Question

Determine the correctness or otherwise of the following Assertion [a] and Reason [r]
Assertion: Application of an optical brightening agent makes the white fabrics appear brighter.
Reason: Optical brightening agents absorb energy in the visible region and radiate back in the UV region.

• A. Both [a] and [r] are true and [r] is the correct reason for [a]
• B. Both [a] and [r] are true and [r] is not the correct reason for [a]
• C. Both [a] and [r] are false
• D. [a] is true but [r] is false

Hint:

Optical brightening agents (OBAs) are used to enhance the brightness of fabrics by re-emitting UV light as visible light.

Answer 1

The Correct Option is D

The assertion is true because optical brightening agents (OBAs) do indeed make fabrics appear brighter by absorbing UV light and emitting visible blue light, which helps to enhance the white appearance. However, the reason provided is incorrect because OBAs do not absorb energy in the visible region and radiate in the UV region; rather, they absorb UV light and re-emit it as visible light. Thus, the correct answer is (D).
Final Answer: \[ \boxed{(D) \, [a] \, \text{is true but} \, [r] \, \text{is false.}} \]