JEE Physics 2023 Question with Solution

Given: Assertion $A$ states that EM waves used for optical communication have longer wavelengths than microwaves used in Radar. Reason $R$ states that infrared EM waves are more energetic than microwaves.

Find: Which option correctly describes the truth values of Assertion $A$ and Reason $R$.

From the solution text: optical communication is performed in the frequency range of $1 \, \text{THz}$ to $1000 \, \text{THz}$. Therefore, the wavelength of EM waves used for optical communication is shorter than that of microwaves used in Radar. Hence, Assertion $A$ is incorrect.

Infrared waves have higher frequency and energy compared to microwaves. Hence, Reason $R$ is true.

Thus, Assertion $A$ is false but Reason $R$ is true.

The solution also displays "The Correct Option is D", but the worked conclusion states "Therefore, the correct answer is $1$." The worked conclusion is consistent with the statements, so the correct option is A.

Given: Optical communication uses higher-frequency EM waves than microwaves used in Radar.

Find: Whether Assertion $A$ and Reason $R$ are true or false.

Use the relation

A higher frequency $\nu$ implies a shorter wavelength $\lambda$.

Since optical communication uses frequencies from about $1 \, \text{THz}$ to $1000 \, \text{THz}$, its wavelength is shorter than that of microwaves. So Assertion $A$ is false.

Also, energy of an EM wave is proportional to frequency:

Infrared waves have higher frequency than microwaves, so they are more energetic. Therefore, Reason $R$ is true.

Hence, the correct choice is: Assertion $A$ is false but Reason $R$ is true, which corresponds to A.