Announcements
• Course bundles should have arrived yesterday in the Book Store?
• No clicker questions today (my computer needs to checked out).
Sorry! No one will lose marks. Will begin as soon as possible!
• Labs start this week:
 Laboratory class sections with an odd number, e.g. B01, B03, etc.
have
Laboratory Exercise #1 on the week of September 12 – 16.
 Laboratory class sections with an even number, e.g. B02, B04, etc.
have Laboratory Exercise #1 on the week of September 19 – 23.
See CourseSpaces to find out when Lab Report#1 is due.
Wave nature of light successfully explains a range of different phenomena:
Examples of interference:
stones and ripples on water,
and reflections of light
from a DVD
Wave model of light useful BUT has some limitations…
1.2 Quantized Energy and Photons
Some phenomena cannot be explained using a simple wave model of light:
1. Blackbody radiation
2. The photoelectric effect
3. Emission spectra
emission of light from hot objects
emission of electrons from metal surfaces
on which light shines
emission of light from electronically
excited gas atoms
Hot Objects and the Quantization of Energy
Heated solids emit radiation (blackbody radiation):
wavelength distribution depends on
temperature
In 1900, Max Planck investigated black
body radiation, and he proposed that
energy can only be absorbed or
released from atoms in certain
amounts
called “quanta”
A quantum is the smallest amount of
energy that can be emitted or absorbed
as electromagnetic radiation
The relationship between energy, E,
and frequency is:
E = hν
where h is the Planck constant
= 6.626 × 10-34 joule-seconds (Js)
Light
emission
by molten
lava (rock)
classical theory =
the “ultraviolet
catastrophe”
T = 7000 K
T = 5000 K
The Photoelectric Effect and Photons
The photoelectric effect (right) provides
evidence for the particle nature of light
and for quantization.
Light shining on the surface of a metal
can cause electrons to be ejected from
the metal.
The electrons will be ejected only
if the photons have sufficient energy.
Below a threshold frequency no electrons
are ejected.
Above the threshold frequency, the
excess energy appears as the kinetic
energy of the ejected electrons.
Einstein proposed that light could have particle-like properties, which he called
photons.
Energy of one photon = E = hν = h c/λ
Light has wave-like AND particle-like properties
1.3 Line Spectra and the Bohr Model
Line spectra
e.g. laser light
Radiation composed of only one wavelength
is called monochromatic.
a whole array of different wavelengths
is called continuous radiation
When radiation from a light source, such as a light bulb, is
separated into its different wavelength components, a
spectrum is produced. White light passed through a prism
provides a continuous spectrum:
White light
spectrum
Hydrogen lamp gives the hydrogen spectrum.
Neon lamp gives the neon spectrum.
Lines are characteristic of the element – a “fingerprint”
Rutherford’s model of the atom
Rutherford assumed that electrons orbited the nucleus
analogous to planets orbiting the sun; however, a charged
particle moving in a circular path should lose energy!
→ the atom should be unstable according to Rutherford’s theory!
Bohr’s model of the atom
Niels Bohr noted the line spectra of certain elements and
assumed that electrons were confined to specific energy states.
These he called orbits.
Bohr’s model is based on three postulates:
1. Only orbits of specific radii are
permitted for electrons in an atom
2. An electron in a permitted
orbit has a specific energy.
an "allowed" energy state
3. Energy is only emitted or
absorbed by an electron
as it moves from one
allowed energy state
to another.
energy is gained or
lost as a photon
These correspond to certain
definite energies.