When light travels from one substance through another, it refracts.

The refraction of light can cause illusions. For example, the refraction can cause an object to seem larger than it actually is.

Strong refractions are created with lenses, which can be made from glass or plastic. A concave lense collects the rays of light into one place, as seen in the image on the left below. A convex lense spreads the rays of light in different directions, as seen in the image on the right below.