Spectroscopy is a science built on three key interactions between light and matter. Light is interacting with all of the matter you can see through the process of Reflection. Light from a source, possibly the sun, reflects from your shirt, and is detected by your eye. Not all of the light survives the encounter though – your shirt greedily traps some of the light in the process of Absorption. Sunlight is made up of a mixture of photons with different energies. Different photons have different wavelengths, which we see as different colours when they are separated apart, but appear as white light when all mixed together. White light adopts colour when it interacts with a pigment; some light is absorbed by the pigment and removed from the white light mixture. When you look at your shirt, the colour you see is the result of absorbed photons and reflected light. What you may not notice while looking at your shirt, but you can test, is the process of Scattering. Try this: sit in a room without the lights on, on an overcast day, with the curtains or blinds pulled. The first thing you will notice is all of the little points of light coming into the room – the corners of the curtains, light from under the door. The second thing you will notice is that you can see the objects in the room, even though they are not being directly lit. This is because light coming into the room is being scattered off all of the surfaces. Consider scattering a sort of recycled reflection (scattering is also known as diffuse reflection).
So there you have it – three interactions between matter and light that form a basis for studying the ancient world. I would highly recommend the Wikipedia pages on Reflection, Absorption and Scattering for more details.