A level Physics, Isaac Physics QWatch
A laser L produces a coherent light beam of wavelength 𝜆 that hits a conveyor belt B at an angle 𝛼. The conveyor belt is moving at speed 𝑣. Some light is scattered from the belt back towards the laser, and it is found that there is a difference in frequency, 𝛿 between the light emitted by the laser and the light which returns to it.
If 𝑣=11cms−1, 𝛼=75∘ and 𝜆=633nm what is the value of 𝛿?
The one you need to use here would be for finding the observed frequency f " of a 'light' source of frequency f ' moving away from the observer.
The only difference in this case is that the beam hits the moving belt at 75 degrees so you need to use the component of this speed in the direction of the beam.