How Electromagnetic Radiation Interacts with Matter

Whenever electromagnetic radiation encounters matter one of three things can happen.

1. The electromagnetic radiation may undergo surface reflection.

All electromagnetic reflections are governed by the same physical laws as reflections of visible light.

Optics describes the general laws of reflection and may be applied to all types of electromagnetic reflections ranging from radio waves to gamma rays.

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2. The electromagnetic radiation may be transmitted completely through the substance it encounters. If absolutely no energy is absorbed by the material, it is said to be transparent to the radiation.

The velocity of the radiation is usually slower in the transparent medium and as a result the radiation usually undergoes refraction

Various materials are transparent at various wavelengths. For example, lead glass is transparent to visible light but not X-rays, whereas several thicknesses of black paper sheets are transparent to X-rays, but not visible light.

No known material is perfectly transparent.

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3. The electromagnetic radiation may be totally or partially absorbed by the substance. In this process energy is transferred to the absorbing medium and this may cause significant changes to occur within the absorbing medium.

Because of the quantum nature of matter on atomic and molecular scales it has been discovered that energy can only be absorbed at the atomic or molecular level if the energy of the incident radiation exceeds a specific threshold value.

At energy levels below the threshold level no physical interaction is possible at the atomic or molecular level.

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Low Energy (Long Wavelength Non-thermal) Electromagnetic Radiation

Radio Station
At low electromagnetic frequencies (long wavelength such as radio and TV), the energy transferred by the electromagnetic radiation is so low that it is only able to oscillate the free electrons that lie within the surface of metals.

This type of radiation is called non-thermal radiation because it is generally produced and radiated by man-made electrical ocscillations in metallic conductors (wires) and not from high temperature objects.

To detect such weak radiation metal conductors are suspended in the air (aerials) and the slight oscillation of the electrons within the aerial is amplified to produce a measurable signal.

The signal is then decoded and further amplified to produce sounds and images or to transfer digital information.

Extraordinarily small (immeasurable) amounts of heat are transferred to the conductor in the process.

Microwave Oven
At higher frequencies (microwaves), the electromagnetic radiation can induce significant molecular oscillations into a substance. These molecular motions manifest themselves as a significant temperature increase in the substance being irradiated.

Microwaves ovens work on this principle.

Although this type of radiation is used to produce heat, it is still considered non-thermal radiation because its source is the non-thermal oscillation of electrons within a small microwave antenna. The antenna is inside the oven and it "illuminates" the inside the oven (and its contents) with microwaves.

High Energy Electromagnetic Radiation
(Short Wavelength, from Both Thermal and Non-thermal Sources)

Now..recall that matter is not really a continuous medium (as shown in the sketches above), but rather it is made of atoms and molecules which are mostly empty space filled by a small dense nucleus of neutrons and protons, surrounded by a "shell" of electrons in distinct and well defined energy levels.


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Electromagnetic radiation with high energy (violet light) but below the ionization threshold of an atom may be absorbed by an orbital electron causing it to "jump" to a higher energy level. This puts the atom in an excited state.

Only photons with exactly the right amount of energy (neither too little nor too much) can do this.

This process results in the formation of absorption lines in the solar spectrum.


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Photons whose energies exceed the ionization potential of a neutral atom will be absorbed and eject an electron from the atom, leaving the atom in an electrically charged state.

The resulting ion may become highly chemically reactive.

Ionization can result from the absorption of either electromagnetic radiation or particle radiation. Because of their highly reactive nature, ionized atoms and ionized molecules are of major importance in our understanding of the biological hazards caused by both types of radiation.

Compton Scattering

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Compton Scattering is generally observed when X-rays illuminate neutral atoms. The effect is an ejected valence electron and lower energy photon.

Since energy is always conserved, the kinetic energy of the ejected electron, its binding (orbital) energy, and the new photon energy all add up to the quantum energy of the original incident photon.

Pair Production

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At very high photon energies, their energy/mass equivalence (E=mc2) is sufficient to produce particles with mass.

Charge, like energy, is also a conserved quantity, and since photons have zero charge, the resulting particles must have zero net charge.

The result of pair production is an electron, negative charge, and a positron (anti-electron) with an equal (but opposite) positive charge.

Photon Energy

All electromagnetic radiation "carries" energy. When electromagnetic energy is absorbed by any substance the absorbed radiation delivers its energy in discrete indivisible bundles called quanta (Also called photons , even when they refer to non-optical radiation).

The amount of energy E contained in each quantum bundle is uniquely characterized by the frequency f of the electromagnetic radiation.

The amount of energy contained in a single quantum of electromagnetic radiation is given by the famous equation

E= hf
h is a called Planck's constant and equals 6.626 x 10-34 J · s

When one is working in the world of physics on an atomic scale, energies are often expressed in eV (electron volts) rather than in joules.
For purposes of comparison, 1 eV = 1.6022 x 10 -19 J

Intensity vs Energy

The intensity of electromagnetic radiation is also called the brightness, the name derived from the optical case. The electromagnetic intensity is related to the number of photons (or quanta) passing through a unit area in space per unit time; or equivalently, the number of photons incident on a unit area (surface) per unit time. For example a beam of radiation illuminating one square metre with 50 photons per second is five times more intense (brighter) than a beam illuminating a one square metre with 10 photons per second.

The energy emitted per photon by a source of electromagnetic radiation depends only upon the wavelength (or alternatively the frequency) of the radiation, not on the brightness of the source.

The total amount of energy absorbed (or emitted) per unit area depends on the product of the intensity (number of photons emitted or absorbed per second) and the energy per photon.

An important consequence of this is that on the atomic and molecular scale, absorption of photons below the threshold energy is impossible, regardless of the intensity of the radiation!

Student Assignment

Requires Data Sheets I & II



Prepared by the YES I Can! Science Team,