Question

ASSERTION: Optical fibers make use of total internal reflection.
REASON: Light goes through successive total internal reflections as it moves through an optical fiber.
(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 the assertion.
(C)Assertion is correct but reason is incorrect
(D)Both assertion and reason are incorrect.

Answer 1

Total internal reflection is the phenomenon of complete reflection where at a particular angle called critical angle, a ray of light is completely reflected and there is no light refracted into the other medium. Optical fibers make use of this phenomenon to transmit light around very large distances.

Complete answer:
Optical fibers are used in the medical industry for a wide number of purposes by doctors. To transmit light through long distances without much loss of light through refraction, transmission or absorption, this phenomenon is made use of in optical fibers to serve the same purpose.

Fig: Ray diagram showing total internal reflection of light through an optical fiber.

So as a fact the assertion is true that optical fibers use total internal reflection. The role of optical fibers is to transport light over large distances without much loss and the phenomenon of total internal reflection serves the purpose very well. Light is made to undergo total internal reflection successive times as the ray of light moves through the optical fiber. Hence, both assertion and reason are correct. But the reason is not the correct explanation to why an optical fiber makes use of total internal reflection.

So, the correct answer is “Option B”.

Additional Information:
Total internal reflection explains many optical phenomenons like the sparkling of diamonds, formation of mirages, etc. In a similar way it is also used for making optical fibers. Two conditions must be necessarily satisfied for total internal reflection to occur. The first is that light must travel from a denser to a rarer medium and the second is that the angle of incidence must be greater than the critical angle. The optical fiber consists of 2 parts – the core and the cladding. The core has a higher refractive index than the cladding.

Note:
In the questions that involve assertion and reason, it is very important to check the validity of both the statements made and then rationalise if the reason is the correct explanation for the assertive statement. In this particular question, both assertion and reason are true but the reason fails to validate the assertion.