E. The Wave Nature of Light · The Wave Nature of Light

Light: Wave or Particle? Testing Two Models

E2.1 E3.2 A1.1 A1.11About 65 minutes

Learning goals

I can describe the particle (corpuscular) model and the wave model of light and the observations each explains.
I can explain why the two models predict different speeds for light in water and how this was tested.
I can explain why diffraction and interference of light are hard to observe, and why they decided the debate in the 1800s.
I can evaluate a scientific model by its explanations and its testable predictions.

Words to know

particle (corpuscular) model
The model, championed by Isaac Newton, in which light is a stream of tiny particles called corpuscles that travel in straight lines.

wave model
The model, proposed by Christiaan Huygens, in which light is a wave that spreads out from its source.

scientific model
A simplified description, often with equations, that explains observations and makes predictions that can be tested.

Poisson's bright spot
A small bright spot at the centre of the shadow of a round disc, predicted by Fresnel's wave theory and observed by Arago (also called the Arago spot).

electromagnetic radiation
Energy carried by changing electric and magnetic fields travelling as a wave at the speed of light; visible light is one kind.

photon
A particle (quantum) of light. The idea returns in the unit Quantum Mechanics.

Two great scientists, two pictures

In the late 1600s two of the greatest scientists of their time looked at the same light and saw different things. Isaac Newton pictured light as a stream of tiny, fast particles. Christiaan Huygens pictured it as a wave, spreading out like ripples. For more than a century, scientists argued over which picture was right.

This lesson follows that argument. It is a model of how science works: two models, each explaining some facts, are tested against new experiments until one wins (and then, as you will see in the unit Quantum Mechanics, the story takes one more twist).

What each model explained

Newton's particle model (described in his book Opticks, 1704):

Straight-line travel and sharp shadows: particles move in straight lines unless pushed.
Reflection: particles bounce off a mirror like a ball off a wall, angle in equals angle out.
Refraction: Newton said a surface such as water attracts the particles as they arrive, speeding up the part of their motion perpendicular to the surface. This bends them toward the normal, but only if light moves faster in water than in air.
Colour: different colours were particles of different kinds, and a prism sorted them.

Huygens' wave model (Treatise on Light, 1690):

Straight-line travel, reflection and refraction, all explained with wavelets (last lesson).
Two beams of light can cross without bouncing off each other, as waves can.
Refraction toward the normal happens because light moves slower in water.

Both models explained reflection and refraction. Newton's reputation was enormous, and two facts seemed to count against waves: light did not appear to bend around corners the way sound does, and nobody knew what medium light waves could travel through across empty space. So the particle model was the favourite for most of the 1700s.

Both models can explain bending toward the normal, but they disagree about speed. Newton's particles must speed up in water; Huygens' waves must slow down. Measuring the speed of light in water can decide between them.

Worked example

Example 1: Two predictions for the same ray

Light goes from air into water, with sin θ₁/sin θ₂ = 1.33. What speed of light in water does each model predict? (Speed in air: 3.00 × 10⁸ m/s.)

Wave model: sin θ₁/sin θ₂ = v_air/v_water, so v_water = 3.00 × 10⁸ ÷ 1.33 = 2.26 × 10⁸ m/s (slower).
Particle model: the parallel part of the velocity is unchanged and the perpendicular part grows. That gives sin θ₁/sin θ₂ = v_water/v_air, so v_water = 1.33 × 3.00 × 10⁸ = 3.99 × 10⁸ m/s (faster).
The two predictions differ by a factor of 1.33² ≈ 1.8, a difference a careful experiment can detect.

Wave: 2.26 × 10⁸ m/s. Particle: about 4.0 × 10⁸ m/s. In 1850 Léon Foucault measured light travelling more slowly in water than in air, as the wave model predicts.

Why was light's wave nature so hard to see?

