Physics › Turning points › The nature of light
The nature of light
For a century the question of whether light is a stream of particles or a wave came down to Newton against Huygens, and Newton's reputation decided it. Young's double slit, Fizeau's measurement of the speed of light and Maxwell's calculation from two electrical constants settled it the other way.
Pick your board and the few notes written for the other boards quietly fold away, here and in the practice players. Nothing is deleted: every folded piece reopens on a tap.
Builds on Interference and Young's double slit and Cathode rays and the electron.
IN THIS TOPIC
- Compare Newton's corpuscular theory with Huygens' wave theory, and say why Newton's was preferred.
- Explain the significance of Young's fringes, and why acceptance of the wave theory was delayed.
- Say what ε₀ and μ₀ each measure, and use c = 1/√(μ₀ε₀).
- Outline Fizeau's and Hertz's measurements, and what each one settled.
COMMON MISCONCEPTION
A theory backed by a great enough scientist counts as proved.
Corpuscles against waves
Newton argued that light is a stream of tiny particles, corpuscles. The theory had genuine successes. Particles travel in straight lines, so shadows come out sharp, and they bounce like billiard balls, so reflection is easy. Refraction it could manage too, provided the glass attracts the corpuscles as they cross the surface, pulling them towards the normal. That requires light to travel faster in glass than in air.
His contemporary Christiaan Huygens proposed the opposite. Light is a wave, with each point on a wavefront acting as a source of new wavelets. Waves reflect and refract just as well, but they bend towards the normal by slowing down in the denser medium. So there stood two theories, both fitting the everyday facts, disagreeing about one number nobody could yet measure, the speed of light in glass.
So why did Newton's version reign for a hundred years? Partly the evidence. Light seemed to cast perfectly sharp shadows; the faint edge-bending Grimaldi had reported in the 1660s was so weak at everyday apertures that it was easy to set aside, and nobody had tied it to an accepted wave account. Partly the man. Newton's authority in science was unmatched, and disagreeing with him was a poor career move. Reputation was standing in for proof, and it held the wave theory down for a century.
Young's fringes
In 1801 Thomas Young lit two narrow slits with one source and looked at what fell on a screen beyond. Not two bright stripes, but a whole ladder of them, alternating bright and dark fringes. The explanation needs no equations, only overlap. Light spreading from the two slits meets on the screen. Where crest arrives with crest the light reinforces and the screen glows. Where crest arrives with trough, the two lights cancel and the screen is dark.
That cancellation is the fatal fact. Two streams of particles can only ever add, since more corpuscles mean more light. Only waves can arrive out of step and produce darkness from two lights. Yet acceptance still limped. Newton's reputation stood guard, the corpuscular school explained fringes away, and the wave theory only conquered as further interference and diffraction results piled up through the following decades, long after Young's demonstration and long after Huygens had died.
Maxwell, Hertz, and light unmasked
The wave theory had one gap left. Waves of what? The answer came from a different subject entirely. By the 1860s electricity and magnetism each carried a constant of proportionality. ε0, the permittivity of free space, sets the electric field strength around a charged object. μ0, the permeability of free space, sets the magnetic flux density around a current-carrying wire. James Clerk Maxwell showed the two fields could sustain each other as a travelling wave, an electromagnetic wave of oscillating electric and magnetic fields at right angles, needing no medium at all, moving at
WORKED EXAMPLE
Maxwell's speed from bench-top constants
Evaluate Maxwell's speed using μ0 = 4π × 10−7 H m−1 and ε0 = 8.85 × 10−12 F m−1.
μ0ε0 = 4π × 10−7 × 8.85 × 10−12 = 1.11 × 10−17, so c = 1/√(1.11 × 10−17) = 3.00 × 108 m s−1.
Both constants come from laboratory measurements on charges and currents, nothing to do with optics. That their combination equals the measured speed of light was Maxwell's thunderbolt. Light is an electromagnetic wave.
The comparison was only possible because the speed of light had finally been measured on Earth. In 1849 Armand Fizeau fired light through a gap in a spinning toothed wheel, off a mirror 8.63 km away and back. Spin the wheel fast enough and the returning light meets the next tooth instead of the gap, and the timing gives the speed.
