Electrons and Light
Light’s relationship to matter
• Atoms can absorb energy, but they
must eventually release it
• When atoms emit energy, it is
released in the form of light =
emission spectrum
• Atoms don’t absorb or emit all
colors, only very specific
wavelengths; the spectrum of
wavelengths can be used to identify
the element
Line Spectra= specific wavelengths are
emitted; characteristic of atoms
• Light has particle-like behavior
• Photons (light) come from electrons
falling from high electron orbits to
low orbits.
The Wave Description of Light
• Light must be a wave,
because…
–Light is refracted in
lenses.
–Light can be
diffracted.
Light is a Particle
• Light can travel through the
vacuum of space, but
waves can’t travel in a
vacuum. So light must be a
particle!
• Light is Both
– light is both a wave and a
particle. Packets of light we call
photons.
• Visible light is a kind of electromagnetic radiation.
• Electromagnetic radiation is a form of energy that
exhibits wavelike behavior as it travels through
space.
• Examples of EMR include X rays, ultraviolet light,
infrared light, microwaves, and radio waves.
• All forms of EMR move at a constant speed.
– 3.0 x 108 m/s = speed of light (c)
1. Which color of light has a longer wavelength, green or
yellow? ______________
2. Which color of light has a higher frequency? Violet or
Red? ______________
3. Which color of light has the highest energy? Violet or
Red?
4. What is the wavelength of red light? ______________
5. What is the wavelength of violet light? _____________
Characteristics of a Wave
• Significance of wave motion is its repetitive nature: waves
have measurable properties of wavelength and frequency.
• Wavelength is the distance between corresponding points
on adjacent waves.
• Frequency is the number of waves that pass a given point in
a specific time (usually one second).
Which wave has a higher frequency? ________
A
B
Frequency and wavelength are
mathematically related
V=f x λ
V = velocity
F = frequency
λ = wavelength
EX 1: A wave has a frequency of 30 Hz and a
wavelength of 5 m, what is its velocity?
EX 2: A wave has a frequency of 10 Hz and a
wavelength of .01 m, what is its velocity?
• Scientists in the early 20th century,
Max Planck and Albert Einstein
showed that electromagnetic
radiation was composed of
particles we call photons (photons
are particles of light energy).
• Each wavelength of light has
photons that have a different
amount of energy.
• the longer the wavelength, the
lower the energy of the photons.
Dual Nature of Light
• From Einstein's famous equation E = mc2
E = mc2
m=E
c2
E= energy
m= mass c2 = speed of light
• Energy has an apparent mass.
• The apparent mass of a photon depends on its
wavelength.
Einstein (1905)
• Explained the photoelectric effect by proposing
that EMR is absorbed by matter only in whole
numbers of photons.
• Concluded that light has properties of both waves
and particles “wave-particle duality”
• A photon carries a quantum of energy.
• The energy of a photon is proportional to its
frequency.
E = h
E: energy (J, joules)
h: Planck’s constant (6.6262  10-34 J·s)
: frequency (Hz)
Practice
EX 1: Find the energy of a red photon with a frequency of
4.57  1014 Hz.
GIVEN:
E=?
 = 4.57  1014 Hz
h = 6.6262  10-34 J·s
Solve:
EX 2: Find the energy of a photon with a frequency of
3.55  1014 Hz.
GIVEN:
E=?
 = 4.57  1014 Hz
h = 6.6262  10-34 J·s