Last lesson you learned that diffraction is noticeable only when an opening is roughly the size of the wavelength. Visible light has wavelengths of only 400 to 700 nm (less than a thousandth of a millimetre). Through a doorway or a window, light's diffraction is far too small to see, so shadows look sharp, just as Newton observed. Light also comes from ordinary sources as a jumble of short, random wave trains, which makes steady interference patterns hard to produce.

In the early 1800s, three key experiments changed everything:

Interference (Thomas Young, about 1801 to 1803): light through two narrow, closely spaced slits made a pattern of bright and dark bands, exactly like the two-source water pattern. Particles could not explain dark bands where two beams overlap. You will repeat his analysis next lesson.
Diffraction and the bright spot (1818): Augustin Fresnel entered a wave theory of diffraction in a competition of the French Academy of Sciences. Siméon Poisson, a judge who favoured particles, showed that Fresnel's theory predicted something that seemed absurd: a bright spot at the centre of the shadow of a small round disc. François Arago, another judge, did the experiment, and the spot was there.
Speed in water (1850): Foucault's measurement showed light is slower in water.

Poisson's bright spot (not to scale). Waves diffracting around the edge of a small round disc all travel the same distance to the centre of the shadow and arrive in phase, so they interfere constructively and make a small bright spot where particles would leave only darkness.

More evidence, and a twist

Polarization (later in this unit) showed that light is a transverse wave.
In the 1860s James Clerk Maxwell showed mathematically that changing electric and magnetic fields travel as waves at almost exactly the measured speed of light, and concluded that light is an electromagnetic wave. In the late 1880s Heinrich Hertz made and detected radio waves, confirming the theory.

By 1900 the wave model seemed complete. Then, in 1905, Albert Einstein explained the photoelectric effect by treating light as packets of energy, now called photons. Light can show both wave and particle behaviour, depending on the experiment. You will study this wave–particle duality in the unit Quantum Mechanics. For everything in this unit (interference, diffraction, thin films, polarization), the wave model works perfectly.

A timeline of the debate. Green boxes mark evidence or theories that supported the wave model; yellow boxes mark the particle model (Newton) and the return of particle ideas (Einstein's photons). Years are approximate for Young.

Observation	Particle model	Wave model
Straight-line travel	explains	explains (λ is tiny)
Reflection	explains	explains
Refraction toward normal	only if light is faster in water	only if light is slower in water
Measured speed in water (slower)	fails	explains
Interference (Young's bright and dark bands)	fails	explains
Diffraction, Poisson's bright spot	fails	explains
Polarization	no simple explanation	explains (transverse wave)
Photoelectric effect (1905)	explains (photons)	fails

How the two models compare. The last row is studied in the unit Quantum Mechanics.

Worked example

Example 2: Sharp shadows

Newton argued that if light were a wave, it would bend around obstacles and shadows would not be sharp. Use λ/w to explain how a wave model can still give sharp shadows of everyday objects, such as a 2 cm wide pencil.

Visible light has λ ≈ 5 × 10⁻⁷ m.
For a 2 cm (0.02 m) pencil, λ/w = 5 × 10⁻⁷ ÷ 0.02 = 2.5 × 10⁻⁵.
Diffraction is only noticeable when λ/w is close to 1, so the bending is tiny: only the very edge of the shadow is slightly blurred.

Sharp shadows do not rule out waves; they show that light's wavelength is very small compared with everyday objects.

Worked example

Example 3: Judging a model

A good scientific model must (1) explain existing observations and (2) make predictions that could prove it wrong. Use these criteria to say why Poisson's bright spot was so important.

Fresnel's wave theory explained the diffraction fringes already seen at shadow edges (criterion 1).
It also made a surprising prediction that nobody had looked for: a bright spot in the middle of a disc's shadow (criterion 2). If the spot had been missing, the theory would have been in serious trouble.
Arago looked, and the spot was there. A risky prediction that comes true is strong evidence for a model.

