How light behaves: reflection, refraction, absorption and scattering

Almost nothing we photograph emits its own light. We see and record objects because light from the sun, a lamp or the sky strikes them and some of it reaches the lens. What happens at each surface, whether light bounces off, passes through, bends, is absorbed or is scattered, determines the color, texture, shine and transparency of everything in an image. Understanding these interactions lets an image maker predict how a subject will look under a given light, control reflections and glare, choose modifiers, and use filters intelligently. This lesson covers specular and diffuse reflection, absorption and the color of objects, transmission and translucency, refraction and its consequences in lenses and nature, scattering in the atmosphere, diffraction, polarization and polarizing filters, and the ideas of subsurface scattering and the Fresnel effect that shape how skin, liquids and glossy surfaces appear.

Reflection

When light meets a surface, part of it reflects. The kind of reflection depends on the surface texture compared with the wavelength of light:

• Specular reflection occurs on smooth surfaces such as mirrors, still water, polished metal and glossy paint. Light reflects at an angle equal to the angle at which it arrived (the law of reflection: angle of incidence equals angle of reflection), producing a mirror image of the source called a specular highlight. Because the reflection follows a fixed angle, the highlight moves as the camera or light moves.
• Diffuse reflection occurs on rough or matte surfaces such as paper, unpolished wood, fabric and skin. Microscopic roughness scatters light in many directions, so the surface looks similar in brightness from different viewpoints and shows no mirror image of the source.

Most real surfaces mix both: a glossy apple shows a diffuse red body and a small white specular highlight; skin shows a diffuse base with specular shine on the forehead and nose. Controlling the balance, for example softening shine with powder or polarizers, or emphasizing gloss for product shots, is a constant task in lighting.

The law of reflection gives a practical tool: to avoid a reflection of a light in glass, a picture frame or eyeglasses, move the light so the reflected angle does not reach the lens, or move the camera. For product photography of reflective objects, photographers deliberately place large light sources where their reflections will define the shape of the object, a technique called lighting the reflection.

Absorption and the color of objects

The light a surface does not reflect or transmit is absorbed, usually becoming heat. Objects have color because they absorb some wavelengths more than others. A red apple absorbs most blue and green light and reflects mostly red; a white sheet reflects most wavelengths about equally; black velvet absorbs nearly everything. Dark surfaces heat up in sunlight because they absorb more energy, which matters for equipment and for black flags and cloth used near hot lights.

An object's color therefore depends on the light illuminating it. Under a light lacking red wavelengths, such as a sodium street lamp emitting mainly yellow-orange or some cheap LEDs with weak red output, a red object can look dull brown or gray, because there is little red light for it to reflect. This is why color rendering of light sources matters (Lesson 7).

Reflectance describes the fraction of light a surface reflects. Fresh snow reflects about 80 % to 90 %, a typical middle gray card is designed to reflect about 18 % (Lesson 11), average skin roughly 25 % to 40 % depending on tone, green grass perhaps 10 % to 20 %, and black velvet a few percent or less. These differences explain why metering a scene full of snow or black fabric can mislead an automatic exposure system.

Transmission and translucency

Some materials let light pass through. Transparent materials such as clear glass, water and acrylic transmit light with little scattering, so objects can be seen clearly through them. Translucent materials such as frosted glass, tracing paper, diffusion fabric, leaves and thin skin transmit light but scatter it, so shapes behind them are blurred. Translucency is the principle behind diffusion materials used to soften light (Lesson 4): a sheet of diffusion fabric in front of a hard light becomes a larger, softer source. Colored transparent materials, such as stained glass, theatrical gels and colored liquids, transmit some wavelengths and absorb others. Backlit translucent subjects such as leaves, petals, ears and drinks glow attractively, a look image makers use deliberately.

Refraction

Light slows down when it enters a denser medium such as glass or water, and if it enters at an angle, it changes direction: refraction. The amount of bending depends on the refractive index of each medium (about 1.0 for air, 1.33 for water and roughly 1.5 to 1.9 for optical glasses) and is described by Snell's law, n₁ sin θ₁ = n₂ sin θ₂. Refraction is how lenses work: curved glass surfaces bend light rays so they converge to form an image.

