Unpolarized light is light whose electric field vibrates randomly in every direction perpendicular to its path. Most light in the world around you — sunlight, lamp light, candle light — is unpolarized. It has no preferred direction of vibration, which is why its behaviour through a polarising filter is the same no matter how you rotate the filter.
The defining characteristic of unpolarized light is that its polarization direction changes randomly and rapidly. Over any measurable time interval, all directions are equally represented. This is fundamentally different from polarized light, where the electric field is confined to a specific orientation or rotates in a predictable way.
Picture a crowd of people milling around a plaza — people walking north, south, east, west, and every direction in between. That is unpolarized light. Now picture a marching band moving in perfect lockstep down one street. That is polarized light. The crowd has no net direction; the band does. The polarising filter acts like a narrow corridor: in a crowd, only those already walking along the corridor make it through (about half), while the marching band either fits perfectly or gets completely blocked, depending on whether their direction matches.
What unpolarized light actually means
At the wave level, unpolarized light means the electric field vector of the light wave points in a random direction at any instant, and this direction changes unpredictably over time.
For monochromatic light (a single wavelength), it is technically impossible for the light to be completely unpolarized — a pure sine wave always has a well-defined polarization. True unpolarized light requires a mixture of many different wave trains with random polarizations and random phase relationships. This is exactly what thermal light sources produce.
The key points to understand:
- Random direction. The electric field vibrates in all planes perpendicular to the beam.
- Rapid fluctuations. The polarization changes on timescales of 10⁻⁹ to 10⁻⁸ seconds.
- No net preference. Over time, all directions average out equally.
- Orientation-independent. Rotating a polariser in the beam makes no difference to transmitted intensity.
Why most light is unpolarized
The reason comes down to how light is produced. In an incandescent bulb, a candle flame, or the sun, light is emitted by billions of independent atoms. Each atom produces a short wave train — a brief pulse of electromagnetic radiation lasting about a nanosecond — with its own random polarization direction.
These wave trains are emitted independently and incoherently. The light reaching your eye is the superposition of billions of these random wave trains. When you average over all of them, there is no preferred polarization direction. The result is unpolarized light.
Only special sources produce polarized light directly. Lasers emit polarized light because the stimulated emission process locks the atoms in phase. Certain natural phenomena — like synchrotron radiation and some astronomical sources — also produce polarized light. But common thermal sources always produce unpolarized light.
For a more detailed introduction to polarization concepts, see our pillar guide on what is polarized light.
Common sources of unpolarized light
| Source | Type | Why it is unpolarized |
|---|---|---|
| Sunlight | Natural thermal | Billions of independent atomic emissions |
| Incandescent bulbs | Thermal (heated filament) | Random thermal vibrations of atoms |
| Fluorescent lamps | Gas discharge | Many independent atomic transitions |
| LED lights | Semiconductor | Random electron-hole recombinations |
| Candle flames | Thermal combustion | Chaotic molecular emissions |
| Halogen lamps | Thermal (heated filament) | Same as incandescent — random thermal emission |
The only common exception is the laser. Laser light is polarized because the emitted photons are stimulated by an existing wave, which forces them to share the same phase and polarization direction.
Unpolarized vs polarised light: detailed comparison
| Property | Unpolarized light | Polarised light |
|---|---|---|
| Electric field direction | Random, changes rapidly | Fixed plane or rotating predictably |
| Common sources | Sun, bulbs, LEDs, flames, candles | Lasers, light through a polariser |
| Behaviour through a polariser | 50% transmitted, independent of orientation | Varies with angle (Malus's law: I = I₀ cos²θ) |
| Can form interference patterns? | No (Fresnel-Arago law) | Yes, if coherent |
| Natural occurrence | Most common type of light | Rare in nature (reflections create it) |
| Degree of polarization | 0% (completely unpolarized) | 100% (fully polarized) — or any value between for partial |
For a full walkthrough of what happens to unpolarized light at a polariser, see our guide on how does polarisation work.
Partially polarized light
Not all light is either fully polarized or fully unpolarized. Most light in the real world is partially polarized — a mixture of the two.
When unpolarized sunlight reflects off a wet road, the reflected light is partially polarized. The horizontal component reflects more strongly, so the reflected beam has more horizontal than vertical polarization — but it is not completely horizontal. Similarly, sunlight scattered by the atmosphere becomes partially polarized, which is why the blue sky has a measurable polarization pattern.
The degree of polarization quantifies this. It ranges from 0% (completely unpolarized) to 100% (fully polarized). A beam that is 50% polarized means half its intensity is polarized in a specific direction and half is random.
This matters for photography. A polarising filter on a camera can reduce the partially polarized glare from a wet road or a shop window by adjusting the filter angle to block the polarized component. But it cannot eliminate all the light because the unpolarized component passes through regardless.
