Question

A source of light and an observer both are moving with uniform speed along the same straight path. If the wavelength of light received by the observer is equal to the actual wavelength of light emitted by the source, then,

• A. the source and the observer are moving with equal speed and towards each other.
• B. the source and the observer are moving with equal speed and away from each other.
• C. the source and the observer are moving with equal speed and in the same direction.
• D. Insufficient information.

Answer 1

The correct option is C the source and the observer are moving with equal speed and in the same direction.

The apparent wavelength will be different from the actual wavelength only when the distance between source and observer is changing.

So, if the apparent and actual wavelengths are equal, it clearly means that the distance between the source and observer is not changing. This is possible only when both are travelling in the same direction and with the same speed.

Hence, $\left(C\right)$ is the correct answer.