Electromagnetic waves and their bands

A radio receives a signal through a wall, an eye sees a lamp, and an X-ray machine reveals an object's internal structure. Why do these different applications all involve waves of the same nature?

Sound passes motion through matter, but sunlight crosses nearly empty space. What propagates here is a coupled change in electric and magnetic fields: an electromagnetic wave. It carries energy without requiring a material medium. Accelerating charges can be sources, as in an oscillating current in an antenna. A field around a stationary charge does not, by itself, imply a wave travelling away from it.

What makes it transverse? In a plane electromagnetic wave in vacuum, the electric and magnetic fields are perpendicular to each other and both are perpendicular to propagation. The sine curves show field values at different positions or times. They are not paths along which particles move up and down. There is no rope in empty space, but the measurable fields change and can act on charges in a receiver.

In vacuum, radio waves, visible light and every other band share the speed c ≈ 3·10⁸ м/с. Thus c = λν: as frequency in hertz rises, wavelength falls. Doubling ν halves λ without changing the speed. In matter, speed and absorption depend on the medium and frequency. The condition 'in vacuum' is what lets us compare the entire spectrum using the same constant c.

Moving towards higher frequencies, we encounter radio waves, microwaves, infrared, visible, ultraviolet, X-rays and gamma rays. These are regions of one scale, not seven different substances. Boundaries are conventional; microwaves are often included in the radio band. X-rays and gamma rays can overlap in energy and are also distinguished by their origin. Nature therefore has no single rigid set of non-overlapping boundaries.

Antennas generate and receive radio waves and microwaves for communication; radar uses reflected signals. Infrared radiation transfers energy from warm objects, and measuring it helps estimate surface temperatures. The eye detects the visible band. Applications depend on how a wave is emitted, travels through a medium and is detected: a signal invisible to the eye may be readily detected by a suitable instrument.

At high frequencies, other interactions with matter become prominent. Ultraviolet can cause photochemical changes. Different tissues and materials attenuate X-rays differently, producing contrast that reveals internal details at the detector. Gamma photons arise, for example, when nuclei change energy states. These different effects do not alter the waves' common electromagnetic nature.

Make a radio transmitter more powerful while keeping its frequency. It may transmit more energy, but radio does not become X-rays. Frequency sets the rhythm of field oscillations; intensity is the energy passing through unit area per unit time. Effects on matter depend on both. Later we will see another distinction: frequency determines the energy of one photon, whereas increasing intensity need not change that frequency.

Why cannot visibility alone tell us the effect of radiation? Ionizing radiation can remove electrons from atoms and change molecules; the consequences depend on radiation type, received dose and conditions. Yet the absence of ionization does not rule out all damage: sufficiently intense non-ionizing radiation can strongly heat matter. What matters is how energy is transferred to tissue, not just the name of the band.

Exposure control depends on the mechanism. Skin and eyes need protection from ultraviolet; X-ray procedures require justified use and dose control. For external sources of ionizing radiation, less time, greater distance and suitable shielding can reduce dose. Suitable shielding depends on radiation type and energy, so no material works universally in all conditions. These are principles of physical control, not a way to estimate dose from time alone.

A transmitter operates at 100 MHz. Estimate the wavelength in air, taking the speed as 3·10⁸ m/s.

First convert frequency to hertz: 100 МГц = 100·10⁶ Гц = 10⁸ Гц. Under the stated approximation, the speed in air is c = 3·10⁸ м/с.

From c = λν, we get λ = c/ν = 3·10⁸/10⁸ = 3 м.

In one period T = 1/ν = 10⁻⁸ с, the wave travels 3 m. This metre-scale wavelength agrees with a frequency far below that of visible light.

The wavelength is approximately 3 m: the wave pattern advances that distance in one transmitter period.

All electromagnetic bands describe propagating fields; a material medium is not essential.

All share one vacuum speed, while frequency and wavelength are inversely proportional: c = λν.

Frequency and intensity affect different aspects of interaction; applications and protection are chosen based on interactions with matter.

Spectra and radiation will help us understand the atom's internal structure.