If Atoms Are Mostly Empty Space, Why Doesn’t Light Pass Through Everything?

The phrase “atoms are mostly empty space” can make matter sound like a collection of tiny hard balls with wide gaps between them. If that picture were right, light might seem able to slip between the balls. But atoms are not miniature solid spheres, and light does not need to hit a nucleus to interact with matter.

The key is that light is an electromagnetic wave (or, in a quantum description, a stream of photons) that interacts mainly with the charged particles in atoms. Whether light gets through depends on how those particles respond at the light’s frequency and on how many atoms the light encounters.

What “mostly empty” means

An atom’s nucleus is extremely small compared with the atom’s overall size. A rough comparison is a nucleus about $10^{-15}$ m across in an atom about $10^{-10}$ m across. That makes the nucleus’s volume tiny compared with the atom’s volume.

But the space around the nucleus is not simply a set of empty gaps between solid objects. Electrons are described by quantum states that give the probability of finding them in different regions. Those states extend across much of the atom. The electron distribution, together with the nucleus and the electromagnetic forces between charges, determines how the atom responds to light.

So light does not need to strike a solid atomic surface. Its electric field can exert forces on an atom’s electrons even when it passes nowhere near the nucleus.

How light interacts with electrons

An electromagnetic wave has an oscillating electric field. That field pushes and pulls charged particles, including electrons. The electrons’ response depends on the wave’s frequency and on the atom’s or material’s available energy states.

If a photon’s energy can drive an allowed transition in the material, it may be absorbed. The absorbed energy can later become heat, be re-emitted as light, or take another path. Light can also be scattered or reflected without being absorbed. These outcomes depend on the material and the light’s wavelength.

In a transparent material, visible light usually does not match a strong absorption transition. The light still interacts with the material: the electrons respond to its field, and the material’s polarization changes how the wave propagates. The transmitted wave travels at a different phase speed than it would in vacuum. This is why a transparent material has a refractive index. In an ideal, uniform transparent material, the response is largely coherent: it changes the transmitted wave rather than making each atom act like a little black obstacle.

Why some materials transmit and others block light

Transparency is not a property of “empty space” inside atoms. It depends on a material’s electronic structure, its arrangement, and the light’s wavelength.

• Transparent materials, such as clear glass in the visible range, have relatively little visible-light absorption. Much of the light is transmitted, though some is reflected at the surfaces.
• Absorbing materials take up light energy at relevant frequencies. A dark surface may absorb much of the visible light that reaches it.
• Reflective materials, such as many metals, have mobile electrons that respond strongly to incident light and send much of it back.
• Scattering materials redirect light in many directions. A material can look opaque or cloudy because of scattering even when its ingredients do not absorb all the light.

A material can behave differently at different wavelengths. For example, a material that looks clear in visible light may absorb infrared or ultraviolet light. “Transparent” therefore means transparent over a specified range of wavelengths, not transparent to every kind of electromagnetic radiation.

Worked example: visible light through glass

Consider green light with a wavelength of $550$ nm. Its photon energy is

$$E = \frac{hc}{\lambda} \approx 2.25\ \text{eV},$$

where $h$ is Planck’s constant, $c$ is the speed of light in vacuum, and $\lambda$ is the wavelength. In ordinary clear glass, this visible-light energy generally does not correspond to strong absorption, so most of the light can travel through the glass.

The light is not completely unaffected. For a simple estimate, take glass with refractive index $n=1.5$ in air and light arriving straight on. The fraction reflected at one air–glass surface is approximately

$$R = \left(\frac{n-1}{n+1}\right)^2 = \left(\frac{0.5}{2.5}\right)^2 = 0.04.$$

That is about 4% reflected at each surface. Ignoring absorption and light reflected back and forth inside the pane, the directly transmitted fraction through two surfaces is approximately $0.96^2=0.9216$, or 92%. The estimate shows how glass can transmit most visible light while still interacting with it at its surfaces and throughout its interior.

Common mistakes

• Treating atoms as hard marbles: Atoms do not have sharp solid boundaries that light must physically hit. Their charged particles respond to electromagnetic fields.
• Assuming all wavelengths behave alike: A material can transmit visible light and absorb other wavelengths.
• Equating transparency with no interaction: Light in glass is refracted, and some is reflected or absorbed. Transmission means much of the light emerges, not that the material has no effect.
• Assuming opacity always means absorption: Scattering and reflection can also prevent light from passing straight through.