Light can be reflected, absorbed, transmitted, refracted, scattered, or transformed in several other ways depending on what it encounters. In a vacuum, light travels at exactly 299,792,458 meters per second, roughly 186,000 miles per second. But the moment it meets matter, things get interesting. Every color you see, every sunset, every warm surface soaking up sunshine is the result of one or more of these interactions playing out at the atomic level.
Reflection: Light Bouncing Off Surfaces
When light hits an object and bounces back, that’s reflection. Very smooth surfaces like mirrors reflect nearly all incoming light, which is why you see a clear image in them. Rougher surfaces scatter reflected light in many directions, producing a duller appearance rather than a mirror image.
The color of every object you see is actually determined by reflection. A red apple absorbs most wavelengths of visible light but reflects the red wavelengths back to your eyes. A white object reflects nearly all wavelengths, while a black object absorbs almost everything and reflects very little. The physical and chemical makeup of a material determines which wavelengths get reflected and which get absorbed.
Absorption: Light Becoming Heat
When light is absorbed, its energy doesn’t disappear. Photons (individual packets of light energy) strike atoms and molecules in the material, causing them to vibrate faster. The more they vibrate, the hotter the material gets. That heat is then released as thermal energy, which is why black asphalt feels scorching on a summer day while a light-colored concrete sidewalk stays noticeably cooler. The dark pavement absorbs most visible and ultraviolet light, converting it to heat, while the lighter surface reflects more of that energy away.
At the atomic level, absorption is remarkably specific. An electron orbiting an atom can absorb a photon only if that photon carries the right amount of energy to bump the electron up to a higher energy level. If the energy doesn’t match any available jump, the photon passes through or reflects instead. Once an electron absorbs that energy and reaches an excited, unstable state, it eventually drops back down and releases a lower-energy photon. This is the basis of fluorescence and phosphorescence.
Fluorescence and Phosphorescence
When absorbed light is re-emitted, the timing and energy of that re-emission vary dramatically. Fluorescence happens almost instantly: an electron absorbs a photon, jumps to a higher energy state, then drops back down within about 3 nanoseconds, releasing a new photon of lower energy (longer wavelength). This is why fluorescent materials seem to glow under ultraviolet light. They absorb UV you can’t see and re-emit visible light you can.
Phosphorescence works on the same principle but on a vastly slower timescale. The electron gets temporarily trapped in an intermediate energy state, delaying the re-emission for milliseconds to seconds, sometimes longer. Glow-in-the-dark stickers and watch dials use phosphorescent materials, which is why they keep glowing after the light source is removed. The difference in timing between the two processes spans roughly eight orders of magnitude.
Refraction: Light Changing Speed and Direction
Light slows down when it enters a denser material like water or glass. Because light interacts with atoms in the material, it can never travel as fast through matter as it does in a vacuum. The degree of slowing is captured by a number called the refractive index. Water has a refractive index of about 1.33, meaning light travels at roughly 75% of its vacuum speed in water. Diamond has a refractive index of about 2.42, slowing light to around 41% of its vacuum speed.
When light crosses from one material into another at an angle, the change in speed causes it to bend. This bending is refraction, and it’s responsible for how a straw looks broken in a glass of water, how lenses focus images, and how prisms split white light into a rainbow. Each wavelength slows by a slightly different amount, so they bend at slightly different angles, separating the colors.
Scattering: Why the Sky Is Blue
Scattering happens when light is redirected in many directions after encountering small particles. The type of scattering depends on the size of the particle relative to the wavelength of light.
When particles are much smaller than the wavelength of light, as nitrogen and oxygen molecules in the atmosphere are, the result is Rayleigh scattering. Shorter wavelengths (blue and violet) scatter far more efficiently than longer wavelengths (red and orange). This is why the sky appears blue: sunlight enters the atmosphere, and blue wavelengths get scattered in all directions, reaching your eyes from every part of the sky. At sunset, sunlight travels through a much thicker slice of atmosphere, scattering away so much blue light that the remaining reds and oranges dominate. The sky isn’t purple, even though violet scatters even more than blue, because our eyes are less sensitive to violet and because sunlight contains less violet to begin with.
When particles are closer to the size of visible light wavelengths or larger, like water droplets in fog or dust, Mie scattering takes over. Mie scattering doesn’t favor short wavelengths the way Rayleigh scattering does, which is why clouds and fog appear white. All wavelengths scatter roughly equally.
Diffraction: Light Bending Around Obstacles
When light passes through a narrow opening or around a sharp edge, it doesn’t just travel in a straight line. It spreads out, a behavior called diffraction. This can only be explained by treating light as a wave. Each point along the wave front acts as a source of new waves, and those waves overlap and combine on the other side of the obstacle.
Where overlapping waves line up peak to peak, they reinforce each other, creating bright spots. This is constructive interference. Where a peak meets a trough, they cancel out, producing dark spots, called destructive interference. The pattern of alternating bright and dark bands is visible when laser light passes through a thin slit or a pair of closely spaced slits. The spacing of the pattern depends on both the wavelength of the light and the size of the opening. Diffraction effects are most noticeable when the opening or obstacle is close in size to the wavelength of the light involved.
Polarization: Filtering Light’s Orientation
Light waves vibrate in all directions perpendicular to their direction of travel. Polarization is the process of filtering those vibrations down to a single plane. A polarizing filter works like a tiny set of parallel slits at the molecular level: it absorbs the component of light vibrating along one direction and passes the component perpendicular to it.
If you place a second polarizing filter behind the first and rotate it, the amount of light that gets through depends on the angle between the two filters. When they’re aligned, nearly all the polarized light passes through. When they’re perpendicular, almost none does. The transmitted intensity follows a precise pattern: it drops off with the square of the cosine of the angle between the filters. At 45 degrees, you get half the light. At 90 degrees, you get essentially zero. This is the principle behind polarized sunglasses, which block glare from horizontal surfaces like roads and water by filtering out horizontally polarized reflections.
The Photoelectric Effect: Light Releasing Electrons
Light can also knock electrons completely free from a material’s surface, a phenomenon called the photoelectric effect. Each photon carries an amount of energy determined by its frequency. If that energy exceeds a material-specific threshold (the minimum energy holding electrons to the surface), the photon is absorbed and an electron is ejected. Any leftover energy becomes the kinetic energy of the freed electron.
Below that threshold frequency, no electrons are released no matter how intense the light is. A dim beam of ultraviolet light can eject electrons from a metal surface while a blindingly bright beam of red light cannot, because the individual red photons don’t carry enough energy. This discovery was key evidence that light behaves as discrete packets of energy, not just as a continuous wave, and it’s the principle behind solar cells and light sensors.
Thermal Emission: Objects Glowing From Heat
Every object with a temperature above absolute zero emits light. Most of the time this emission is in the infrared range, invisible to your eyes, which is how thermal cameras work. As an object gets hotter, the peak wavelength of light it emits shifts to shorter, higher-energy wavelengths. A warm body emits infrared. Heat metal to around 500°C and it begins to glow a dull red. At thousands of degrees, objects glow white or bluish-white because they emit strongly across the visible spectrum and into shorter wavelengths. The surface of the sun, at roughly 5,500°C, peaks in the visible range, which is no coincidence: human vision evolved to be most sensitive to the wavelengths our star emits most strongly.