Electromagnetic wave

In physics, an electromagnetic wave is a change, periodic in space and time, of an electric field E(r,t) and a magnetic field B(r,t). Examples of electromagnetic waves (in increasing wavelength) are: gamma rays, X-rays, ultraviolet light, visible light, infrared, microwaves, and radio waves. All these waves propagate with the same speed c, the speed of light.

Electromagnetic wave. Electric component (red) in plane of drawing; magnetic component (blue) in orthogonal plane; propagation to the right. The wavelength is λ.

In the figure we see a snapshot (i.e., a picture at a certain point in time) of the magnetic and electric fields in adjacent points of space. In each point, the vector E is perpendicular to the vector B. The wave propagates to the right, along an axis which we conveniently refer to as y-axis. Assume that the snapshot is taken at time t, then at a point y we see an arrow of certain length representing E(y,t). At a point in time Δt later, the same value of E (same arrow) is seen at y + c Δt. The arrow seems to have propagated to the right with a speed c.

Schematic overview of electromagnetic spectrum. Vertical axis: wavelengths in meter. Examples: infrared extends from 8 ·10⁻⁷ to 10⁻³ m; radio from 10⁻⁴ to about 10⁴ m.

If we focus on a fixed point y, then in progressing time we see the two vectors E(y,t) and B(y,t) grow to a maximum value, then shrink to zero, become negative, going to a minimum value, and grow again, passing through zero, and growing to maximum value. In other words, when we plot E and B in the fixed point y as a function of time t, we see the same sine-type function as in the figure.

External link

ISO 21348 Definitions of Solar Irradiance Spectral Categories

To be continued