AIIMS 2007 Physics Total Internal Reflection Assertion Reason Question

To tackle the question effectively, let’s analyze both the Assertion and the Reason provided.

Assertion:

Optical fibres are used for telecommunication. This statement is true. Optical fibres are widely utilized in telecommunications due to their ability to transmit data over long distances with minimal loss. They are preferred over traditional copper wires because they can carry more data at higher speeds and are less susceptible to electromagnetic interference.

Reason:

Optical fibres are based on the phenomenon of total internal reflection. This statement is also true. Optical fibres operate on the principle of total internal reflection (TIR), which occurs when light travels from a medium with a higher refractive index to one with a lower refractive index at an angle greater than the critical angle. This phenomenon ensures that light is effectively trapped within the core of the fibre, allowing it to travel long distances with very little loss in signal strength.

Explanation of the Correct Answer:

Correct Answer: A) Both Assertion and Reason are true and Reason is the correct explanation of Assertion. Both the assertion and the reason are indeed true. The use of optical fibres in telecommunications is fundamentally based on the principle of total internal reflection. In an optical fibre, light signals are transmitted through a core surrounded by a cladding of lower refractive index. The critical angle for TIR can be determined using Snell's law:

$n_1 \sin(\theta_1) = n_2 \sin(\theta_2)$

For total internal reflection to occur, the light must hit the boundary at an angle greater than the critical angle, which is defined as:

$\sin(\theta_c) = \frac{n_2}{n_1}$

where:

• $n_1$ is the refractive index of the core,
• $n_2$ is the refractive index of the cladding,
• $\theta_c$ is the critical angle.

When this condition is satisfied, light is reflected back into the core, allowing it to propagate through the fibre.

Clarification of Incorrect Options:

• B) Both Assertion and Reason are true but Reason is not the correct explanation of Assertion. This option is incorrect because the reason directly explains why optical fibres are used in telecommunications. The principle of TIR is not just related; it is the foundational mechanism that enables the functioning of optical fibres.

• C) Assertion is true but Reason is false. This option is incorrect because both statements are true. The assertion about the use of optical fibres in telecommunications is correct, and so is the reason regarding total internal reflection.

• D) Both Assertion and Reason are false. This option is incorrect because both the assertion and the reason are true.

Conclusion:

In summary, the correct answer is A, as both the assertion and reason are true, and the reason provides a clear explanation for the assertion. Understanding the principle of total internal reflection is key to grasping how optical fibres function in telecommunications.