1. 3.4 Energy levels in atoms
Electrons in atoms
Electrons are attracted to a positive
nucleus by electrostatic force
Electrons move about in allowed orbits
or shells with different energy levels
The lowest energy state of an atom
is called its ground state
When an atom in its ‘ground’ state
absorbs energy, one of its electrons
moves to a shell at higher energy.
The atom is now in an ‘excited’ state
leaving a vacancy in the shell.

2. 3.4 Energy levels in atoms
Electrons in atoms
Electrons are attracted to a positive
nucleus by electrostatic force
Electrons move about in allowed orbits
or shells with different energy levels
The lowest energy state of an atom
is called its ground state
When an atom in its ‘ground’ state
absorbs energy, one of its electrons
moves to a shell at higher energy.
The atom is now in an ‘excited’ state
leaving a vacancy in the shell.

3. 3.4 Energy levels in atoms
Electrons in atoms
Electrons are attracted to a positive
nucleus by electrostatic force
Electrons move about in allowed orbits
or shells with different energy levels
The lowest energy state of an atom
is called its ground state
When an atom in its ‘ground’ state
absorbs energy, one of its electrons
moves to a shell at higher energy.
The atom is now in an ‘excited’ state
leaving a vacancy in the shell.

4. 3.4 Energy levels in atoms
Electrons in atoms
Electrons are attracted to a positive
nucleus by electrostatic force
Electrons move about in allowed orbits
or shells with different energy levels
The lowest energy state of an atom
is called its ground state
When an atom in its ‘ground’ state
absorbs energy, one of its electrons
moves to a shell at higher energy.
The atom is now in an ‘excited’ state
leaving a vacancy in the shell.

5. 3.4 Energy levels in atoms
Electrons in atoms
Electrons are attracted to a positive
nucleus by electrostatic force
Electrons move about in allowed orbits
or shells with different energy levels
The lowest energy state of an atom
is called its ground state
When an atom in its ‘ground’ state
absorbs energy, one of its electrons
moves to a shell at higher energy.
The atom is now in an ‘excited’ state
leaving a vacancy in the shell.

6. De-excitation
An excited atom is unstable and the vacancy in the shell is soon filled by an
electron from an outer shell dropping to a lower energy level emitting a photon.
De excitation of a mercury atom
may proceed indirectly to the
ground state via an intermediate
state.
5.7 eV
4.9 eV
O eV
0.8 eV photon
4.9 eV photon

7. De-excitation
An excited atom is unstable and the vacancy in the shell is soon filled by an
electron from an outer shell dropping to a lower energy level emitting a photon.
De excitation of a mercury atom
may proceed indirectly to the
ground state via an intermediate
state.
5.7 eV
4.9 eV
O eV
0.8 eV photon
4.9 eV photon

8. De-excitation
An excited atom is unstable and the vacancy in the shell is soon filled by an
electron from an outer shell dropping to a lower energy level emitting a photon.
De excitation of a mercury atom
may proceed indirectly to the
ground state via an intermediate
state.
5.7 eV
4.9 eV
O eV
0.8 eV photon
4.9 eV photon

9. De-excitation
An excited atom is unstable and the vacancy in the shell is soon filled by an
electron from an outer shell dropping to a lower energy level emitting a photon.
De excitation of a mercury atom
may proceed indirectly to the
ground state via an intermediate
state.
5.7 eV
4.9 eV
O eV
0.8 eV photon
4.9 eV photon

10. Excitation using photons
An electron in an atom can absorb an incident photon’s energy if the photons
energy exactly matches the quantum of energy required for excitation.

11. Excitation using photons
An electron in an atom can absorb an incident photon’s energy if the photons
energy exactly matches the quantum of energy required for excitation.