Refraction also explains everyday effects: a straw appears bent in a glass of water, swimming pools look shallower than they are, and heat shimmer over hot roads distorts distant objects because layers of air with different temperatures have slightly different refractive indices. Underwater photographers must account for refraction at the camera housing port: a flat port makes objects appear larger and narrows the field of view, while a dome port corrects for this.

Because the refractive index varies slightly with wavelength, different colors bend by different amounts: dispersion. A prism separates white light into a spectrum, and rainbows form when sunlight refracts and reflects inside raindrops. In lenses, dispersion causes chromatic aberration, color fringes at high-contrast edges, which lens designers reduce with combinations of glasses, including low-dispersion elements, and which can also be corrected in software.

Scattering in the atmosphere

Small particles scatter light. Air molecules, much smaller than the wavelength of light, scatter short wavelengths far more strongly than long ones (Rayleigh scattering, proportional roughly to 1/λ⁴). This is why the clear sky is blue: blue sunlight is scattered across the sky toward us from all directions. At sunrise and sunset, sunlight travels through much more atmosphere, so most blue is scattered away before reaching us, and the direct sun and the light it casts become warm orange and red.

Larger particles such as water droplets in clouds, fog and haze scatter all wavelengths about equally (Mie scattering), so clouds and fog look white or gray and soften contrast. Haze reduces contrast and saturation with distance, creating aerial perspective: distant hills look lighter and bluer, which painters and photographers use to convey depth. Smoke and haze machines on film sets exploit scattering to make light beams visible and to add atmosphere.

Diffraction

When light passes through a small opening or past an edge, it spreads slightly: diffraction. In photography, diffraction limits sharpness at small apertures. Stopping a lens down from f/8 to f/22 increases depth of field but spreads each point of light into a larger blur (the Airy disk), softening fine detail, especially on high-resolution sensors with small photosites. Each lens and sensor has a range of apertures where sharpness is best, often around f/5.6 to f/8 on full-frame cameras. Diffraction at the blades of the aperture also creates the starburst effect around bright points of light at small apertures.

Polarization

Light waves oscillate in directions perpendicular to their travel. Ordinary sunlight and lamplight are unpolarized, oscillating in all such directions. Reflection from non-metallic surfaces such as water, glass, leaves and paint partially polarizes light, strongly so at a particular angle (Brewster's angle, about 53° from perpendicular for water and 56° for glass). Light from the blue sky is also polarized, most strongly at 90° from the sun.

A polarizing filter passes light oscillating in one direction and blocks light oscillating at right angles. Rotating a circular polarizer on a lens can remove reflections from water and glass (revealing what lies beneath or behind), cut glare from foliage to increase color saturation, and darken a blue sky at 90° from the sun. Cinematographers and product photographers use polarizers on both lights and lenses (cross-polarization) to eliminate specular reflections entirely, for example when photographing artwork or reducing shine on skin. Polarizers cost one to two stops of light, can make wide-angle skies look uneven, and do not remove reflections from bare metal, which does not polarize light.

Subsurface scattering and the Fresnel effect

Two further effects strongly influence how real materials look. In subsurface scattering, light enters a translucent material, scatters inside, and exits at a different point, giving skin, wax, marble, milk and leaves their soft glow; it is why skin lit from behind glows red at the edges and why harsh light on skin looks less harsh than on plaster. In the Fresnel effect, surfaces reflect more light at grazing angles than when viewed straight on: a lake reflects the sky strongly near the horizon but lets you see into the water at your feet, and the edges of a glossy object show stronger reflections. Computer graphics and modern visual effects simulate both effects to make digital materials look real, and image makers who notice them can light real materials more convincingly.

Using material properties in practice

Knowing how materials interact with light helps solve everyday problems. Glossy magazine pages and framed artwork reflect lights into the lens, so lights are placed at angles where their reflections miss the camera. Glassware is often lit from behind through diffusion, so its edges appear as dark or bright outlines. Water in a glass glows when backlit. Metal objects reflect their surroundings rather than showing their own color, so photographers surround them with white and black cards to create clean reflections that describe their shape.

When something in an image looks wrong, a strange color cast, an unwanted reflection, a soft or fringed edge, a washed-out distant landscape, the cause can usually be traced to one of the interactions in this lesson. Identifying which one is the first step to fixing it, whether by moving a light, changing the angle, adding a filter, choosing a different aperture or waiting for different atmospheric conditions.