Materials can be categorised based on their composition, atomic structure, hardness and several other parameters. One of the key factors of categorisation is based on the extent of light they transmit. Those materials that allow complete transmission of light are called transparent materials. A typical example is a glass. Materials that permit the partial transmission of light are translucent, and some examples are plastics like polyethene sheets, oily paper, etc. Materials that don’t allow light transmission are called Opaque materials, examples being wood, stone, etc.
Example: The above picture shows three different glasses and the extent of light transmitted through them, as seen in the shadow behind them. Hence they are transparent, translucent and opaque glasses, respectively – viewed from left to right.
Now let’s look at the science behind why some materials allow light transmission- fully or partially, and some don’t allow any light to be transmitted through them.
We’ll briefly look at key laws or principles relevant to the study of transparent, translucent and opaque materials and why they exhibit their unique properties.
Refractive Index: Refractive Index measures the bending of a light ray when it passes from one medium to another. It is also called as Index of Refraction.
Distinguishing between Transparent, Translucent and Opaque Objects
Let us look at How to distinguish between transparent, translucent and opaque objects.
Transparency is the property of allowing light to pass through the material with a negligible scattering of light.
In transparent materials, light rays pass through them. Most of the light that falls on them gets transmitted. The Refractive Index There is little scattering of light as it passes through a transparent material. Because of this factor, a clear image can be seen on the other side of the transparent object.
Example: Lenses used in Spectacles; glasses used in windows, and Glass Jug and crockery glasses are transparent. You can recollect several examples of transparent materials used in your daily life.
Translucency is the property of allowing light to pass through the material, with light rays getting scattered.
A material is called translucent when it allows partial transmission of light through it. The light rays passing through it get scattered and emerge out from the materials in random directions. Because of this, any object seen through translucent materials appears blurred or fuzzy.
Translucency in materials is because of the following properties:
- Non-uniform density: When the matter is not uniformly distributed in material, its density is not uniform. This non-uniform density results in irregular light transmission and refraction across the material. Light rays get scattered at points of such density variation.
- Crystallographic Imperfections or defects: Variation in the composition of the crystalline structure can also result in the scattering of light rays in a material.
- Grain Boundaries: Grain boundaries in the crystalline structure act as scattering medium. Examples: Butter paper used in baking; Tissue paper, Plastic – polyethene sheet; Frosted glass in bathroom windows.
Opacity is the property of not allowing any light to transmit through a material.
Opaque materials do not allow light to be transmitted through them. Light rays can penetrate the material to a certain depth. Light passing through such material either gets reflected, scattered or absorbed.
Reasons for opacity are:
- Absorption: The light rays passing through a material or object can be absorbed inside the medium. This results in a significant reduction in the intensity of light rays.
- Scattering: Molecules of the material can scatter the light rays in different directions. The total of all light rays passing through a material getting scattered means the energy of the light is completely dissipated before it emerges from the other side.
- Reflection: Light rays falling on a material surface gets reflected. The material appears coloured because of the reflection of particular wavelengths. Other wavelengths may either be scattered or absorbed.
Examples: Wood used in furniture; Stone used for construction; concrete roofs and concrete roads; Utensils of various metals.
Factors that help Distinguish between transparent and translucent materials:
The main difference between transparent and translucent materials is the physical properties that determine the amount of light allowed to pass through them. The table below shows the key differences between transparent and translucent materials.
|Allows all light rays to pass through it without being scattered||Allows part of the light rays to pass through it, most of it being scattered|
|Follows laws of refraction||They do not follow laws of refraction|
|Allows full images to be formed when seen on the other side||Allows only partial images to be seen on the other side|
|Transparent material appears clearly||Translucent material appears blurred, frosty or stained.|
Key facts to know:
- A materials’ atomic and molecular structure determines its light transmission or absorption characteristics.
- Some sea animals like Jellyfish are transparent. This gives them the ability to ward off potential predators.
- Blue veins in humans can be seen due to translucency of the skin.
- Translucent and opaque glasses are used in house windows to give privacy. Some of these window glasses are transparent from the inside and opaque from the outside.
- Metals have lots of free electrons. Light rays pass through them or are absorbed or re-emitted, making them opaque to radiation.
We use transparent, translucent and opaque materials in our daily life, sometimes without understanding adequately the reasons that make them exhibit these unique properties. This article should help better understand the properties that provide unique characteristics and enhance their utility value.
- Why are transparent materials colourless?
The colour of any material depends on the frequency or wavelength of the light it reflects. A red cricket ball appears Red because it reflects red light and absorbs all other wavelengths. Transparent material doesn’t much reflect (allowing light to pass through it). Hence transparent material appears colourless.
- What is the difference between transparent and translucent materials
The property of materials to transmit, scatter or prevent light through them determine this.
Transparent materials allow all the light to pass through them;
Translucent materials allow only partial light to pass through them;
Opaque materials prevent light from passing through them