Sometimes we use the wave model for light …
• λ (lambda)= wavelength

Wavelength increases, Frequency ?
decreases
C

c = speed of light = 3.00 x
10 8 m s -1 c = λ ν λ = wavelength m lambda
ν = frequency Hz
( or s -1 ) nu
c
 
λ  c
υ
  c

Use m for λ

C = λν problems….
c = 3.00x10
8 m s -1
Work out λ:
1. If ν is 3.00 m
2. If ν is 30.0 cm
3. If ν is 3.00 mm
Work out λ:
4. If ν

Sometimes we use the particle
E
 h
 model …  
4 .
5 x 10
14 Hz h = 6.63 x 10 -34 J Hz -1
Planck constant
• Packages of energy are called photons
• Light is a stream of photons
• E = 6.63 x10 -34 x 4.5 x 10 14 = 3.0 x 10 -19 J

h
E
E
 h

 
E h


• h = 6.63 x 10-34 J Hz -1
• Planck constant
5. If the frequency is 1.089 x 10 6 Hz, what is the energy of each photon?
6. If E = 3.65 x 10 -20 J per photon, what is the frequency ( ν) of the radiation?

Emission Spectrum of hydrogen.
• Black background
• Coloured lines.

• Prism
• Sample of hydrogen
• High voltage

 

E h
ΔE
• Electrons can only exist in fixed energy levels.
• Electrons absorb energy and move to a higher energy level.
• The electrons drop back to a lower energy level and emit energy.
• The frequency of the radiation emitted depends on ΔE.

3  2
4  2
?

7 etc
6
5
4
3
2
1
Why no
• 7  2
• 8  2
• 9  2?

visible ultra violet
E increases

E increasing
Balmer  2
Lyman  ?
1
6
4
3
2
1
4
3
2
1

Absorption Spectrum of hydrogen.
• Continuous spectrum
• Black lines.

How do the 2 compare?

• Electrons absorb energy
• Electrons are excited
• Electrons move to higher energy level
• E = h v
4
3
2
1

• Particles in the chromosphere absorb some of the light
• Photosphere of hot stars emit visible or
UV light

• Every element has a different absorption spectrum
H
He
Fe
• Have a look at assignment 5

Ionisation energy

+
-
Ground state electron?
4
3
2
1
Energy needed to remove one e from 1 mole of atoms of a gaseous element

Convergence limit = 3.27 x 10 15 Hz
7. Use ΔE = hν to calculate the energy per photon corresponding to this frequency (in J). h = 6.63 x 10-34 J Hz -1 .
8. Ionisation energy has units of kJ mol -1 . You have calculated the energy required to ionise a single atom. Work out the ionisation energy for hydrogen.
L = 6.02 x 10 23 mol -1 .