1.3 Interactions between Matter and Electro-magnetic Radiation

All matter reflects, absorbs, penetrates and emits electro-magnetic radiation in a unique way. For example, the reason why a leaf looks green is that the chlorophyll absorbs blue and red spectra and reflects the green spectrum (see 1.9). The unique characteristics of matter are called spectral characteristics (see 1.6). Why does an object have a peculiar characteristic of reflection, absorption or emission? In order to answer the question, one has to study the relation between molecular, atomic and electro-magnetic radiation. In this section, the interaction between hydrogen atom and absorption of electro-magnetic radiation is explained for simplification.

A hydrogen atom has a nucleus and an electron as shown in Figure 1.3.1. The inner state of an atom depends on the inherent and discrete energy level. The electron's orbit is determined by the energy level. If electro-magnetic radiation is incident on an atom of H with a lower energy level (E1), a part of the energy is absorbed, and an electron is induced by excitation to rise to the energy level (E2) resulting in the upper orbit.

The electro-magnetic energy E is given as follow.
E = hc /
where h : Plank's constant
c : velocity of light
: wavelength

The difference of energy level
E = E2 - E1 = hc / H is absorbed.

In other words, the change of the inner state in an H-atom is only realized when electro-magnetic radiation at the peculiar wavelength lH is absorbed in an H-atom. Conversely electro-magnetic radiation at the wavelength H is radiated from an H-atom when the energy level changes from E2 to E1.

All matter is composed of atoms and molecules with a particular composition. Therefore, matter will emit or absorb electro-magnetic radiation at a particular wavelength with respect to the inner state.

The types of inner state are classified into several classes, such as ionization, excitation, molecular vibration, molecular rotation etc. as shown in Figure 1.3.2 and Table 1.3.1, which will radiate the associated electro-magnetic radiation. For example, visible light is radiated by excitation of valence electrons, while infrared is radiated by molecular vibration or lattice vibration.

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