The bright spot was a risky, testable prediction that came true, which convinced many scientists that light is a wave.

Just for fun

Newton and Huygens walk into a bar. Newton bounces off. Huygens diffracts around the door and is already at a table, interfering with everyone's conversation.

Hands-on activity

See diffraction with your own fingers, then build the case

Part A (2 minutes). Look at a bright, evenly lit surface, such as a frosted light bulb that is switched on, a lampshade or a bright window (never look at the Sun). Hold two fingers right in front of one eye and squeeze them together until there is only a very thin gap between them. Look through the gap.

What to look for: thin dark lines running parallel to the gap. They appear only when the gap is extremely narrow, which is exactly what the wave model predicts. A particle model gives no reason for dark lines.

Part B (20 minutes): a mini debate. Pick a side: the year is 1810, and you must convince the scientific community about the nature of light. Write a one-page argument with (1) at least three observations your model explains, (2) the strongest objection to your side and your reply, and (3) one experiment that could prove your side wrong. Then write a short paragraph, as yourself in 2026, saying what actually happened.

What to look for: a particle-side argument should use straight-line travel, sharp shadows and Newton's explanation of refraction; a wave-side argument should use Young's interference and the λ/w idea to explain sharp shadows. The 2026 paragraph should mention Foucault's speed measurement and the bright spot, and note that photons brought particle ideas back in 1905.

Your turn

State two observations that both the particle and wave models could explain.

What did the particle model predict about the speed of light in water compared with air?

Who measured the speed of light in water, about when, and what was found?

Why do everyday objects cast sharp shadows even though light is a wave?

What is Poisson's bright spot?

Which phenomenon in Young's experiment could the particle model not explain?

Which 1905 discovery brought back a particle picture of light?

More practice: warm-up

Who proposed the wave model of light in 1690?

What name did Newton give the particles of light?

What range of wavelengths does visible light have?

Name the scientist who showed light is an electromagnetic wave.

True or false: Arago observed the bright spot that Poisson thought disproved Fresnel's theory.

More practice: on your own

Light enters glass where sin θ₁/sin θ₂ = 1.50. What speed in glass does each model predict?

Explain why the particle model was favoured for most of the 1700s.

Why is a prediction that could fail more valuable than an explanation of something already known?

A beam of particles from one opening and a second beam from another overlap on a screen. What would the particle model predict for the brightness where they overlap, and how does this differ from what Young saw?

Find λ/w for red light (λ = 650 nm) passing through a 0.10 mm slit. Would you expect noticeable diffraction?

Challenge

Using the particle model, derive that sin θ₁/sin θ₂ = v₂/v₁ when the component of velocity parallel to the surface does not change.

A modern student says, "The particle model was just wrong, so it was useless." Give a balanced response.

Why did the discovery that light is a transverse wave make the medium problem harder for 1800s scientists?

Check your understanding

Quiz

What did Newton's particle model predict about light entering water?

Which observation could the particle model NOT explain?

Why are the wave properties of light hard to notice in daily life?

Poisson's bright spot was important because…

Foucault's 1850 measurement showed that light…

Which discovery brought a particle picture of light back in 1905?

For parents and teachers

Big idea: this lesson covers the historical debate between the particle and wave models of light (expectation E3.2 with terminology from E2.1) and shows how scientists judge models (strand A: A1.1 formulating questions and predictions; A1.11 communicating ideas and arguments).

How to help: the finger-gap observation takes two minutes and genuinely shows diffraction. The debate write-up is a good dinner-table conversation: ask your teen to argue the side they think is wrong.

Common difficulties:

Thinking the particle model was foolish. It explained a lot and made a testable prediction.
Confusing the two models' speed predictions (particle: faster in water; wave: slower).
Thinking the story ended with waves. Photons (unit Quantum Mechanics) show light also behaves like particles.
Dates: exact years are less important than the order of events and why each experiment mattered.