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Oil-Slick Colours

Why a Nearly Colourless Film Makes a Rainbow

Big Question: How can a transparent film only a few wavelengths thick sort white light into visible colours without using a prism or pigment?

Quick Answer

White light contains many visible wavelengths.

At a thin film, part of each wavelength reflects from the first boundary and part travels through the film before reflecting from the second boundary.

When the two reflected waves rejoin, their crests and troughs may line up or misalign.

• constructive interference: waves reinforce;
• destructive interference: waves weaken or cancel.

Because different colours have different wavelengths, the same film thickness can reinforce one colour while reducing another.

Film thickness also varies from place to place, so different parts of the slick or bubble show different colours.

What You Will Learn

• Why white light contains many wavelengths.
• Why a thin film creates two main reflected paths.
• What interference means.
• What constructive and destructive interference are.
• Why film thickness controls colour.
• Why viewing angle changes the colours.
• Why one reflection can undergo a half-wave phase change.
• Why the thinnest region of a soap film can become dark.
• Why soap bubbles, oil films and anti-reflection coatings share the same wave principle.
• Why pigment colour and structural colour are different scientific jobs.

Part 1 — Light Has Wavelength

Visible light is electromagnetic radiation.

Different visible wavelengths are associated with different perceived colours. Violet light has shorter wavelength than red light.

White light from the Sun or a lamp contains a range of wavelengths at once.

Part 2 — One Beam Becomes Two Reflected Beams

Suppose white light reaches a thin oil film on water.

1. Some light reflects from the air–oil boundary.
2. Some enters the oil by refraction.
3. That light travels through the film.
4. Some reflects from the oil–water boundary.
5. It travels back through the film.
6. It emerges into the air and overlaps the first reflection.

Now two light waves from the same original source reach your eye along closely related directions.

Part 3 — The Second Path Is Longer

The first reflected wave turns around near the top surface.

The second travels down through the film and back up again.

That extra optical path changes the phase relationship between the waves.

For normal incidence in a simple film, the travel contribution is related to roughly twice the optical thickness, 2nt, where n is refractive index and t is film thickness.

Part 4 — Waves Can Add or Cancel

If crest arrives with crest and trough with trough, amplitudes add.

in phase → constructive interference → stronger reflected light.

If crest arrives with trough, the waves partly or strongly cancel.

out of phase → destructive interference → weaker reflected light.

Part 5 — Why Different Colours Behave Differently

A path difference that equals one full wavelength for green light is not one full wavelength for red light.

So the same film can reinforce green while reducing red, or reinforce red while reducing blue.

Your eye then receives a reflected spectrum with some wavelengths stronger than others.

The film appears coloured even though it contains no matching green, red or blue pigment.

Part 6 — Why Thickness Changes the Colour

Change film thickness and the second ray’s path changes.

That changes which wavelengths return in phase.

Soap films drain under gravity, so their thickness can vary continuously from top to bottom. Oil films spread unevenly. Both create moving bands or patches of colour.

Part 7 — Reflection Can Flip the Wave Phase

There is another piece.

When light reflects from a boundary leading to a higher refractive index, the reflected electromagnetic wave undergoes a phase change equivalent to half a wavelength.

Reflection from a lower-index boundary does not have the same phase reversal.

Therefore the full interference condition depends on both:

• extra travel through the film;
• phase changes at the reflecting boundaries.

Part 8 — Why the Thinnest Soap Film Can Look Black

For an extremely thin soap film in air, the geometric path difference approaches zero.

But the reflection from the air-to-film boundary undergoes a half-wave phase shift while the reflection from the film-to-air boundary does not.

The two reflected waves can therefore interfere destructively across much of the visible spectrum.

A soap bubble often develops a dark region near the top just before rupture because the film there has become extremely thin.

Part 9 — Why Viewing Angle Matters

At an angle, the ray travels a different distance through the film before returning.

The effective optical path difference therefore changes with viewing direction.

A patch that looks green from one angle can look purple or gold from another.

thin-film colour belongs to the light–film–observer geometry, not to the material alone.

Part 10 — Why Oil on Water Produces Complex Patterns

Real oil films are not perfectly uniform.

