Science › Classical physics
Light is an electromagnetic wave
1865· Discovery / experiment
ProvenTested many times; part of working physics.
Maxwell's discovery, completed in 1865, that light is a ripple of electric and magnetic fields travelling through space.
See it on the Science map →How it works
Maxwell computed how fast his electromagnetic waves should move, using only constants measured with magnets and electric charges on lab benches. The answer came out near 310,000 km per second, matching measured speeds of light. He concluded light itself is an electromagnetic wave. Visible colours are one narrow band; the same waves at longer lengths are radio and microwaves, at shorter lengths ultraviolet and X-rays.
Why it matters
It merged optics with electricity and magnetism and opened the whole spectrum to use, from radio to medical X-rays.
How we know
Hertz produced invisible electromagnetic waves in 1887–1888 that reflected, refracted and travelled at light speed. X-rays (1895) and radio broadcasting followed.
Still a mystery
In 1905 Einstein showed light also arrives in particle-like packets (photons). Quantum theory handles this dual nature, but how to picture it is still debated.
Connections (5)
▸Electromagnetism (Maxwell's equations) predicted Light is an electromagnetic waveProven
The relationship
Maxwell's equations let a changing electric field create a magnetic field, which creates an electric field, and so on, travelling outward as a wave. The equations fix that wave's speed from two lab-measured constants describing electric and magnetic strength. The speed matched light's, implying light is this wave.
How the link was found
Maxwell noticed the match in 1862 and stated the conclusion in his 1865 paper.
Why it's proven
Hertz made electromagnetic waves with electric sparks in 1887–1888 and showed they reflect, refract and travel at light speed, as predicted.
Open questions and mysteries
A wave seemed to need a medium, and the search for that 'ether' failed. Quantum theory later showed light also comes in particle-like packets.
▸Light is an electromagnetic wave first hinted at The speed of light is constantProven
The relationship
Maxwell's equations predicted electromagnetic waves with one speed, calculated from two electrical constants measured in labs. That speed matched light, so light was an electromagnetic wave. The equations gave a single fixed speed without saying relative to what, which raised the question of what the speed was measured against. Einstein answered: the same against everything.
How the link was found
Maxwell calculated the speed in 1862-1865. Hertz produced radio waves in 1887-1888, confirming electromagnetic waves travel at light's speed.
Why it's proven
Measured light speed equals the value from electrical constants. Radio, microwaves and X-rays all travel at the same speed.
Open questions and mysteries
Why the electrical constants have the values they do, and therefore why light has this speed, is unexplained.
▸Light is an electromagnetic wave sent physicists hunting for the ether in Michelson–Morley: no "ether wind"Proven
The relationship
Sound travels through air and ocean waves through water, so 19th-century physicists assumed light waves also travel through a medium, an invisible ether. If Earth moves through it, light's measured speed should vary with direction. Michelson and Morley built a sensitive instrument to detect that variation, a direct test of the wave-in-a-medium picture.
How the link was found
Albert Michelson first tried in Berlin and Potsdam in 1881, then refined the test with Edward Morley in Cleveland, Ohio, in 1887.
Why it's proven
Their null result has been repeated with ever greater precision. Einstein's 1905 relativity explained it: light needs no medium, and its speed is the same for every observer.
Open questions and mysteries
George FitzGerald and Hendrik Lorentz first explained the result with shrinking lengths, an idea relativity later reinterpreted. How much it directly influenced Einstein is debated.
▸Light is an electromagnetic wave is the wave half of Wave–particle dualityProven
The relationship
Maxwell's 1865 theory showed light is an electromagnetic wave, explaining interference, diffraction and, later, radio. That wave behavior did not go away when light's particles were discovered. Wave–particle duality keeps it as one side: light spreads and interferes as a wave but deposits its energy in single packets.
How the link was found
Thomas Young's interference (about 1801), Maxwell's equations (1865) and Heinrich Hertz's radio waves (1887) built the wave side; Einstein's 1905 quanta added the particle side.
Why it's proven
Interference and radio technology confirm wave behavior daily; photon counters confirm particle behavior. Single-photon interference experiments show both in one setup.
Open questions and mysteries
A single photon interferes with itself yet is detected whole, in one spot. Why it lands where it does is the measurement problem in another form.
▸James Clerk Maxwell calculated the speed of light, revealing Light is an electromagnetic waveProven
The relationship
Maxwell used electric and magnetic constants measured by Wilhelm Weber and Rudolf Kohlrausch in 1856 to compute how fast his waves travel. The result, about 310,000 km per second, was very close to light speeds measured by Fizeau and Foucault. He concluded light is an electromagnetic disturbance, uniting optics with electricity.
How the link was found
Maxwell, in 'On Physical Lines of Force' (1861–1862), and fully in his 1865 paper.
Why it's proven
Hertz confirmed in 1887–1888 that electromagnetic waves travel at light speed. Today light's speed is fixed by definition at exactly 299,792,458 metres per second.
Open questions and mysteries
Maxwell assumed an ether. Why the speed is the same for every observer puzzled physicists until Einstein in 1905.