What happens when light meets a matter
In this report, we will see what happens when light meets a matter.
Relationship between electron and photon in a matter
A matter composes of atoms. And atom is made up of the central nucleus and the electrons that orbit outer discrete energy orbits. If a photon with energy equal to the energy difference between the electronic orbits is incident, the electron absorbs the photon and moves from the low orbit to the high orbit, but if the energy of the photon is greater or less than this, the electron do not absorb the photon. This is like you can't catch a horizontal bar if you jump too high or too low to catch it. Although a matter consists of many atoms and the electronic energy level of a matter is more complex than that of atom, the principle of photon absorption in a matter is same.

Figure 1. Electronic energy levels of the atom and the matter
Light absorption and reflection in a matter
It can be explained why leaves look green using electronic energy levels. This is because leaves do not have an electronic energy level that can absorb green light, so green light is emitted without being used. In other words, the chlorophyll of the leaves absorbs the blue and red light and reflects the green light, so the leaves look green. We perceive the color of reflected light by matter as the color of a matter.

Figure 2. Green leaves
The electrons emission from a metal by light
If light with energy that is higher than the bonding energy between electrons and a metal is incident to a metal, metal emits electrons as well as light. This is because an incident light with higher energy makes the electrons separate and escape from the metal. The solar cells in an electronic calculator generate electricity from light using this phenomenon.
Sung Soo Kim Ph.D

* Next week’s topic: Effect of Artificial Lighting on Amphibians
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