How Light Interacts with Materials: Transmission, Absorption, and Reflection
You must have studied that light is a wave and the color of an object is related to the frequency of light waves it emits.
High-frequency light means a light with high-energy, so, red light is a high-frequency wave light, whereas violet is a low-frequency wave light.
In between these two frequencies, we have yellow, green, orange, and blue.
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We call these colors as physical colors because the color is a physical property of light.
These colors don’t rely on human perception.
For example, you might have seen this picture earlier.
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As you can see, the region where the blue and yellow lights overlap is green.
However, you are aware of the fact that light is a wave, and two different frequencies can’t interact with each other at all.
They should coexist like musicians playing bands in harmony.
So, we conclude that in this green looking region, the blue and the yellow color are present; however, green color is not present in this region.
Now, let’s see why we see a green color and how we perceive colors.
How do we Perceive Color?
We know that light perception happens in a hairline layer of cells called the retina which covers the back of our eye; it has two types of light-detecting cells, rods, and cones.
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Rods: Used for seeing low-light conditions (It’s of one kind).
Cones: Comprises of three kinds, each corresponding to red, green, and blue color respectively.
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When we see any color, each color sends its distinctive color signal to our brain.
Let’s say, my electric bulb is emitting yellow light with its particular frequency and my cones can’t detect this color, but it is kind of close to green and also to red.
So each color sends signals to my brain.
Therefore, it is possible to activate red and green cones when red and green lights are present at the same time.
However, the brain still perceives it as a yellow color, whether it is a mixture of green or red frequencies.
That’s why for light, red + green = yellow.
When it’s dark, our rods, which are of one kind can detect only one color at a time, either light or no light because only one signal gets into our brain.
How do Humans See Color?
Since there is an infinite number of colors and we have only three cones to detect the light; this is the matter where the mind gets tricked into seeing any color by carefully adding the colors to get the right combination of three colors i.e., red, green, and blue.
You can use such properties discussed above in day-to-day activities.
For example, a TV set has three colors: RGB.
Reflection Refraction Absorption Transmission
When any light passes from rarer medium to the denser medium, one of the three things happen:
Reflection: The light may get reflected from the surface.
Absorption: This happens when the light gets converted to another form of energy. This occurs when light disappears as it passes through another medium.
Transmission and Refraction: The light could be transmitted, which means it may pass easily through another medium or may get refracted.
So, when the light of any color interacts with the medium; some could be reflected, absorbed, transmitted, or refracted.
So, this is how we can see the color of an object.
If you look at the figure below:
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Draw a perpendicular line, i.e., normal.
As the light bounces off on the surface, the angle of incidence becomes equal to the angle of reflection.
This is what you can see in the reflection of trees over the lake:
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Here, the light from the top of the tree is reflected at an angle off the surface of the water which is going to our eye and that’s why it appears as it is down in the lake.
Absorption happens when the light hits the surface and gets converted to another form of energy.
For example, the color of the leaf is green because it reflects the green color while the other colors like red and blue light are absorbed.
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If we look at the absorption spectrum of the leaf.
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There are two pigments, chlorophyll A and B. They absorb most of the bluish-purple light and most of the reddish-yellow light but they absorb only a fraction of the green light. This is the reason green light gets reflected.
We might have seen leaves having a reddish-blue or purple color at the bottom.
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The reason they have layers is to reflect the backup of light through the leaf so they can get more of the heat energy from the sunlight.
The refraction of moonlight or sunlight by ice crystals results in numerous beautiful optical effects like halos that are produced when sunlight or moonlight is refracted by the pencil-shaped ice crystals of cirriform clouds.
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FAQs on Transmission, Absorption, and Reflection of Light: Key Concepts
1. What is the fundamental difference between the transmission, absorption, and reflection of light?
When light strikes an object, it can interact in three primary ways. The key difference lies in the path the light energy takes:
- Transmission: This occurs when light passes through a material. Materials that allow this are called transparent (e.g., glass) or translucent (e.g., frosted glass).
- Absorption: This happens when the light energy is taken in by the material and converted into another form, usually heat. The object neither reflects nor transmits this light. A black surface is a good example of high absorption.
- Reflection: This is when light bounces off the surface of an object. This is how we see non-luminous objects, like a book or a table.
2. How are transparent, translucent, and opaque materials defined by their interaction with light?
These terms describe how much light can pass through a material:
- Transparent materials, like clear glass or water, allow almost all light to pass through them, enabling clear vision of objects on the other side. They exhibit high transmission and low reflection and absorption.
- Translucent materials, such as butter paper or frosted glass, allow some light to pass through but scatter it in the process. You can see light through them, but not clear images. They exhibit partial transmission and partial reflection/scattering.
- Opaque materials, like a wooden block or a metal sheet, do not allow any light to pass through. Light that strikes them is either reflected or absorbed.
3. What is the Law of Reflection?
The Law of Reflection governs how light bounces off a smooth, reflective surface. It consists of two key principles:
1. The angle of incidence (the angle between the incoming light ray and the normal) is equal to the angle of reflection (the angle between the reflected ray and the normal).
2. The incident ray, the reflected ray, and the normal to the surface all lie in the same plane. This principle is fundamental to understanding how mirrors and other reflective surfaces work.
4. What are some real-world examples of light transmission?
Light transmission is a common phenomenon we observe daily. Key examples include:
- Light from the sun passing through the Earth's atmosphere.
- Vision through eyeglasses, where light is transmitted and refracted by lenses to focus on the retina.
- Light passing through a glass of water, which can also cause the light to bend or refract.
- The operation of fibre optic cables, which transmit light signals over long distances with minimal loss.
5. How does the absorption and reflection of light determine the colour of an object?
The colour we perceive is determined by the wavelengths of visible light an object reflects. White light is a mixture of all colours (wavelengths). When it hits an object:
- If the object appears red, it's because it absorbs all other colours of the spectrum and reflects only the red light, which reaches our eyes.
- A white object reflects all colours, so we see a combination of them, which appears as white.
- A black object absorbs all colours and reflects very little or no light, which is why it appears black.
Therefore, colour is not a property of the object itself, but of the light it reflects.
6. What happens to the energy of light when it is absorbed by a surface?
According to the law of conservation of energy, the absorbed light energy does not disappear; it is converted into other forms. Most commonly, it transforms into thermal energy (heat). This is why dark-coloured objects, which absorb more light, get hotter in the sun than light-coloured ones. In other specific cases, the energy can be converted into:
- Chemical energy during photosynthesis in plants.
- Electrical energy in solar panels (photovoltaic cells).
7. Can a single object exhibit reflection, absorption, and transmission at the same time?
Yes, most real-world objects interact with light in all three ways simultaneously, though one interaction may be dominant. A perfect example is a tinted car window or a piece of coloured glass. When light hits it:
- A small portion is reflected from the surface.
- A significant portion is absorbed by the tinting material, which causes the glass to warm up.
- The remaining light is transmitted through the glass, allowing visibility but with reduced intensity.
The balance between these three processes defines the optical properties of the material.
8. What is the difference between specular and diffuse reflection?
The key difference lies in the texture of the reflecting surface:
- Specular Reflection occurs on smooth, polished surfaces like a mirror or calm water. Parallel incident light rays are reflected as parallel rays in a single direction, creating a clear, mirror-like image.
- Diffuse Reflection occurs on rough or uneven surfaces like paper, wood, or a wall. When parallel incident light rays strike the surface, they are reflected in many different directions. This is why you can't see your reflection in a piece of paper, but it allows the surface to be visible from any angle.