Fizeau's result, close to 3.1 × 108 m s−1, mattered twice over. It made c a terrestrial, checkable quantity. And Foucault's rotating-mirror experiment, descended from the same race-the-light idea, soon showed light travelling slower in water than in air, exactly as Huygens required and Newton forbade. Then in 1887 Heinrich Hertz closed the case from the other side. Sparks in his laboratory generated invisible waves that reflected, refracted and formed stationary waves, and their measured speed came out at Maxwell's c. Radio waves existed. Light now had a family, and the wave theory had waves that needed no medium.
GUIDED PRACTICE
Hertz's stationary waves
Hertz set up stationary radio waves with adjacent nodes 2.5 m apart, from an oscillator of frequency 6.0 × 107 Hz. Find the speed of his waves and state the significance.
Show the working
Adjacent nodes sit half a wavelength apart, so λ = 5.0 m, and c = fλ = 6.0 × 107 × 5.0 = 3.0 × 108 m s−1.
Invisible waves made from electricity, travelling at exactly the speed of light. Maxwell's prediction stood confirmed, and the electromagnetic spectrum was thrown open beyond the visible.
INDEPENDENT PRACTICE
Fizeau's arithmetic
Fizeau's wheel had 720 teeth and the mirror stood 8.63 km away. The returning light was first blocked at 12.6 revolutions per second. Estimate the speed of light.
Show the working
First blocking means the wheel advanced half a tooth during the round trip, a rotation of 1/1440 of a turn, taking t = 1/(1440 × 12.6) = 5.51 × 10−5 s.
c = 2d/t = 2 × 8630 / (5.51 × 10−5) = 3.1 × 108 m s−1. Within a few per cent of the modern value, from a cogwheel and a distant hill.
ASSESSMENT FOCUS
- Compare the theories as a table in prose. Both explain reflection and refraction. Corpuscles need light faster in glass, waves need it slower, and that speed disagreement is the sentence most often asked for.
- Two reasons are wanted for Newton's theory being preferred, and one alone loses a mark. No convincing diffraction of light had been demonstrated, its known edge effects being too weak to force a wave reading, and Newton's scientific authority was enormous.
- Young's fringes score through cancellation. Only waves can arrive out of step and produce darkness from two sources, whereas particle streams can only add.
- "Delayed acceptance" is a separate mark from the physics, and it cites Newton's standing plus the decades of further interference work needed before the wave picture won.
- Know what each constant means before you use c = 1/√(μ0ε0). ε0 comes from the electric field of a charged object, μ0 from the flux density around a current-carrying wire. Both are measured in electrical experiments, and that is the force of the argument.
- Fizeau's implications come in two parts. He made c a terrestrial, repeatable measurement, and the related rotating-mirror work showed light slower in water, so the waves won. Hertz then supplied radio waves at the same speed.
CHECK YOURSELF
Explain why Young's 1801 experiment did not immediately overturn the corpuscular theory, and name the two later results that settled the wave theory's victory.
Show a hint
One reason is about people. The two results are a speed comparison and a prediction confirmed.
Show the answer
Newton's authority kept the corpuscular theory respectable, and one experiment, however clean, could not displace a century of consensus. Acceptance of the wave picture was delayed for decades.
Measurements descended from Fizeau's method showed light travelling slower in water than in air, as the wave theory required and the corpuscular theory forbade.
Maxwell's c = 1/√(μ0ε0) then matched the measured speed of light, and Hertz produced electromagnetic waves directly at that same speed. Light was a wave, and an electromagnetic one.
Corpuscles need light faster in glass. Waves need it slower. The measured speed decides between them.
Young's dark fringes are two lights cancelling, which particles cannot do.
Maxwell computed c from two electrical constants, and light turned out to be his wave.
Hertz made those waves in a laboratory and measured their speed at c.
WORKBOOK
Printable practice for this topic: original exam-style questions with room to work, and a fully worked answer book. Free to use; please do not redistribute or sell.
Or read them with their mark schemes on the nature of light questions page.
CHECK YOUR PROGRESS
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- Compare Newton's corpuscular theory with Huygens' wave theory, and say why Newton's was preferred.
- Explain the significance of Young's fringes, and why acceptance of the wave theory was delayed.
- Say what ε₀ and μ₀ each measure, and use c = 1/√(μ₀ε₀).
- Outline Fizeau's and Hertz's measurements, and what each one settled.
Open the full revision checklist to track your progress across the whole unit.