12. These atoms then de-excite emitting visible light.
Fluorescence
The atoms of a fluorescent substance may get excited by incident uv light.
These atoms then de-excite emitting visible light.
IR nm
Uv 217 nm
UV nm
In the Starter ( a time delay switch): an electric current warms the filament electrodes.
During the first second argon vapor in the starter conducts and warms up a bimetallic
strip which bends and switches off the current flowing through the filament electrodes.
The mains voltage then acts across the filament electrodes causing the gas to glow.

13. These atoms then de-excite emitting visible light.
Fluorescence
The atoms of a fluorescent substance may get excited by incident uv light.
These atoms then de-excite emitting visible light.
In the Starter ( a time delay switch): an electric current warms the filament electrodes.
During the first second argon vapor in the starter conducts and warms up a bimetallic
strip which bends and switches off the current flowing through the filament electrodes.
The mains voltage then acts across the filament electrodes causing the gas to glow.

14. These atoms then de-excite emitting visible light.
Fluorescence
The atoms of a fluorescent substance may get excited by incident uv light.
These atoms then de-excite emitting visible light.
In the Starter ( a time delay switch): an electric current warms the filament electrodes.
During the first second argon vapor in the starter conducts and warms up a bimetallic
strip which bends and switches off the current flowing through the filament electrodes.
The mains voltage then acts across the filament electrodes causing the gas to glow.

15. These atoms then de-excite emitting visible light.
Fluorescence
The atoms of a fluorescent substance may get excited by incident uv light.
These atoms then de-excite emitting visible light.
In the Starter ( a time delay switch): an electric current warms the filament electrodes.
During the first second argon vapor in the starter conducts and warms up a bimetallic
strip which bends and switches off the current flowing through the filament electrodes.
The mains voltage then acts across the filament electrodes causing the gas to glow.

16. These atoms then de-excite emitting visible light.
Fluorescence
The atoms of a fluorescent substance may get excited by incident uv light.
These atoms then de-excite emitting visible light.
IR nm
Uv 217 nm
UV nm
In the Starter ( a time delay switch): an electric current warms the filament electrodes.
During the first second argon vapor in the starter conducts and warms up a bimetallic
strip which bends and switches off the current flowing through the filament electrodes.
The mains voltage then acts across the filament electrodes causing the gas to glow.

17. These atoms then de-excite emitting visible light.
Fluorescence
The atoms of a fluorescent substance may get excited by incident uv light.
These atoms then de-excite emitting visible light.
IR nm
Uv 217 nm
UV nm
In the Starter (a time delay switch): an electric current warms the filament electrodes.
During the first second argon vapor in the starter conducts and warms up a bimetallic
strip which bends and switches off the current flowing through the filament electrodes.
The mains voltage then acts across the filament electrodes causing the gas to glow.

18. These atoms then de-excite emitting visible light.
Fluorescence
The atoms of a fluorescent substance may get excited by incident uv light.
These atoms then de-excite emitting visible light.
IR nm
Uv 217 nm
UV nm
In the Starter (a time delay switch): an electric current warms the filament electrodes.
During the first second argon vapor in the starter conducts and warms up a bimetallic
strip which bends and switches off the current flowing through the filament electrodes.
The mains voltage then acts across the filament electrodes causing the gas to glow.

19. These atoms then de-excite emitting visible light.
Fluorescence
The atoms of a fluorescent substance may get excited by incident uv light.
These atoms then de-excite emitting visible light.
IR nm
Uv 217 nm
UV nm
In the Starter (a time delay switch): an electric current warms the filament electrodes.
During the first second argon vapor in the starter conducts and warms up a bimetallic
strip which bends and switches off the current flowing through the filament electrodes.
The mains voltage then acts across the filament electrodes causing the gas to glow.

21. Fluorescence
In the Starter ( a time delay switch): an electric current warms the filament electrodes.
During the first second argon vapor in the starter conducts and warms up a bimetallic
strip which bends and switches off the current flowing through the filament electrodes.
The mains voltage then acts across the filament electrodes causing the gas to glow.