Diffraction occurs when a wave encounters an obstacle or slit and spreads out as it passes around the edge of the obstacle or through the slit. If a slit is sufficiently small and light passes through it, a diffraction pattern is formed. This is because the light waves spread out as they pass through the slit, and this spreading out of the light creates a pattern of light and dark areas on the other side of the slit.

Reflection occurs when a wave encounters a surface and bounces off of it. The angle of reflection is equal to the angle of incidence. This means that if a wave hits a surface at a 45-degree angle, it will bounce off at a 45-degree angle.

Refraction occurs when a wave passes from one medium to another and bends. The amount of bending depends on the refractive indices of the two media. The refractive index is a measure of how fast light travels through a medium. The higher the refractive index, the slower light travels through a medium.

The main difference between diffraction and reflection or refraction is that diffraction occurs when a wave encounters an obstacle or slit, while reflection and refraction occur when a wave encounters a surface. Diffraction is also the only one of the three that involves the spreading out of a wave.