Transmission, Absorption and Reflection of Light

Light is the only electromagnetic radiation that is visible to the human eye. It is made of little packets of energy known as photons. When a light wave is incident on an object, a number of things can happen. It is either absorbed, transmitted or reflected. The manner in which visible light interacts with an object depends on the frequency of light and the nature of the atoms of the object. In this article, we will discuss how and why light of certain frequencies are selectively absorbed, reflected and transmitted.

What causes the Absorption of Light?

At specific frequencies, the electrons of the atom tend to vibrate. When a light wave of the same natural frequency is incident on the atom, the electrons of that atom will be set into vibrational motion. During vibration, electrons interact with neighbouring atoms in such a manner it converts the vibrational energy into thermal energy. Hence, we can conclude that the selective absorption of light occurs when the frequency of the light matches the frequency at which the electrons in the atoms vibrate. Different atoms and molecules possess different natural frequencies of vibration, hence they will selectively absorb different frequencies of visible light.

Light Reflection and Transmission

Contrary to the absorption of light, transmission and reflection of light occurs when the natural frequency of the vibration of electrons do not match the frequencies of the incident light. In such cases, when the light wave strikes the objects, the electrons of the object begin to vibrate. The electrons vibrate for a brief period of time with small amplitude after which the energy is reemitted as a light wave. If the object under consideration is transparent, the vibrations of the electrons are passed on to the neighbouring atoms and are reemitted from the opposite side of the object. In such cases, the light is said to be transmitted. If the object is opaque, the vibrations of the electrons are not passed in bulk, rather the electrons of the atoms vibrate for a short period of time and are reemitted as a reflected light wave from the surface of the object.

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How do we see Colours?

The colour of the objects that we see is largely due to interaction of the object with the incident light. The colour of the object is not within the object itself. Rather, it is in the light that shines upon it. When the light of a certain frequency falls on the object, the light frequencies that are absorbed never make it to our eyes. Only the frequencies of incident light that are reflected or transmitted contributes to the colour of the object. So if an object absorbs all the frequencies of the light wave except the frequency associated with green, then the object appears green.

Check Your Understanding

1. What colour would a blue shirt appear when the lights are turned off and the room is entirely dark?

When the room lights are turned off, any object present in the room appears black. The colour of the object depends on the frequency of the light that is reflected to the observer’s eye. Without light, there is no reflected light as a result of which the green shirt appears black.

2. One common historical belief was that coloured objects in nature produce small particles (perhaps light particles) that subsequently reach our eyes. Different objects produce different coloured particles, thus contributing to their different appearance. Is this belief accurate or not? Justify your answer.

This view presumes that the appearance of an object is independent of the colours of light which illuminate the object. It must be noted that the same objects appear to be of different colour under different light. Hence, the colour of an object is strictly not due the object’s ability to produce a colour. The only role the object plays in determining its appearance is by absorbing certain wavelengths of incident light.

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