MHTCET 23 Apr 2025 Shift 1 Paper

Concept:When an observer moves towards a stationary sound source, the apparent frequency increases due to the Doppler effect.Explanation:For a moving observer and a stationary source, the apparent frequency is given by:$$f' = f\left(\frac{v + v_o}{v}\right)$$Here, $f$ is the actual frequency, $f'$ is the apparent frequency, $v$ is the velocity of sound, and $v_o$ is the observer's velocity towards the source.Given, $v_o = \frac{1}{5}v$.Substituting this value into the formula:$$f' = f\left(\frac{v + \frac{1}{5}v}{v}\right) = f\left(1 + \frac{1}{5}\right) = \frac{6}{5}f$$The increase in frequency is:$$\Delta f = f' - f = \frac{6}{5}f - f = \frac{1}{5}f$$Therefore, the percentage increase in apparent frequency is:$$\frac{\Delta f}{f} \times 100\% = \frac{1}{5} \times 100\% = 20\%$$Answer:Option C — $20\%$