Properties of Light
Light is weird. It does something no everyday object does — it behaves like a wave and a particle at the same time.
Shine light through two narrow slits and it produces an interference pattern, exactly like water waves overlapping in a pond (very wave-like). Shine the same light at a metal surface and it knocks individual electrons off the metal, one at a time, like a stream of bullets (very particle-like). Neither model alone explains everything light does — only the combined “wave-particle duality” picture does.
This is also where modern physics (Einstein’s photoelectric effect, quantum mechanics) was born. For the MCAT, you don’t need to resolve the philosophical paradox — you just need to know when to use each model.
Wave-Particle Duality
Light is both a wave and a stream of particles called photons. Which behavior you see depends on the experiment:
- Wave behavior: interference, diffraction, polarization (covered in §§8.11–8.12).
- Particle behavior: the photoelectric effect, Compton scattering, emission and absorption of light by atoms.
The MCAT-friendly rule of thumb: wave properties explain how light bends, interferes, and spreads. Particle properties explain how light transfers energy to matter.
Photon Energy
Each photon carries a specific amount of energy determined by its frequency:
This equation is the bridge between the wave model and the particle model. Frequency (a wave property) determines the energy of each photon (a particle property).
The Visible Spectrum
Visible light is the narrow band of the EM spectrum that human eyes can detect — about 400 nm (violet) to 700 nm (red).
| Color | Approximate wavelength | Relative energy |
|---|---|---|
| Red | ~700 nm | Lowest |
| Orange | ~620 nm | |
| Yellow | ~580 nm | |
| Green | ~530 nm | |
| Blue | ~470 nm | |
| Violet | ~400 nm | Highest |
Color Perception
When white light (which contains all visible wavelengths) hits an object, some wavelengths get absorbed and others get reflected. The reflected wavelengths are what your eyes detect as color:
- A red apple absorbs most wavelengths and reflects red (~700 nm).
- A white shirt reflects all visible wavelengths.
- A black shirt absorbs all visible wavelengths (which is why black surfaces get hotter in the sun).
This is subtractive color — the object subtracts certain wavelengths and you see what remains. Screens and projectors use additive color, combining red, green, and blue light to create the full color range your eye can perceive. (RGB on a screen vs. CMYK in a printer is exactly this distinction.)
Intensity vs. Photon Energy
Two properties of a light beam that students often confuse — and the MCAT loves the confusion:
- Intensity (brightness) depends on the number of photons per second hitting a surface.
- Photon energy depends on the frequency of each individual photon.
Turning up the brightness of a red laser pours out more red photons per second but doesn’t change the energy of each one. Switching from a red laser to a blue laser increases the energy per photon but says nothing about total brightness.
This distinction matters in biology too: UV light damages DNA not because it’s bright, but because each individual UV photon has enough energy to break a covalent bond. Bright red light might pour vastly more total energy onto your skin without causing any damage, because no single red photon is energetic enough to break a bond.