Is light composed of particles or waves?

Atomic electrons have specific allowed energy states. Why?

If atoms occupy only certain allowed levels, how do they get from one allowed level to another?

Why do atoms emit or absorb only certain specific colors of light?

How can you tell that there is calcium in the atmosphere of the star Betelgeuse, which is about 310 light years away from the earth?

Why is it that you are transparent to radio waves, more and more opaque as the frequency increases to the visible and ultraviolet part of the spectrum, and then suddenly transparent again when you get to the xray part of the electromagnetic spectrum?

Why do you say that ultraviolet exposure might involve some risk of skin cancer, whereas exposure from your microwave oven carries no such risk?

What types of radiation make up the ionizing radiation which we might be exposed to?

What is a laser?

What are the characteristic properties which make the light from a laser unique?

What is a hologram? What properties distinguish a hologram from an ordinary photograph?