Transmission and absorption

Waves arriving at the interface (boundary) between two materials can be reflected, transmitted or absorbed.

The type of wave, its properties such as wavelength, and the difference between the two media will determine what happens to most of the wave energy.

Common examples of absorption of wave energy:

  • waves hitting the beach usually give most of their energy to the sand
  • sunlight landing on a face is mostly absorbed, warming the skin
  • sound waves hitting thick curtains give up their energy and the sound is muffled

Common examples of transmission of wave energy:

  • sea waves passing a shallow area continue with their energy mostly unchanged
  • light passing through a glass window continues with over 95% of its energy
  • ultrasound waves scanning a baby pass from flesh into bone and continue with enough energy for the machine to detect the echo

Effect of wavelength on absorption

Wavelength has a big effect on how much of a wave's energy is reflected, absorbed or transmitted at an interface.

For example, a greenhouse uses these differences to raise the temperature inside.

Ultraviolet light has a very short wavelength, and this is mostly transmitted by the glass. The plants, soil and floor in the greenhouse mostly absorb ultraviolet, and their temperature rises.

The plants, soil and floor emit infrared light which has a longer wavelength that is mostly reflected by glass. The infrared cannot escape the greenhouse and reflects until it is re-absorbed by objects inside the greenhouse. The amount of energy in the greenhouse rises so its temperature increases.

Differences in wavelength have a big effect on how much of a wave’s energy is reflected, absorbed or transmitted at an interface. Diagram shows the path 3 types of radiation take through a greenhouse.
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