Doppler Effect

The apparent frequency of sound heard by the passengers depends on the relative motion between the source and the observer. Since the passenger is moving with the train, the frequency heard by them is the same as the frequency emitted by the whistle, .

Concept: According to the Doppler Effect, when an observer moves towards a stationary source, the apparent frequency is given by where

• = apparent frequency
• = actual frequency
• = velocity of sound
• = velocity of the observer

The percentage increase in frequency is calculated from the ratio .
Step 1: {Substitute the given velocity.} The observer moves with velocity Thus
Step 2: {Simplify the expression.}
Step 3: {Find the increase in frequency.}
Step 4: {Calculate percentage increase.}