# Electromagnetic Waves

## Key Questions

See below

#### Explanation:

Electromagnetic waves require no medium to travel through, while mechanical waves do.

Electromagnetic waves also have a fixed velocity of about $3 \times {10}^{8} \frac{m}{s}$ in a vacuum, while mechanical waves cannot possibly travel through a vacuum.

• High frequency electromagnetic radiation can ionise atoms/molecules in cells which then cause abnormal chemical reactions to take place in cells. DNA molecules may be ionised and break down also.

1. Somatic effects of radiation are due to damage of a person's tissues. Only the person who is irradiated suffers these effects.
2. Genetic effects of radiation are due to damage of DNA in the reproductive organs. These effects can be passed onto children via genes.

Electromagnetic waves can transfer energy to orbital electrons. If they transfer enough energy electrons can escape their atom leaving behind a positive ion (either an ionised atom or an ionised molecule).

The energy is transferred in the form of photons, so there is a 1:1 interaction taking place and the energy is equal to: $E = h f$. Photons need to have sufficient energy to remove electrons, therefore only high frequency EM radiation causes biological harm – UV, X-rays and γ-rays.

One such abnormal chemical change that can occur in a cell due to ionisation would be the production of hydrogen peroxide when water molecules are ionised. The peroxide may then attack organic molecules in the cell.

During cell division DNA molecules are more exposed and thus more susceptible to ionisation from EM radiation. It is for this reason that types of cells that reproduce more frequently are more radiosensitive, for example blood forming cells. You may know that after radiotherapy a patient must be kept isolated as their immune system has practically been wiped out, this is why.

• Electromagnetic waves are non elastic waves in the sense that they don't really need a material medium for their propagation like sound waves do.

EM waves, as I shall call them are basically perpendicularly oscillating electric and magnetic fields propagating in space.

Their velocity in a particular medium is determined thus by the electrical and magnetic properties of the medium.

In vacuum, they propagate with a velocity approximately given as $c = 3 \cdot {10}^{8} m {s}^{- 1}$

In presence of material medium their velocity is smaller than this and is given as $v = {\left(\epsilon \mu\right)}^{- \frac{1}{2}}$ where $\epsilon$ is the electrical permittivity and $\mu$ is the magnetic permeability.