The Fresnel-Arago law: unpolarized light cannot interfere
Here is a surprising consequence of unpolarized light: it cannot form interference patterns.
This is known as the Fresnel-Arago law, formulated by Augustin-Jean Fresnel and François Arago in the early 1800s. They discovered that two beams of unpolarized light cannot produce stable interference fringes, even if they come from the same source.
The reason is that interference requires a fixed phase relationship between the two waves. Unpolarized light's polarization direction fluctuates so rapidly that any phase relationship is destroyed. For interference to occur, the two beams must have the same polarization state and be mutually coherent.
This is one reason why Young's double-slit experiment uses a single light source split into two paths — it ensures both the polarization and phase relationship are preserved. Using two independent unpolarized sources would produce no fringes at all.
For more on how polarization affects interference and wave behaviour, see our guide on destructive interference formula.
Key takeaways
- Unpolarized light has no preferred polarization direction — its electric field vibrates randomly.
- Almost all natural and artificial light sources (sun, bulbs, LEDs, flames) produce unpolarized light.
- The randomness comes from billions of independent atomic emissions, each with its own polarization.
- Unpolarized light passes through a polariser with 50% intensity, regardless of the filter's orientation.
- Partially polarized light is a mixture — most real-world light falls somewhere between fully polarized and fully unpolarized.
- The Fresnel-Arago law states that unpolarized light cannot produce interference patterns.
- Lasers are the main exception — they produce coherent, polarized light directly.
External resources
- Britannica: Unpolarized Light — authoritative description of why thermal sources produce unpolarized light and how polarization arises
- RP Photonics: Unpolarized Light — technical encyclopedia entry covering definition, partial polarization, and the difference from circularly polarized light
- The Physics Classroom: Polarization — clear tutorial with diagrams on how polarising filters work and Malus's law
Frequently Asked Questions
What is unpolarized light in simple terms?
Unpolarized light is light whose electric field vibrates randomly in all directions perpendicular to the direction it travels. Think of it like a crowd of people walking every which way. Polarized light is like a marching band — everyone moving in the same direction. Most light from the sun, light bulbs, and candles is unpolarized.
What are common sources of unpolarized light?
Common sources include sunlight, incandescent light bulbs, fluorescent lamps, LED lights, candle flames, and any thermal light source. These produce light from many independent atoms emitting randomly, so the overall polarization direction averages out to no preferred orientation. The only common source of naturally polarized light is a laser.
What is the difference between unpolarized and polarized light?
Unpolarized light has electric field vectors vibrating randomly in all planes perpendicular to the direction of travel. Polarized light has electric field vectors restricted to a single plane (linear), or rotating in a controlled way (circular/elliptical). Unpolarized light passes through a polarizer with 50% intensity loss regardless of orientation. Polarized light transmission depends on the angle between its polarization and the polarizer's axis, following Malus's law.
Can unpolarized light be converted to polarized light?
Yes. Passing unpolarized light through a polarising filter converts it to polarized light. The filter contains long-chain molecules aligned in one direction that absorb light vibrating parallel to them. Light vibrating perpendicular passes through. The result is linearly polarized light with roughly half the original intensity. Reflection from a non-metallic surface at Brewster's angle also produces polarized light from unpolarized incident light.
Why is most natural light unpolarized?
Most natural light comes from thermal sources where billions of atoms emit independently. Each atom produces a short wave train lasting about 10⁻⁹ to 10⁻⁸ seconds with a random polarization direction. The sum of all these random emissions produces light with no net polarization. For sunlight, additional scattering in the atmosphere can introduce partial polarization, but direct sunlight remains essentially unpolarized.
What is partially polarized light?
Partially polarized light is a mixture of unpolarized and polarized light. It occurs when unpolarized light reflects off a surface, scatters in the atmosphere, or passes through certain materials. The degree of polarization quantifies what fraction of the light is polarized. For example, light reflecting from a wet road at certain angles may be 50% polarized — meaning half the intensity is polarized horizontally and half is random.
Can unpolarized light produce interference patterns?
No. This is the Fresnel-Arago law: two beams of unpolarized light cannot form a stable interference pattern because their polarization directions fluctuate randomly and independently. Even if you rotate them into alignment, the random phase relationship prevents stable interference. Both beams must have a fixed polarization relationship — they must be coherent and have the same polarization state — to produce interference fringes.
How much light passes through a polarizer from unpolarized light?
Exactly half the intensity passes through an ideal polarizer from unpolarized light. This is because unpolarized light contains equal components of every polarization direction. Only the component aligned with the polarizer's axis passes through. The other half is absorbed. This is why polarising filters reduce overall brightness by about one stop in photography.
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