What is defined as the spreading out of light rays after they encounter an opening?

The concept described—light rays spreading out after encountering an opening—best aligns with diffraction. Diffraction occurs when waves, including light waves, encounter an obstacle or slit that is comparable in size to their wavelength, causing the waves to bend and spread out. This phenomenon is particularly noticeable with light when it passes through narrow apertures, leading to patterns of interference and the characteristic spreading effect.

Refraction, on the other hand, involves the bending of light as it passes from one medium to another with a different density. While this still changes the direction of light, it does not involve the spreading effect tied specifically to slits or openings.

Reflection refers to the bouncing back of light rays when they hit a surface, while dispersion describes the process of separating light into its constituent colors, such as through a prism. Both of these processes do not pertain to the spreading effect after passing through an opening, solidifying diffraction as the correct choice.