Chapter 7 Physics

Chapter 7.1

Emission & Absorbtion Spectra, and Energy Levels

When white light from the sun passes through a prism, the light is dispersed into a continuous spectrum. Substances emit light when they are supplied with energy to heat them. Each element will have its own emission spectrum.


This can be done by passing a large potential difference across the two electrodes in a tube. Examination of the light with a spectroscope shows the emission spectrum is made up of a few specific wavelengths.

If broad spectrum of white light is passed through a sample of gaseous molecules of an element, light of certain wavelengths will appear to be ’missing’. This pattern of lines is identical to the same element’s emission spectrum.

Emission and absorption spectra

Photons

All electromagnetic radiation travels in discrete energy packets called photons. This means that the transfer of energy is not continuous. The term quantum is used to describe the smallest possible quantity of any energy.

The energy carried by one photon depends only on its frequency, \(f\), as follows:

\begin{equation} E=hf\nonumber \\ \end{equation}

The value of \(h\), plank’s constant, is \(6.63\times 10^{-34}\).


The equation above may also be written as:

\begin{equation} \lambda=\frac{hc}{E}\nonumber \\ \end{equation}

Transition between energy levels

The emission of a light photon transfers energy away from the atom, meaning that the energy within the atom must have decreased. As energy can only be released in the form of photons, energy can only be emitted from atoms in discrete amounts. Therefore, the emission of a photon must be when an electron changes from precise energy level to another precise energy level.

The emission of photons of different energies must mean that each atom has many possible energy levels. If there is a difference in energy, ΔE, between two levels, the frequency of the photon released is given by E = hf.

The lowest possible energy level is called the ground state. Electrons that are in their ground state must be excited to reach to raise them to a higher energy level. When an electron returns from the excited state to the ground state, a photon is released.
When energy is absorbed into an atom, its electric potential energy is increased. When photon released, the electric potential energy decreases. Absorption spectra may be explained by electrons being raised to higher energy levels by absorbing photons with the same energy that can be emitted by that atom.
The atom will quickly release the photon it has absorbed, but in a random direction, meaning that the intensity of the radiation traveling ’through’ the atom has decreased, comparative to those photons that are not absorbed traveling in the same direction.