Transparency and translucency

In the field of optics, transparency is a special property that lets light pass through a material without much scattering. Materials that are transparent, like plate glass and clean water, allow most light to go straight through them and reflect very little. This makes them appear clear and bright.

Translucency is similar but different. It also lets light pass through, but the light may scatter as it moves through the material. This means the light does not always follow the same path, and the material might look cloudy or milky instead of clear.

When light meets a material, it can reflect, absorb, or transmit depending on the material and the light's wavelength. Materials that do not let light through are called opaque. The way light interacts with a material gives us the colors and appearances we see every day.

Some marine animals, like jellyfish, are highly transparent, which helps them hide in the water. Transparency can act like camouflage, especially in dim or murky seawater, making it harder for others to see them.

Etymology

The words we use for see-through materials have long histories. The word "transparent" comes from old Latin words meaning "visible through," combining "through" and "be visible." The word "translucent" also comes from Latin, meaning "shining through," from "through" and "to shine." The word "opaque," meaning not see-through, also has roots in Latin, where it meant "darkened."

Introduction

When we look at objects, we see them because light bounces off them. Some materials let light pass through clearly, which we call transparent. Others let light pass but scatter it, making things look fuzzy—this is called translucent.

Materials let light through based on how their tiny parts are arranged. For example, pure glass lets light pass easily because its atoms are arranged in a way that doesn’t catch the light. But if there are tiny bumps or spots inside a material, they can scatter the light, making it look less clear. Scientists study these tiny details to create materials that are very clear, like special ceramics used in lasers and protective windows.

Absorption of light in solids

When light hits an object, it usually contains many different colors or frequencies. Objects can absorb, reflect, or let through certain colors of light. For example, one object might reflect green light but absorb all other colors, while another might let blue light pass through but absorb the rest.

Some materials let most light pass through without reflecting it. These are called optically transparent materials. Pure glass and clean water are good examples. Materials that don’t let any light through are called opaque. They may have special parts in their chemistry that absorb light. Most materials absorb only certain colors, which is why we see different colors — the colors we see are the ones not absorbed.

Different mechanisms cause absorption. In electronic absorption, light can change the energy levels of electrons in atoms. In vibrational absorption, the light’s energy matches the natural vibrations of atoms or molecules, turning light into heat. Metals usually reflect most light because of their free electrons, while many non-metal materials let visible light pass through unless they have added colorants. Most liquids, like water or oil, are clear because their structure allows light to pass through easily.

Optical waveguides

Main article: Optical fiber

Materials that let light pass through are important for how light waves behave. Special glass can carry many different colors of light at once without mixing them up, which helps send information over long distances.

An optical fiber is a thin glass tube that guides light by keeping it bouncing inside. It has a center part called the core and a surrounding layer called the cladding. Light stays in the core because it moves slower there than in the cladding. When light hits the edge of the core at a sharp angle, it bounces back inside, which keeps it moving along the fiber.

Mechanisms of attenuation

When light travels through a fiber, its strength can get weaker over distance. This happens mainly because of tiny bumps in the glass and because some light gets caught by small bits of material inside the fiber. Bending the fiber or connecting pieces together can also make the light weaker.

As camouflage

Further information: List of camouflage methods

Many animals in the sea that float near the surface are very clear, which helps them hide very well. However, being clear is tricky for animals made of things that see light differently than seawater does. Some sea animals like jellyfish have bodies mostly made of water, which makes them see-through. This helps them stay up in the water, but it also means they are big compared to their muscles and cannot swim fast. So, hiding this way has a cost — they cannot move quickly.

Small floating sea animals are between 50 and 90 percent clear. Being about 50 percent clear is enough to hide from a fish like cod deep in the water, but clearer is needed closer to the surface where there is more light and predators can see better. For example, a cod can see food that is 98 percent clear in shallow water with good light. Because of this, hiding by being clear works better in deep water. Being clear in air is even harder, but some glass frogs in South American rain forests have skin you can see through, as well as pale green legs. Some butterflies in Central America and many dragonflies and other insects also have wings that are mostly clear, which helps them hide from animals that might eat them.