions (MCQ) - Chapter 11 Class 10 - Human Eye and Colourful World

Assertion (A): In case of a rainbow, light at the inner surface of the water drop gets internally reflected.  

Reason (R): The angle between the refracted ray  and normal to the drop surface is greater than the  critical angle.  

Answer

Let’s look at how a rainbow is formed

• After rain , there are many tiny droplets of water still in the air
• When sunlight falls on these droplets
• These droplets act as tiny prisms
• When sun rays fall on these droplets, the rays get first refracted , then internally reflected and then refracted again
• Due to this different colors of sunlight are bent at different angles and we are able to see a spectrum in the form of a rainbow

Since light at the inner surface gets internally reflected, Assertion is true

For critical angle,

• Total internal reflection only takes place if the light from a denser medium to a lighter medium strikes at an angle of incidence larger than the critical angle . 
• If the angle between the refracted light and the normal to the drop surface is greater than a critical angle ( which is 45 degrees), the light reflects off the back of the drop .
• If the angle between the refracted light and the normal to the drop surface is less than the critical angle , the light will simply pass through .

Therefore, r easoning is also a true and correct explanation of assertion .  

Thus,

• Assertion is True
• Reason is True
• And reason is the correct explanation for the assertion.

So, the correct answer is (a)