If the recieved frequency is greater than the one that they emitted, then that means the recieved wavelength must be shorter, and the car is approaching the police unit. The greater the change, then the greater the speed of your car. Transmission, absorption, and scattering We are made of matter, therefore the only way that we can detect electromagnetic radiation is by observing its interaction with matter. There are 3 ways in which this can occur. The light interacts with the atoms in the object and instead of passing through, it gets converted to heat, and the energy stops being transmitted.
Dark objects absorb visible light.
E.g. black pavement gets hot. The electrons then pop back down in energy and re-emit the light. A mirror re-emits all the light. If the color gets changed then it is because some light is absorbed or transmitted and not re-emitted.