Recall that different substances may absorb, transmit, refract or reflect…

The idea

When electromagnetic waves meet a substance, one or more of four things happens: the substance absorbs the wave, transmits it through, refracts it as it changes speed at a boundary, or reflects it from a surface. Which of these dominates depends on the substance and on the wavelength. A red filter transmits red light but absorbs most other visible wavelengths, which is why it looks red. Glass transmits visible light well yet absorbs much ultraviolet. A shiny metal reflects a broad range of wavelengths, while a radio dish reflects long radio waves. Because behaviour varies with wavelength, one material can be transparent to one region of the spectrum and opaque to another. In a multiple-choice question, match each option to the correct process and check that the wavelength dependence is respected.

What earns the marks

• Identifies absorption, transmission, refraction and reflection as the four possible outcomes when electromagnetic waves meet a substance.
• States that the way a substance absorbs, transmits, refracts or reflects electromagnetic waves depends on the wavelength.
• Selects an example in which a material behaves differently for two different wavelengths, such as a filter transmitting one colour and absorbing another.
• Rejects an option that claims a given substance behaves identically for all wavelengths of electromagnetic radiation.

Common mistakes and how to put them right

• Treats transmission and refraction as the same process.

  Put it right: Distinguish passing through unchanged from changing direction because the speed changes.

• Assumes a material that transmits visible light also transmits all other wavelengths.

  Put it right: Check the wavelength dependence.

• Confuses absorption with reflection.

  Put it right: Absorbed energy is taken in by the substance, whereas reflected energy bounces off the surface.

Exam tips

• Read each option and decide which of the four processes (absorb, transmit, refract, reflect) it describes before choosing.
• Look for the word wavelength in the question and eliminate any option that ignores how behaviour changes with wavelength.
• Use a familiar example such as a coloured filter or a metal surface to test whether an option is physically sensible.