Final answer:
The correct answer is 2) interference.
Step-by-step explanation:
When two light waves meet under the correct conditions, they can produce regions where their effects reinforce each other, known as constructive interference, and regions where they cancel each other out, known as destructive interference. This phenomenon is referred to as interference. Interference results from the superposition of two or more traveling waves, which causes variations in the resultant wave amplitude. Constructive interference increases amplitude where wave peaks coincide, whereas destructive interference decreases amplitude where a peak and trough coincide.
Examples of interference include the patterns observed when light passes through narrow slits in a double-slit experiment. Such patterns cannot be explained by classical mechanics and are a result of the wave aspect of light, confirming that light behaves as a wave.