Both rods look just the same, but… one disappears into the liquid, the other doesn’t. Why’s that?
First you need to know how our eyes are able to see. We see objects when they give light, or when they reflect light. In the dark there is no light, and therefore we can’t see anything.
In fact, light also travels through things. For example, through the liquid in this container. As a result, we can also see the stripes at the back of the box. However, light does not travel at the same speed through everything. Light goes faster through air than through water, for example. It’s the same with us in fact: we can walk quickly through air, but much slower through water.
Because the speed of light changes, the light breaks — or refracts — the moment it passes through another substance. This causes the light rays to bend.
That’s also what’s happening here. There is glycerine in the container. One rod is made of polycarbonate. The light travels more slowly through polycarbonate than through glycerine. At the point where the light refracts, it also reflects back to our eye. As a result, we clearly see the distinction between the liquid and the rod.
So why don’t we see the other rod? Because it’s made of quartz glass. And light travels through glycerine almost as fast as it does through quartz glass. This way the light barely refracts through the quartz glass and also doesn’t reflect back to our eyes. As a result, the rod seems to have disappeared.
Light refraction in a rainbow
A rainbow is created by sunlight shining on water droplets in the air. As a result, the light refracts, reflects on the back of the droplets and refracts again. That sunlight is — contrary to what you might think — not just white, but a mixture of many colours. And these suddenly become visible due to refraction in the droplets.