• thickness changes across the surface;
• the oil refractive index differs among mixtures;
• the water surface moves;
• film flow redistributes material;
• sunlight arrives over a range of angles;
• surface contamination changes boundaries.

The colourful map is therefore also a map of film thickness and viewing geometry.

Part 11 — Why a Rainbow Is a Different Mechanism

A rainbow forms mainly through refraction, internal reflection and wavelength-dependent bending inside water droplets.

Thin-film colours come from interference between multiple reflected waves separated by a film only a few wavelengths thick.

Both separate colours from white light, but they do different scientific jobs.

Part 12 — Structural Colour Is Not Pigment Colour

A pigment absorbs some wavelengths and reflects or transmits others because of molecular electronic structure.

Structural colour arises because physical structures comparable in size to light wavelengths alter interference, diffraction or scattering.

Thin films are one important form of structural colour.

Part 13 — Anti-Reflection Coatings Use the Same Physics on Purpose

Engineers place carefully chosen thin coatings on lenses and screens.

If reflections from the coating’s two boundaries are arranged to interfere destructively at selected wavelengths, unwanted reflected light is reduced and more light enters the device.

The same wave principle that makes an oil slick colourful can make a camera lens look unusually dark.

Part 14 — Why Soap Bubbles Change Colour Before They Burst

Liquid drains downward and evaporates from the film.

As thickness changes, the constructive-interference condition sweeps through different visible wavelengths.

The colours therefore move.

When the top becomes extremely thin, the dark-film condition can appear just before rupture.

Part 15 — Thin Films Can Measure Tiny Thicknesses

If refractive index, wavelength and geometry are known, interference fringes can reveal film thickness changes far smaller than can be measured directly with a ruler.

Scientists and engineers use interference in metrology, coatings, semiconductor manufacture and surface measurements.

Follow One Green Wave

1. White light reaches an oil film.
2. The green component arrives at the top boundary.
3. One part reflects immediately.
4. Another part enters the oil.
5. It travels to the bottom boundary.
6. Part reflects.
7. It travels back upward.
8. It emerges and overlaps the first green reflection.
9. The two waves arrive with a particular phase difference.
10. If nearly in phase, green reflection is strengthened.
11. Another wavelength may arrive out of phase and be reduced.
12. Your eye receives a coloured reflection.

A Text Ray Diagram You Can Draw Anywhere

            reflected ray 1
                 ↗
white light ↘  /  air
-------------/------------- top of film
            ↘
             ↘ thin film thickness t
              ↘
---------------\----------- bottom of film
                ↗ reflected ray 2
               ↗
ray 2 travels farther
+ reflection phase changes
→ interference

Think Like a Scientist — Soap Film Thickness Without a Ruler

Make a soap film across a small wire loop or bubble wand.

1. Illuminate it with broad white light.
2. Hold the viewing angle steady.
3. Record the colour bands over time.
4. Watch liquid drain downward.
5. Note whether the top becomes dark before rupture.
6. Repeat with a different orientation.

The aim is not to assign exact thickness from colour without calibration. The aim is to link changing interference pattern to changing optical thickness.

How Do We Know Interference Is the Cause?

• changing film thickness shifts the reflected colours predictably;
• changing viewing angle shifts the pattern;
• monochromatic light creates bright and dark fringes rather than a rainbow;
• wave calculations predict constructive and destructive conditions;
• anti-reflection coatings deliberately use the same principle;
• thin-film thickness can be inferred quantitatively from measured interference fringes.

Observation vs Inference

• Observation: a nearly colourless film reflects vivid colours.
• Observation: colours move when thickness or viewing angle changes.
• Observation: extremely thin soap film can become dark.
• Inference: reflected waves interfere with wavelength-dependent phase differences.
• Model test: calculate expected bright wavelengths from film optical thickness and compare with spectra.

Common Misconceptions and How to Repair Them

Misconception	Better model
The oil contains rainbow pigments.	The colours can arise from interference in a nearly transparent film.
A thin film acts exactly like a prism.	Its colours are mainly interference, not ordinary dispersion by refraction alone.
Only path length matters.	Reflection phase changes can also matter.
Each thickness has one permanent colour.	Viewing angle and illumination spectrum also affect the result.
Dark means no light reached the film.	Reflected waves can destructively interfere.
Structural colour and pigment colour are the same.	They select wavelengths through different mechanisms.

