In physics, how do we know that electromagnetic waves travel as a sine wave if we cannot see the shape of them?
The understanding that electromagnetic (EM) waves travel as sine waves is rooted in theoretical principles, experimental evidence, and mathematical formulations. Here’s a breakdown of how we know this:
- Maxwell’s Equations
James Clerk Maxwell formulated a set of equations in the mid-19th century that describe how electric and magnetic fields interact and propagate. These equations predict that a changing electric field generates a magnetic field and vice versa. When solved under certain conditions, these equations reveal that EM waves propagate through space as sinusoidal waves.
- Wave Equation
From Maxwell’s equations, one can derive the wave equation for electromagnetic waves. The solutions to this wave equation are sinusoidal functions (sine and cosine), which describe how the electric and magnetic fields vary in space and time.
- Superposition Principle
The principle of superposition states that when two or more waves overlap, the resultant wave is the sum of the individual waves. Sinusoidal waves are particularly useful in this context because they can be easily added together. This property leads to phenomena like interference and diffraction, which can be observed experimentally.
- Experiments
Numerous experiments have confirmed the sinusoidal nature of EM waves:
- Interference Patterns: Experiments like the double-slit experiment demonstrate that light behaves like a wave, producing interference patterns consistent with sinusoidal waves.
- Polarization: The behavior of polarized light can also be explained by the sinusoidal nature of EM waves.
- Spectroscopy: The interaction of EM waves with matter provides evidence for their wave-like properties, as seen in the emission and absorption spectra of atoms.
- Fourier Analysis
Any periodic wave can be decomposed into a series of sine and cosine waves through Fourier analysis. This mathematical tool shows that even complex waveforms can be understood in terms of their sinusoidal components, reinforcing the idea that many physical phenomena, including EM waves, can be represented as sine waves.
Conclusion
While we cannot ‘see’ electromagnetic waves directly, the combination of theoretical predictions from Maxwell’s equations, experimental validations, and mathematical analyses provides strong evidence that they propagate as sinusoidal waves. This understanding is foundational in both classical and modern physics, influencing technologies like radio, microwaves, and optics."
"Electromagnetic waves in free space must be solutions of Maxwell’s electromagnetic wave equation. Two main classes of solutions are known, namely plane waves and spherical waves. The plane waves may be viewed as the limiting case of spherical waves at a very large (ideally infinite) distance from the source. Both types of waves can have a waveform which is an arbitrary time function (so long as it is sufficiently differentiable to conform to the wave equation). As with any time function, this can be decomposed by means of Fourier analysis into its frequency spectrum, or individual sinusoidal components, each of which contains a single frequency, amplitude, and phase. Such a component wave is said to be monochromatic.
Interference is the superposition of two or more waves resulting in a new wave pattern. If the fields have components in the same direction, they constructively interfere, while opposite directions cause destructive interference. ~ Wikipedia"
"Are Electromagnetic Waves Sinusoidal?
Most recent answer: 05/23/2018
Q:
‘Are all electromagnetic waves sinusoidal in nature? I know that we can generate any waveshape (say, a square wave) using Fourier series from sinusoidal harmonics. Whenever I see a representation of an EM wave, whether it be the E or B vector, it’s always in form of a sinusoid. Is this the fundamental nature of EM waves (derived from mathematical models) or is there another answer? Hopefully my question was clear enough.’ ~ Vaibhav Singh
A:
'As you say, EM waves can follow all sorts of patterns. What’s special about sine waves?
In a vacuum, plane waves of any shape propagate keeping the same shape. In materials (e.g. glass or water) the shape of the wave in general changes as it propagates. Each sine-wave component, however, keeps its sinusoidal shape. (I’m assuming that the intensity is low enough that the propagation is linear, typically an excellent approximation.) The shape of a non-sinusoidal wave changes because the sine-wave components propagate at slightly different speeds. So the sine-wave shape is special because it’s the shape that doesn’t change while propagating through materials.
Closely related to that, it’s the single-frequency sinusoidal waves that transfer well-defined quanta of energy to materials. The size of the energy quatum is hf, where f is the frequency and h is the universal Planck’s constant. Waves wiith other shapes are made of mixtures of different frequencies and thus have a range of different possible sizes of quantum energy transfers.’ " ~ Mike W., University of Illinois Urbana-Champaign, The Grainger College of Engineering
