Physics 5.13 - 5.14 - Electromagnetic waves in matter

How materials respond

Higher tier only

Both specification points in this lesson are Higher tier only.

How substances respond

When an electromagnetic wave meets a substance, the outcome depends on both the substance and the wavelength:

One incident wave can be partly reflected, partly transmitted and partly absorbed. Do not add refraction as a separate fourth energy share: the transmitted part can also be refracted.

Different substances can give different balances at the same wavelength. One substance can also respond differently at different wavelengths. For example, tests on one sheet might give:

The sheet mostly transmits A but mostly absorbs B. It is therefore incomplete to call the sheet simply "transparent" or "opaque" without naming the wavelength range.

Refraction can be wavelength-dependent too: the path of a transmitted ray can change by a different amount at a different wavelength. The claim must still name or compare the substance, because different substances need not produce the same response.

Velocity, wavelength and direction

Electromagnetic waves can have different velocities in different substances. At a stationary boundary, the source continues to set the wave's frequency, so frequency does not change. The wave equation then requires the wavelength to change:

$v = f \lambda$

• lower velocity with the same $f$ means a shorter $\lambda$;
• higher velocity with the same $f$ means a longer $\lambda$.

For oblique incidence, different parts of a wavefront reach the new substance at different times. The part that enters first changes velocity first, so the wavefront pivots: