Atomic Spectra
Physics: IB Diploma Higher Level, topic 5.1.1
Absorption Spectra
Electrons in atoms can only move between discrete energy levels. So an atom can only absorb or emit photons with particular frequencies (or wavelengths).
White light - continuous spectrum
- White light contains all the colors in the visible part of the spectrum.
- When white light is passed through a prism or a diffraction grating, the colors are split up to produce a continuous spectrum.
- Continuous means there aren't any gaps or black lines.
Cool gas - absorption spectra
- A cool gas means a gas containing atoms in their ground state.
- When white light is passed through a cool gas, electrons in the ground state of the atom absorb certain frequencies of light and become excited.
- Most frequencies of the white light are not absorbed because they do not correspond to the difference between two energy levels in the atom.
Absorption spectra 2
- If the light coming out of the cool gas is split by a prism (or diffraction grating), we see a continuous spectrum with black lines.
- These lines are called absorption lines.
- They are unique to each type of atom.
Emission Spectra
Electrons in atoms can only move between discrete energy levels. So an atom can only absorb or emit photons with particular frequencies (or wavelengths).
Excited gas
- An excited gas contains atoms in excited energy states.
- These excited atoms contain electrons in high energy levels.
- As these electrons de-excite and fall back to lower energy levels, photons are emitted.
Photon emission
- These photons carry away energy from the atoms.
- The amount of energy (and so what frequency) the photons have depends on the difference in energy levels in the atom.
- For every possible electron transition, there will be a unique frequency photon emitted.
- We see a series of bright lines when we pass the emitted light through a prism or diffraction grating.
Emission vs absorption
- Compare the emission and absorption spectra produced by this gas.
- We can see that the emission lines exactly match the frequencies of the absorption lines.
- This is because the lines correspond to the same energy differences in the atoms. The same energy photons are either emitted or absorbed.
Test yourself on Atomic Spectra
When white light passes through a cool gas what happens?
Which of these statements about absorption spectra are true?
When an electron falls to a lower energy level what happens?
Which of these is true about emission spectra?