A Level Physics Practice Exam

What does the term coherent sources imply concerning the waves?

1. Waves are of different frequencies

2. Waves lose energy over time

3. Waves have a constant phase relationship

4. Waves travel in opposite directions

The correct answer is: Waves have a constant phase relationship

The term "coherent sources" refers to sources of waves that maintain a constant phase relationship with each other. This means that the waves produced by these sources have a fixed phase difference, which leads to consistent constructive and destructive interference patterns when the waves overlap. In practical terms, coherent sources produce waves that oscillate at the same frequency and have a consistent time relationship, allowing for phenomena such as stable interference patterns that are crucial in applications like lasers and Young's double-slit experiment. The other options do not accurately define coherent sources. For example, having different frequencies would imply a varying phase relationship, while energy loss over time is not a characteristic of coherence but rather affects the amplitude of waves generally over distance. Lastly, the direction of wave travel does not determine coherence; coherent sources can produce waves traveling in the same or different directions. Thus, the defining characteristic that makes C the correct choice is the maintenance of a constant phase relationship among the waves.