Wavelength changes interactions
Definition
Wavelength
The distance between consecutive points on a wave that are at the same stage of an oscillation.
- The same substance can interact differently with waves of different wavelengths.
- For each wavelength, a material may absorb, transmit, reflect or refract a different proportion of the incident wave energy.
- A statement that a material is transparent or reflective is incomplete unless the relevant part of the electromagnetic spectrum is clear.
Selective absorption and transmission
- A material transmits a wavelength well when only a small fraction of that wave energy is absorbed or reflected.
- Visible glass transmits much visible light, but different glass compositions can absorb more ultraviolet or infrared radiation.
- A coloured filter transmits a limited range of visible wavelengths and absorbs many of the others.
- An object appears coloured because the wavelengths reaching the eye depend on which wavelengths the object reflects or transmits.
Reflection depends on wavelength
- A surface can be smooth compared with one wavelength but rough compared with a shorter wavelength.
- This difference changes whether reflection is concentrated in one direction or scattered over many directions.
- Metallic surfaces reflect many radio wavelengths effectively, which allows metal dishes to direct radio and microwave signals.
Refraction depends on wavelength
- The speed of electromagnetic waves in a material can depend on wavelength.
- Different wavelengths therefore change speed by different amounts at the same boundary and are refracted through different angles.
- In a prism, visible wavelengths spread into a spectrum because each wavelength is refracted by a different amount.
- This separation of wavelengths is called dispersion.
Comparing materials
- A valid comparison must keep wave intensity, angle of incidence, material thickness and detector arrangement controlled.
- The detector must be suitable for the wavelength being tested because the human eye detects only visible light.
- The measured incident, reflected and transmitted signals can be compared, while absorption is inferred from the missing energy.
Example
Interpreting wavelength data
- A sheet transmits 82%82\%82% of incident visible radiation but only 12%12\%12% of incident infrared radiation.
- The sheet is much more transparent to visible wavelengths than to infrared wavelengths.
- The remaining energy at each wavelength is reflected or absorbed, so transmission data alone cannot distinguish those two outcomes.
Exam technique
- When comparing two wavelengths, name both the interaction and the direction of change, such as more infrared is absorbed but more visible light is transmitted.
- Use evidence from the values or graph rather than describing a material as simply transparent or opaque.
Common Mistake
- Do not assume that a material behaves in the same way for every wavelength.
- Do not treat colour as a property independent of illumination; the wavelengths present in the incident light affect what is observed.
Self review
- Why can one material transmit visible light but absorb infrared radiation?
- How can wavelength affect reflection from a surface?
- Why does a prism separate visible light into colours?
- Which variables should be controlled when comparing transmission by wavelength?
- Why can transmission data alone not determine absorption?
