Look at the spectrum above. You will see red, orange, yellow, green, blue, and violet (you may see indigo, depending on who taught this to you, although our eyes are so insensitive to the higher frequencies, it's difficult for us to see indigo and violet.)
Now look through
the filters. What is missing? See
how each filter blocks out different parts of the spectrum? Which
filter is the narrowest (the one which lets through the slimmest section
of the spectrum.) Why do you think most people use red filters to
make secret messages? Cool, huh?
Now here's how small our visible spectrum is compared to the other forms of electromagnetic radiation. There are many other kinds of radiation and you can use them to look at the universe as well as visible light. It wasn't until long after all these different types of radiation were discovered that we realized that they are all forms of "light."
