When a wave reaches a new material, three things can happen to its energy, and usually all three happen at once in some mix. This is true of light, sound and water waves alike.
Key idea
A wave meeting a new material can be reflected (bounce back), absorbed (its energy taken in, usually as heat) or transmitted (pass through). The three shares always add up to 100% of the energy that arrives.
Why it works
Energy can't vanish, so whatever doesn't bounce back or pass through must be taken in by the material. A black shirt in sunlight absorbs most of the light and warms up. A window transmits most light. A white wall reflects most of it, scattered in every direction, which is why the whole room looks bright.
Worked example: Where did the light go?
A sheet of tinted glass transmits 70% of the light that hits it and reflects 10%. What percent does it absorb?
- The three shares add to 100%: $70 + 10 + x = 100$.
- $x = 20\%$ is absorbed.
Answer: 20%
Worked example: Sound in a music studio
Why are the walls of a recording studio covered with soft foam?
- Hard, smooth walls reflect sound, making echoes.
- Soft foam absorbs much of the sound energy instead, so the room stays quiet and clear.
Answer: The foam absorbs sound instead of reflecting it
Common mistake
Thinking light only reflects from shiny surfaces like mirrors. Paper, skin and walls reflect light too, scattered in all directions; that is the only reason you can see them. A mirror is special only because it reflects light in one neat direction, so it forms an image.
Shine lasers at mirrors, lenses and prisms on the Optics Bench in the Science Lab.