Checkpoint Questions

1. Why are there two main reflected rays from a thin film?
2. What is constructive interference?
3. What is destructive interference?
4. Why do different wavelengths behave differently?
5. Why does film thickness change colour?
6. Why does viewing angle change colour?
7. What phase change can happen on reflection?
8. Why can a very thin soap film look dark?
9. How is a rainbow different?
10. How do anti-reflection coatings use interference?

Apply It — Three Films

• A: film with uniform thickness.
• B: film whose thickness gradually increases from top to bottom.
• C: same as A but viewed from a much more oblique angle.

Predict which should show broad uniform colour, which should show bands, and why changing angle can shift the colour even without changing the film itself.

Answer Key

Open after attempting the application

A can show a relatively uniform reflected colour if thickness and illumination are sufficiently uniform. B should show colour bands because different positions have different optical thicknesses. C can shift colour because the path through the film and phase relationship change with angle. Exact colours require refractive index, thickness and wavelength information.

Can You Explain WHY?

• Why can two weak reflections become bright together?
• Why can reflected light become weaker even though both surfaces reflect?
• Why does white light make colour while single-colour light makes bright/dark fringes?
• Why does a bubble’s changing thickness make its colours move?
• Why can the same film look different from another viewing direction?

Singapore Everyday Connection

After rain, thin films on puddles, carpark surfaces or wet machinery can show interference colours. Soap bubbles and detergent films provide a safer clean demonstration.

Do not touch unknown oily films outdoors; they may be contaminated. Observe from a safe distance or recreate the phenomenon with soap solution.

Primary Science / PSLE Bridge

• light can be reflected and refracted;
• white light contains many colours;
• changing a material’s thickness can change what we observe;
• models can explain effects too small to see directly;
• the same material can appear different under different viewing conditions;
• observations must separate pigment from structure.

Go Beyond Primary Science

Primary idea	Higher-resolution science
Two reflections overlap	Coherent wave superposition
Some colours brighten	Constructive interference
Some colours vanish	Destructive interference
Film thickness matters	Optical path length 2nt cosθ
Reflection can flip phase	Boundary-condition phase shifts
Coatings control reflection	Quarter-wave multilayer optics

Deep Science Window — Colour Can Be Geometry at Nanometre Scale

Visible wavelengths are hundreds of nanometres long. A film only a fraction of a micrometre thick can therefore change which wavelengths reinforce.

This is why structures too small to resolve with the eye can control macroscopic colour.

Deep Science Window — One Thin Film Is Only the Beginning

Stack many carefully designed layers and interference can become extremely selective.

Multilayer coatings are used in mirrors, lasers, filters, solar cells and optical instruments. The same basic phase bookkeeping scales into advanced photonics.

Evidence Boundaries

• Oil-slick colours ≠ all coloured oil films are purely interference. Pigments and contaminants can also contribute.
• 2nt ≈ path contribution ≠ full interference condition at every angle.
• Reflection phase shift ≠ every reflection flips phase.
• Thin-film colour ≠ ordinary rainbow mechanism.
• Dark reflection ≠ no electromagnetic field exists. Destructive interference redistributes energy.
• Observed colour ≠ unique thickness without knowing refractive index, angle and spectrum.

Manual Summary — KNOW → CONNECT → EXPLAIN → APPLY → CHECK

KNOW: wavelength, reflection, interference, phase, optical thickness and structural colour.

CONNECT: white light → two reflected paths → phase difference → wavelength-selective reinforcement/cancellation → colour.

EXPLAIN: a nearly colourless thin film can make colour by controlling wave interference.

APPLY: oil films, soap bubbles, anti-reflection coatings and optical filters.

CHECK: distinguish interference colour from pigment and from rainbow dispersion.

Where to Go Next

• The Physical World
• The Rainbow
• Mirrors and Reflection

Research Sources and Further Reading

• OpenStax University Physics — Interference in Thin Films
• OpenStax College Physics — Thin-Film Interference

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