A material's response depends on wavelength
Definition
Absorption
The transfer of energy from electromagnetic radiation to a material when the radiation is taken in rather than reflected or transmitted.
- A substance can absorb, transmit, reflect or refract electromagnetic waves, and its response can change with wavelength.
- Absorbed radiation transfers energy to the material and may increase its thermal energy store or cause changes in its particles.
- Transmitted radiation passes through the material, although its speed and wavelength may differ from their values in a vacuum.
- Reflected radiation changes direction at the surface and returns into the original substance.
- Refracted radiation changes direction because its speed changes at a boundary.
The same material treats wavelengths differently
Definition
Transmission
The passage of electromagnetic radiation through a substance or across a boundary.
- Ordinary window glass transmits much of the visible range, which is why objects can be seen through it.
- The same glass absorbs much of the ultraviolet reaching it, so transmission cannot be predicted from transparency to visible light alone.
- A red filter transmits a band of red wavelengths and absorbs much of the other visible wavelengths.
- A shiny metal surface reflects a large proportion of visible and infrared radiation but may respond differently at other wavelengths.
- Water can transmit visible light over short distances while absorbing parts of the infrared range strongly.
Example
Selecting radiation for a material
- A detector behind a material records a strong visible-light signal but a weak ultraviolet signal from equally intense sources.
- The evidence supports the conclusion that the material transmits visible wavelengths more effectively and absorbs or reflects more ultraviolet.
Evidence must identify the interaction
- A lower detector reading after a wave meets a material shows that less radiation was transmitted.
- The missing transmitted energy may have been absorbed or reflected, so transmission data alone cannot separate those two processes.
- A temperature rise in the material provides evidence that some radiation was absorbed.
- A detector placed on the incident side can provide evidence for reflection.
Common Mistake
Do not claim that a material is completely transparent or opaque without naming the wavelength or electromagnetic group.
Exam technique
- Use comparative language such as more strongly absorbed, a greater proportion transmitted or less reflected.
- Do not use the vague statement that different waves behave differently without naming the interaction and wavelength range.
Self review
- What four interactions can occur when electromagnetic radiation meets a material?
- What happens to energy when radiation is absorbed?
- Why can a material transmit visible light but absorb ultraviolet?
- What extra evidence distinguishes absorption from reflection?
- How should a wavelength-dependent comparison be written?
