Transmission, Absorption, and Reflection of Light

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.

(image will be uploaded soon)

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.

(image will be uploaded soon)

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.

(image will be uploaded soon)

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.

(image will be uploaded soon)

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:

1. Reflection: The light may get reflected from the surface.

2. Absorption: This happens when the light gets converted to another form of energy. This occurs when light disappears as it passes through another medium.

3. 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:

(image will be uploaded soon)

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:

(image will be uploaded soon)

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.

(image will be uploaded soon)

If we look at the absorption spectrum of the leaf.

(image will be uploaded soon)

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.

(image will be uploaded soon)

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.

(image will be uploaded soon)