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Every substance such as glass or plastic that is transparent to light has a characteristic called the index of refraction. This index is simply a number, from 1 on up, that tells us how that substance slows down light. A vacuum is where there is no substance at all, sort of like modern political debates. The index of refraction of a vacuum is simply 1.00. In a vacuum, light travels at a very high speed. Light of any color (red, blue, pink) and even the light that we can’t see all travel at 186,000 miles per second. A beam of light can go around the entire planet Earth seven times every second. Santa Claus is the only comparable entity.
Curiously, anything other than nothing is called matter. I have sometimes called it “stuff,” but was met with severe criticism from my English teachers. When light hits something made up of stuff (matter), it slows down. If light goes directly into matter, it keeps going in a straight line and slows down, so when it comes out the other side, it has only lost a little time. Compared with some light traveling along side, without going through the stuff, the first beam is said to be delayed. Much of this was explained by great minds hundreds of years ago.
What’s really neat is when the light hits the matter, but not directly head‐on. If it comes into contact (figuratively speaking) with the surface of the matter at an angle, the slowing‐down property (the index of refraction) makes the light beam appear to bend or change direction when it enters the stuff. This property of matter, being able to bend light, is almost always useful. We can make prisms and see the wonderful colors of the rainbow without having to stand outside in the rain. We can make lenses of various shapes to focus light onto digital cameras and take really cool pictures. We can use other lenses to help us see clearly when our eyes are somehow not able to do it for themselves.
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