Electromagnetic radiation part 2

  Interaction of electromagnetic radiation with matter

It is well known that all matter is comprised of atoms. But subatomically, matter is 

made up of mostly empty space. For example, consider the hydrogen atom with its one 

proton, and one electron. The diameter of a single proton has been measured to be 

about 10-15 meters. The diameter of a single hydrogen atom has been determined to be 

10-10 meters; therefore the ratio of the size of a hydrogen atom to the size of the proton 

is 100,000:1. Consider this in terms of something more easily pictured in your mind. If 

the nucleus of the atom could be enlarged to the size of a softball (about 10 cm), its 

electron would be approximately 10 kilometers away. Therefore, when 

electromagnetic waves pass through a material, they primarily move through free 

space, but may have a chance to encounter with the nucleus or an electron of an atom.

Because the encounters of photons with sub atomic particles are by chance, a given 

photon has a finite probability of passing completely through the medium it is 

traversing. The probability that a photon will pass completely through a medium 

depends on numerous factors including the photon’s energy and the composition and 

thickness of the medium. The more densely packed a medium’s atoms, the more likely 

the photon will encounter an atomic particle. In other words, the more subatomic 

particles in a material (higher Z number), the greater the likelihood that interactions 

will occur Similarly, the more material a photon must cross through, the more likely 

the chance of an encounter.

When a photon does encounter an atomic particle, it transfers energy to the particle. 

The energy may be reemitted back the way it came (reflected), scattered in a different 

direction or transmitted forward into the material. Let us first consider the interaction 

of visible light. Reflection and transmission of light waves occur because the light 

waves transfer energy to the electrons of the material and cause them to vibrate. If the

material is transparent, then the vibrations of the electrons are passed on to 

neighboring atoms through the bulk of the material and reemitted on the opposite side 

of the object. If the material is opaque, then the vibrations of the electrons are not 

passed from atom to atom through the bulk of the material, but rather the electrons 

vibrate for short periods of time and then reemit the energy as a reflected light wave. 

The light may be reemitted from the surface of the material at a different wavelength, 

thus changing its color.

The interactions between electromagnetic radiation and matter cause changes in the 

energy states of the electrons in matter.

Electrons can be transferred from one energy level to another, while absorbing or 

emitting a certain amount of energy. This amount of energy is equal to the energy 

difference between these two energy levels (E2

-E1

).

When this energy is absorbed or emitted in a form of electromagnetic radiation, the 

energy difference between these two energy levels (E2

-E1

) determines uniquely the 

frequency (λ) of the electromagnetic radiation:

(  E) = E2

-E1 = h

1.4.1 Absorption

Absorption of electromagnetic radiation is the way in which the energy of a photon is 

taken up by matter, typically the electrons of an atom. Atoms or molecules absorb 

light, the incoming energy excites a quantized structure to a higher energy level. The 

type of excitation depends on the wavelength of the light. Electrons are promoted to 

higher orbitals by ultraviolet or visible light, vibrations are excited by infrared light, 

and rotations are excited by microwaves.

An absorption spectrum is the absorption of light as a function of wavelength. The 

spectrum of an atom or molecule depends on its energy level structure, and absorption 

spectra are useful for identifying of compounds.

Emission

The emission spectrum of a chemical element or chemical compound is the spectrum 

of frequencies of electromagnetic radiation emitted due to an atom's electrons making 

a transition from a high energy state to a lower energy state. Atoms or molecules that 

are excited to high energy levels can decay to lower levels by emitting radiation 

(emission or luminescence). For atoms excited by a high-temperature energy source 

this light emission is commonly called atomic or optical emission, and for atoms 

excited with light it is called atomic fluorescence. For molecules it is called 

fluorescence if the transition is between states of the same spin (singlet to singlet 

transition) and phosphorescence if the transition occurs between states of different spin

(triplet to singlet transition).

The emission intensity of an emitting substance is linearly proportional to analyte 

concentration at low concentrations, and is useful for quantitating emitting species.

1.4.3 Transmission

Transmission happens when light goes through a surface or object. There are 3 types 

of transmission: direct, diffuse or selective.

1. Direct transmission: it is when light goes through an object and no change in 

direction or quality takes place. For example, through glass or air.

2. Diffuse transmission: it is produced when light goes through a transparent or semi-

transparent object with texture. For example, frosted glass or drafting paper. Light, 

instead of going in one direction, is redirected to other directions. Light which is 

transmitted in a diffused manner tends to be softer; it will have less contrast and 

less intensity; it will generate clearer shades; and it will have a smoother transition 

between highlights and shadows than direct light.

Selective transmission: it is produced when light goes through a coloured object. A 

portion of light will be absorbed and another portion will be transmitted through this 

object. In the example below, white light (red, green and blue) goes through a red 

surface. The green and blue are absorbed and only red is transmitted. As a result, we 

will only see red light on the other side of this surface.

Filters or gels, which we mentioned in the lesson about colour temperature, work 

through selective transmission. Colour filters will only allow one colour to go through 

(a blue filter allows only blue light go through) and it will absorb the rest of the 

colours. A blue filter lets blue wave lengths through and absorbs red and green wave 

lengths.

1.4.4. Reflection

Reflection happens when light reaches an object and it bounces or is reflected, 

partially or totally, from this object. Light can be reflected directly or in a diffused 

manner.

1. Direct reflection: it is produced when light is reflected from a flat or smooth

surface such as, for example, a mirror. Light will be reflected in the same angle as it 

reached this surface (law of reflection).