Transparent, Translucent and Opaque Objects

Materials can be classified based on the amount of light they transmit. Materials, which allow complete transmission of light, are called transparent. Any object can be seen through the transparent material. One example of transparent material is pure glass.

Opaque materials either reflect or absorb any incident light. As a result, light rays cannot pass through opaque materials. Wood, stone, etc., are opaque materials. Translucent materials allow partial transmission of light through them. A part of the incident light may get reflected or scattered as it passes through the interior of the material. Any object, seen through a translucent material, appears fuzzy or blurred. Some examples are oily paper, tissue, some plastics, etc.

Objects and its Properties

Various objects that we see around exhibit some property. Some objects allow light to pass through clear glass. Or there are objects that create an obstruction in the path of light and are not as clear reflector of light as clear glass does. 

However, on the other hand, some objects don't even allow light to pass through. Instead, they block the light. What these objects are, and why do we study them in physics? Well, physics is all about understanding the science behind real-life objects around us.

So, the first object that allows 100% light transmission is a transparent object. The second that has < 100% light transmission is a translucent object, while that completely blocks the light is opaque by nature.

All these materials have some properties that we will understand on this page in detail.

Comparison of Transparent, Translucent, and Opaque Objects

Transparent Substances

Light rays can pass through these substances. The refractive index of transparent substances is nearly uniform. Most of the light, incident on a transparent object, transmits through it. Light rays follow Snell’s law of refraction. While passing through a transparent object, the amount of scattering is very less. Therefore, a clear image is seen on the other side of the substance. Window glasses are transparent in nature. Transparent materials (e.g. glass) are used for lenses, spherical mirrors, spectacles, and many more.

Translucent Objects

A substance is called translucent if it allows partial transmission. The light rays get scattered in the interior of such objects. Consequently, the light rays emerge in random directions. If any object is seen through a translucent material, the image appears fuzzy or blurred. Translucency can occur due to the following properties,

• Non-uniform Density: If a material has a non-uniform distribution of matter, its density is different at different parts. Such density distribution can result in irregular refraction and transmission. Density fluctuations may cause scattering centers. At the points of fluctuations, the light rays get scattered. 

• Crystallographic Defects: Defects (e.g. fluctuation in composition) in a crystal structure can give rise to a scattering of light.

• Boundaries: Grain boundaries (in a polycrystalline structure) and cell boundaries (in an organism) can behave as scattering centers.

Some examples of translucent objects are frosted glass, butter paper, tissue, various plastics, and so on.

Opaque Objects 

Opaque substances do not allow the transmission of light. Any incident light gets reflected, absorbed, or scattered. Light rays can penetrate the substances to some depth. The causes behind opacity are,

• Absorption: The light rays can be absorbed inside the medium. This process highly reduces the intensity of the incident light.

• Scattering: The molecules of the medium can absorb and scatter light in random directions. Due to cumulative scattering, the energy of the wave can be completely dissipated before the emergence of light from the other side.

• Reflection: The incident light rays may get reflected at the surface of an opaque object. The objects appear colored because of the reflection of a particular wavelength. The rest of the wavelengths get absorbed or scattered. 

Common examples of opaque objects are wood, stone, metals, concrete, etc.

Difference Between Transparent and Translucent Materials

Both substances allow light to pass through them. Transparent objects can transmit a significant part of the incoming light. The light is hardly reflected or scattered. Transparent objects appear to be colorless as they do not reflect light. Translucent materials allow partial transmission of light rays. These materials form hazy or blurred images of objects seen through them.

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However, opaque objects are free from all the efforts that transparent and translucent objects make on light; they simply block all the light that is exposed to them.

Did You Know?

• The absorption of light in materials depends on the atomic and molecular structure of the material. Electrons can make transitions into different energy levels by absorbing corresponding wavelengths of light. Energy can also be absorbed due to resonance in molecular vibrations. 

• Metals have plenty of free electrons. When light rays tend to pass through a metallic object, these free electrons absorb and reemit the light rays frequently. This causes a quick attenuation of the incident light, making the substance opaque to radiation.

• Optical fibers, used in communication, have transparent core and cladding. Electromagnetic waves of specific frequencies can transmit through an optical fiber with minimal energy dissipation. The phenomenon of total internal reflection is used in the fibers. 

• Some marine animals (e.g. jellyfish) are nearly transparent. Transparency gives these animals protection from predators. 

• Due to the translucency of pale skin, the blue veins of the human body can be seen through the skin.

• Sometimes opaque and translucent glasses are used in windows to protect privacy. The glasses are transparent from the inside and opaque from outside. 

• Smoke and fog are translucent substances. Objects appear blurred when seen through smoke or fog.