Question: What is the formula for the observed frequency (f\') in the Doppler effect when the source is moving towards a stationary observer?

Options:

1. f\' = f (v / (v - vs))
2. f\' = f (v + vs) / v
3. f\' = f (v - vs) / v
4. f\' = f (v / (v + vs))

Correct Answer: f\' = f (v / (v - vs))

Solution:

The formula for the observed frequency when the source is moving towards the observer is f\' = f (v / (v - vs)).