Physics › Turning points › The nature of light
The nature of light
For a century, what light actually is came down to Newton's word against a Dutchman's, and Newton won on reputation. Two slits, a spinning cog and a pair of laboratory constants then overturned him, and the answer arrived from somewhere nobody expected: electricity.
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.
- Use c = 1/√(μ₀ε₀), and outline Fizeau's and Hertz's measurements and what they implied.
WHAT YOU PROBABLY THINK
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 earned its keep: particles travel in straight lines, which explains sharp shadows, and they bounce like billiard balls, which explains reflection. Refraction it could manage too, provided the glass attracts the corpuscles as they cross the surface, pulling them toward the normal, which requires light to travel faster in glass than in air.
His contemporary Christiaan Huygens proposed the opposite: light is a wave, each point on a wavefront acting as a source of new wavelets. Waves reflect and refract just as well, but bend toward the normal by slowing down in the denser medium. Two theories, both fitting the everyday facts, disagreeing about one unmeasurable number: the speed of light in glass.
So why did Newton's version reign for a hundred years? Partly evidence: light seemed to cast perfectly sharp shadows, with none of the bending around edges that waves should show, and nobody could then detect light's diffraction. 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, and 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: more corpuscles, 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's last gap was embarrassing: 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, and 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
Two bench-top constants, one astonishing number
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 refinements of the method 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 had a family, and the wave theory had its medium-free waves.
YOUR TURN
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, before opening the working.
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 confirmed, and the electromagnetic spectrum thrown open beyond the visible.
TRY IT UNSEEN
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.
THE EXAM BIT
- Compare the theories as a table in prose: both explain reflection and refraction, but corpuscles need light faster in glass and waves need it slower. That speed disagreement is the examiner's favourite sentence.
- The reasons Newton's theory was preferred are two: no diffraction of light had been observed, and Newton's enormous scientific authority. Give both.
- Young's fringes score through cancellation: only waves can arrive out of step and produce darkness from two sources; particle streams can only add. "Delayed acceptance" then cites Newton's standing.
- Know what each constant means before using c = 1/√(μ0ε0): ε0 from the electric field of a charged object, μ0 from the flux density of a current-carrying wire.
- Fizeau's implications: a terrestrial, repeatable c, later comparisons showing light slower in water (waves win), and agreement with Maxwell. Hertz then supplies 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, was not enough to 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 contradicted.
Maxwell's c = 1/√(μ0ε0) matched the measured speed of light, and Hertz then 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 speed decides.
Maxwell computed c from two electrical constants, and light turned out to be his wave.
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.
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.
- Use c = 1/√(μ₀ε₀), and outline Fizeau's and Hertz's measurements and what they implied.
Open the full revision checklist to track your progress across the whole unit.
No animated video for this topic yet; these notes stand alone. InkPhysics on YouTube.