Look at a window and a mirror side by side. Both are smooth, flat surfaces. Both were made in factories using silica-based materials. Yet light treats them in fundamentally opposite ways—passing through one while bouncing off the other. This isn't a quirk of manufacturing. It's a consequence of how electrons are arranged at the atomic level.

The distinction between reflection and transmission comes down to a single question: can electrons move freely, or are they locked in place? In metals, electrons roam like a fluid through the material, ready to respond to incoming electromagnetic waves. In glass, electrons are bound tightly to their parent atoms, unable to participate in the dance.

Understanding this divide explains far more than just mirrors and windows. It reveals why copper wires conduct electricity, why your microwave has a metal mesh in the door, and why X-ray machines can image bones through flesh. The optical properties of materials are direct consequences of their electronic structure—and once you see the pattern, you'll recognize it everywhere.

Metals possess a remarkable property that sets them apart from most materials: their outermost electrons aren't bound to individual atoms. Instead, these valence electrons detach and form what physicists call an electron sea—a fluid of negative charge that permeates the entire material. The metal atoms themselves become positive ions, arranged in a crystal lattice, but the electrons move freely between them like water flowing around rocks.

When an electromagnetic wave—visible light, for instance—strikes a metal surface, its oscillating electric field encounters this electron sea. The field pushes electrons one direction, then pulls them back, then pushes again, thousands of trillions of times per second for visible light. Because the electrons are free to move, they respond immediately, sloshing back and forth in sync with the incoming wave.

Here's the critical physics: accelerating charges emit electromagnetic radiation. As these electrons oscillate, they don't just absorb energy—they re-radiate it. The collective oscillation of countless surface electrons produces a new electromagnetic wave traveling back out of the metal. This re-radiated wave is what we perceive as reflection. The process is so efficient that polished metals can reflect over 95% of incident visible light.

The electrons respond so quickly and uniformly that the reflected wave maintains the phase relationships of the original light. This is why metal mirrors preserve image quality—the reflected wavefront is a faithful reproduction of the incident one, just traveling in the opposite direction. The free electron sea acts as a near-perfect electromagnetic shield, preventing light from penetrating more than a few nanometers into the material.

Takeaway

Reflection occurs because free electrons in metals can oscillate with incoming light waves and re-radiate the energy back out—materials reflect when their electrons are mobile enough to respond to electromagnetic fields.

Glass presents electrons with a fundamentally different situation. In silicon dioxide—the primary component of common glass—every electron is locked into chemical bonds between silicon and oxygen atoms. These bonds represent specific energy states, and electrons cannot leave them without absorbing a precise amount of energy. The electrons aren't free; they're prisoners of their atomic orbitals.

When visible light strikes glass, its oscillating electric field tries to push these bound electrons around, just as it does in metals. But bound electrons can only respond in limited ways. They can stretch slightly within their bonds, like masses on stiff springs, but they cannot break free and flow through the material. More importantly, visible light photons don't carry enough energy to promote electrons to higher orbital states in glass.

This energy mismatch is the key to transparency. The electrons in glass have resonant frequencies—specific frequencies at which they would absorb energy strongly—but these frequencies lie in the ultraviolet range, not the visible spectrum. Visible light is essentially invisible to glass's electrons. The wave passes through as if the material weren't there, experiencing only a slight slowing due to weak interactions with the bound electrons (which is why light bends when entering glass).

The electrons do interact weakly with passing light, creating the phenomenon we call refractive index. They oscillate slightly, re-radiate, and their emissions combine with the original wave in a way that effectively slows light's propagation speed. But crucially, they don't absorb or scatter the energy. The wave exits the far side of the glass with almost all its original energy intact—the definition of transparency.

Takeaway

Transparency occurs when a material's electrons cannot absorb photons at the incoming frequency—visible light passes through glass because its photon energy falls between the resonant frequencies where glass electrons would absorb.

The story of metal reflection has a surprising twist: metals become transparent at sufficiently high frequencies. This critical transition point is called the plasma frequency, and it reveals the ultimate limits of how fast free electrons can respond to electromagnetic fields. Above this frequency, even metals with their mobile electron seas cannot keep up with the oscillating field.

The plasma frequency depends on the density of free electrons in the metal. For aluminum, it falls in the ultraviolet range, around 15 electronvolts. For visible light, which ranges from about 1.7 to 3.3 electronvolts, aluminum's electrons respond easily—hence aluminum's excellent reflectivity and its use in mirrors. But ultraviolet light above aluminum's plasma frequency passes right through as if the metal weren't there.

X-rays, with photon energies measured in thousands of electronvolts, far exceed the plasma frequency of all common metals. This is why X-rays penetrate aluminum foil, steel plates, and even lead (though lead absorbs through a different mechanism). At X-ray frequencies, the electromagnetic oscillation happens so fast that free electrons can't respond significantly—they barely move before the field reverses. The wave passes through the electron sea like it's passing through vacuum.

This principle has profound practical implications. The radio waves that carry FM broadcasts (around 100 MHz) reflect beautifully off the ionosphere—a layer of free electrons in Earth's upper atmosphere—allowing signals to bounce around the planet. But satellite communications use microwave frequencies that pass straight through. Every material has frequencies where it reflects, frequencies where it absorbs, and frequencies where it transmits—the plasma frequency marks one of these critical transitions.

Takeaway

Every conductor has a plasma frequency above which it becomes transparent—this explains why X-rays penetrate metal, why radio waves bounce off the ionosphere, and why the same material can reflect one frequency while transmitting another.

The optical divide between metals and insulators comes down to electron mobility. Free electrons reflect by oscillating and re-radiating; bound electrons transmit by ignoring frequencies that don't match their resonant transitions. This simple distinction creates mirrors and windows, radio antennas and fiber optics.

But the boundary isn't absolute. Semiconductors fall between the extremes, with tunable electronic properties that power solar cells and LEDs. And every material has its frequency limits—metals that perfectly reflect visible light become transparent to X-rays.

Next time you look through a window at your reflection, you're witnessing this fundamental divide. The glass transmits because its electrons are trapped. The faint reflection comes from the small refractive index mismatch. The physics of electron confinement determines what you see.