Electromagnetic Waves

The One Thing

Electromagnetic waves are self-sustaining transverse disturbances consisting of mutually perpendicular oscillating electric and magnetic fields. All electromagnetic waves have the same fundamental nature and travel through vacuum at $3.00\times 10^8\mathrm{m}{\mathrm{s}}^{-1}$, but differ in wavelength, frequency, energy, origin, and applications.

Reactions, Processes and Experiments

Key Terms

• Electromagnetic wave: A travelling disturbance consisting of time-varying electric and magnetic fields that can propagate through vacuum without requiring a material medium.
• Electric field: The region around a charge in which another charge experiences an electric force; in an electromagnetic wave it oscillates perpendicular to the direction of travel.
• Magnetic field: The region in which a moving charge or current experiences a magnetic force; in an electromagnetic wave it oscillates perpendicular to both the electric field and the direction of travel.
• Transverse wave: A wave in which the oscillations are perpendicular to the direction of propagation.
• Displacement current: The effective current associated with a changing electric field, introduced to complete the connection between electric and magnetic fields.
• Electromagnetic spectrum: The complete range of electromagnetic radiation arranged according to wavelength, frequency, or photon energy.
• Wavelength: The distance between two successive points in the same phase of a wave, represented by the symbol lambda.
• Frequency: The number of complete oscillations made per second, measured in hertz.
• Photon: A discrete packet or quantum of electromagnetic energy.
• Polarisation: The restriction of the vibrations of a transverse electromagnetic wave to a particular direction.
• Radiation pressure: The pressure exerted by electromagnetic radiation when it transfers momentum to a surface.
• Refractive index: A quantity approximately given by $n=c/v$ in a non-magnetic medium.

Easily Confused

• Frequency and wavelength: In vacuum, $c=\nu \lambda$; therefore, higher frequency means shorter wavelength, not longer wavelength.
• Speed and frequency when radiation enters a medium: The frequency remains unchanged, while the speed and wavelength may change.
• Electromagnetic waves and longitudinal mechanical waves: Electromagnetic waves are transverse and can be polarised; longitudinal mechanical waves such as sound in air cannot be polarised in this way.
• Electric field and magnetic field: Both oscillate in an electromagnetic wave, but the electric field, magnetic field, and direction of propagation are mutually perpendicular.
• Photon energy and electromagnetic intensity: Photon energy is $E=h\nu =hc/\lambda$, whereas average intensity is proportional to the square of the electric-field amplitude: $I_{\mathrm{avg}}=(1/2)c{ϵ}_0{E}_0^2$.
• Spectrum order: From increasing frequency and energy, and decreasing wavelength, the order is radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.

What Gets Asked

• Explaining how electromagnetic waves propagate: Questions may require the mutual generation of electric and magnetic fields. The mark-losing error is stating that electromagnetic waves require a material medium.
• Describing the orientation of fields: Students may be asked to identify the relationship between $E$, $B$, and the direction of propagation. The fields must be described as mutually perpendicular.
• Using wave equations: Questions may require $c=1/\sqrt{{\mu }_0{ϵ}_0}$, $c=\nu \lambda$, or $E_0/B_0=c$. A common error is confusing frequency with wavelength.
• Ordering the electromagnetic spectrum: The required order is radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. The order must be linked correctly to increasing frequency and energy or decreasing wavelength.
• Comparing photon energies: Questions may use $E=h\nu =hc/\lambda$. Higher frequency and shorter wavelength mean greater photon energy, particularly for ultraviolet, X-rays, and gamma rays.
• Identifying wave properties and applications: Reflection, refraction, interference, diffraction, and polarisation may be tested alongside applications of each spectral region. In refraction questions, frequency remains unchanged even though speed and wavelength may change.