Everything Science Grade 12

CHAPTER 16. OPTICAL PHENOMENA; PROPERTIES OF MATTER 16.4

Figure ?? shows an example of this happening. When anelectron in an atom fallsfrom
a higher energy level toa lower energy level, itemits a photon to carryoff the extra
energy. This photon’s energy is equal to the energy difference between the two energy
levels. As we previously discussed, the frequency of a photon is relatedto its energy
through the equation E = hf. Since a specific photonfrequency (or wavelength) gives
us a specific colour, wecan see how each coloured line is associated with a specific
transition.

Energy

Level

1

2

3

4

5

6

∞

0 J

16 , 3 × 10 −^19 J

19 , 4 × 10 −^19 J

20 , 5 × 10 −^19 J

21 , 0 × 10 −^19 J

21 , 8 × 10 −^19 J

21 , 3 × 10 −^19 J

ultraviolet

visible light

infrared

ground state

Energy

Figure 16.5: In this diagram are shown someof the electron energylevels for the
hydrogen atom. The arrows show the electron transitions from higher energy levels to
lower energy levels. The energies of the emitted photons are the sameas the energy
difference between twoenergy levels. You canthink of absorption as the opposite
process. The arrows would point upwards and the electrons would jump up to higher
levels when they absorba photon of the right energy.

Visible light is not the only kind of electromagnetic radiation emitted. More energetic
or less energetic transitions can produce ultraviolet or infrared radiation. However,
because each atom hasits own distinct set of energy levels (its fingerprint!), each atom
has its own distinct emission spectrum.

Absorption spectra ESCHK

As you know, atoms donot only emit photons;they also absorb photons. If a photon
hits an atom and the energy of the photon is thesame as the gap between two electron
energy levels in the atom, then the electron can absorb the photon and jump up to the
higher energy level. If the atom has no energy level differences that equalthe incoming
photon’s energy, it cannot absorb the photon, and can only scatter it.

Using this effect, if we have a source of photonsof various energies we can obtain the