Light absorption

Title : Light absorption

Writer : Charles university 2nd faculty of medicine, Junghoo Lee

Table of contents

-Definition of "Light" -Mechanism, example(phenomenon) of Light absorption -References

-DEFINITION OF "LIGHT"

First, before talking about mechanism of light absorption we need to know what is "Light". Since many centuries ago, lots of theories have been suggested by scientists. The main arguments about "what is light?" is between Particle theory and Wave theory. At first, "Particle theory" was proposed and supported by Pierre Gassendi, Isaac Newton. Especially, Newton stated that light was composed of particles which were emitted in all directions from a source. One of Newton's main arguments against the wave nature of light was that if the light was wave, then that would bend around obstacles, but light moved only in straight directions. However, he explained the phenomenon of the diffraction of light by allowing that a light particle could create a localized wave in the aether. So that tells us this theory cannot cover whole phenomenon.

In the 1660s, on the other hand, Robert Hooke proposed "Wave theory" of light. Which means, literally light is composed of waves and have the nature of wave. He proposed that light was emitted in all directions as a series of "waves". As like waves do, the wave theory predicted that light waves could interfere with each other like sound waves which is nature of general wave, and that light could be polarized, if it were a transverse wave. Thomas Young(He also supported wave theory) showed by means of a diffraction experiment that light behaved as like waves. Also, Young suggested that different wavelength of light cause different color of light. That explains how the three colored receptors in our eyes works.

After the Wave theory proposed, still there are lots of arguments between them. In 1900s, Max Plank suggested the new concept which is called "Black body radiation". Which is a starting point of "Quantum Theory". This new concept also suggested that although light was "wave", this wave could gain or lose energy only in finite amounts related to their frequency. Plank defined "quanta" which is a energy of light. Then, Albert Einstein used idea of quanta to explain the photoelectric effect, and proposed that these quanta had a existence for real. But like others, Quantum theory also couldn't cover whole character of light.

Finally, the newest "Electromagnetic theory" which explains all visible light and all Electromagnetic radiation is proposed by James Clerk Maxwell. Maxwell discovered that self-propagating electromagnetic waves would move through space at a constant speed, which happened to be same to the speed of light which was previously measured. From these results, Maxwell concluded that light was a form of EM radiation. Therefore, now we can conclude that the light is a form of electromagnetic radiation.

-MECHANISM, EXAMPLE(PHENOMENON) OF LIGHT ABSORPTION

light is absorbed and converted into energy. In the case of plants, this process is known as photosynthesis. However, they are by no means the only species or objects that do this. Actually, all creatures or inorganic are capable of absorbing light. In all cases, absorption depends on the electromagnetic frequency of the light being transmitted (For example, the color) and the nature of the atoms of the object. Which means the absorption of light is related to the frequency very closely. If they are complimentary, light will be absorbed. If they are not complimentary, then the light will be transmitted or reflected. In most cases, these processes occur at the same time. Since light is usually transmitted at various frequencies. (For example, the "natural light" which is made from Sun is also composed of lights of various frequencies; around 400~800nm EM radiation) So, most of objects will selectively absorb the light and also transmitting or reflecting some of the light. If absorption happened, heat energy will be generated.

Let's imagine that we are walking around the park with lots of grass and plenty of beautiful flowers. As you already know, all living things have their own color. We can infer from this, that all livings or inorganic things are reflecting and also absorbing the light simultaneously. As like the above, every matters have their own specific frequency so if the frequencies are complimentary then it will be absorbed and on the other hand, if the frequencies are not complimentary light will be reflected or transmitted. The colors we can see around is the result of transmission or reflection of light which is caused by non complimentary frequencies.

Absorption depends upon the state of an objects electrons. All electrons are known to vibrate at some-which is specific-frequencies, what is commonly known as their "natural" frequency. When light interacts with an atom with the same natural frequency, the electrons of that atom will become excited and set into a natural vibrational motion. During this vibration, the electrons of the atom interact with neighboring atoms in such a way as to convert this vibrational energy into thermal energy. Subsequently, the light energy is not to be seen again, hence why absorption is differentiated from reflection and transmission. And since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies of visible light.

Ultimately, absorption of electromagnetic radiation requires the generation of the opposite field, in other words, the field which has the opposite coefficient in the same mode. A good demonstration of this is color. If a material or matter absorbs light of certain wavelengths (or colors) of the spectrum, an observer will not see these colors in the reflected light. On the other hand if certain wavelengths of colors are reflected from the material, an observer will see them and see the material in those colors. For example, the leaves of green plants contain a pigment called chlorophyll, which absorbs the blue and red colors of the spectrum and reflects the green. Leaves therefore appear green, whereas reflected light often appears to the naked eye to be refracted into several colors of the spectrum. As a result, light absorption is related to matter's frequency(and frequency of light also) and wavelength of light.

-REFERENCES

- http://en.wikipedia.org/wiki/Absorption_%28electromagnetic_radiation%29 - http://navercast.naver.com/contents.nhn?contents_id=930 (Journal written by Dr. Joonwoo Park)