Which of the following is an example of the Doppler effect? (2 points)
a
A water bug on the surface of a pond is producing small ripples in the water. The bug is not moving.
b
A police car with its siren on is driving towards you, and you perceive the pitch of the siren to increase.
c
Scientists measure the speed of light and find that it is constant.
d
The gravitational pull of two objects on each other
