Refraction

Another property of light is that it refracts, which means that it bends when passing from one medium to another. Moreover, when light enters a more dense medium from one that is less dense, it bends towards a line normal to the boundary between the two media. This is illustrated in the figure below.

 

Figure 10.2: Refraction of light

The greater the density difference between the two materials, the more the light bends. One place where this is used is in lenses for a variety of optical devices, such as microscopes, magnifying glasses, and glasses for correcting vision. An example of an image formed from a lens is shown below.

 

Figure 10.3: Image formed from a lens

In this case the light from the object passes through the lens and is bent, forming an image on the other side of the lens which is magnified and inverted.

Many types of optical illusions are due, at least in part, to the refraction of light. One such example is the fact that if you look down while standing in a swimming pool, your feet appear closer to the surface than they actually are. This is due to the fact that light is bent when passing from water to air, as indicated below. Note that since air is less dense than water, the light bends away from the normal as it emerges.

 

Figure 10.4: Apparent depth of an object under water

The illusion comes from the fact that our eye doesn't know that the light has been refracted when it comes from water into air, and so thinks that it has originated from a point closer to the surface.