A Level Physics Practice Exam

What is a key characteristic of coherent sources of light?

1. They produce light of different colors

2. They have the same frequency and wavelength

3. They are always in phase

4. They emit light continuously

The correct answer is: They have the same frequency and wavelength

Coherent sources of light are defined by their ability to maintain a constant phase relationship over time. This characteristic arises because coherent sources emit light waves that have the same frequency and wavelength. When the frequencies match, the periodic nature of the waves allows them to interfere constructively or destructively in a predictable manner, which is foundational in phenomena such as interference patterns and diffraction. Having the same frequency ensures that the light waves oscillate at the same rate, while matching wavelengths guarantees that the distance between consecutive crests or troughs is consistent. This combination of frequency and wavelength is crucial for creating stable interference patterns. Therefore, the key characteristic that defines coherent sources of light is that they share these same frequency and wavelength values, which allows for synchronized wavefronts essential for coherence.