Reflection and refraction use the ray model of light. But, when light goes through a very small opening the ray model is no longer sufficient. The wave properties of light become important. Suppose you pass light through a round aperture whose diameter you can change. Let the light fall on a screen. As you first begin reducing the size of the hole you’ll find the size of the bright spot shrinking. But, when the hole becomes very small a strange thing happens. The spot no longer shrinks, but its outer edge becomes fuzzy. Light begins to bend around the edge of the hole. Diffraction also occurs when light is sent through narrow slits or if there are small objects that cast shadows in a broader beam of light. Diffraction occurs with all types of waves. You can often see it in water waves and it is one reason that sound waves spread when they come through a door or window.