What is Light, Electromagnetic Radiation

1
What is Light, Electromagnetic Radiation?
Christian Huygens (1629 - 1695) Light has a
wave character. Sir Isaac Newton (1642 - 1727)
Light is a corpuscle, a packet of energy

that we call a photon.
2
l
1,2,3,4,
Motion of the Wave
Frequency number of cycles, wavelengths,
passing a given point, the observer, in 1
second. Units of f are cycles/second called
Hertz, Hz in honor of Heinrich Rudolph Hertz
who was the first to provide experimental
evidence of electromagnetic radiation
supporting James Clerk Maxwell's theory of
electromagnetic radiation.
3
Light or electromagnetic radiation can be
characterized as a wave
l
or as a packet of energy know as a photon
In either case speed of the wave in a vacuum
is c lf And the
energy of a photon is
E hf hc
l
4
Credit The National Radio Astronomy Observatory
(NRAO) http//www.nrao.edu/whatisra/me
chanisms.shtml
5
(No Transcript)
6
Electromagnetic Spectrum
Long l
Short l
400 nm
700 nm
Visible Light
10-3 nm
10-i nm
10 nm
105 nm
107 nm
109 nm 1 meter
g rays
X rays
Micro wave
UV
Radio
Infrared
Opacity of the Atmosphere

Radio Window
H2O 02
0zone Layer
Ionosphere reflects radio.
V i s i b l e
C02 H2O CH4
400nm - 700 nm 1cm - 10 m
7
How is Electromagnetic Radiation Produced? All
objects emit radiation due to - the motion of
charged particles amount of motion is determined
by its temperature
Thermal or Blackbody Radiation Radiation is
emitted over some portion of the E/M Spectrum -
energy is maximized at one particular
frequency - the higher the temperature, the
higher the frequency
the shorter the l
8
Bohr Model of the Atom
nucleus
The electron orbiting the Hydrogen atom nucleus
is shown orbiting in the ground state. It can be
excited and reach a higher orbit if it absorbs
precisely the correct amount of energy from a
photon or thermally, i.e from a collision.
9
If the electron receives precisely the correct
amount of energy it will "jump" to a higher orbit
(rhs). It will remain there about 10-8 seconds
as it returns to a lower energy level,
ultimately the ground state it will give up a
photon (or photons) which equal the energy of
the photon that was orginally absorbed. On the
left the energy does not match and the electron
"ignores" the photon and the photon passes
unabsorbed.
10
(No Transcript)
11
Absorption Spectrum
12
Kirchhoffs Laws
Absorption
Continuous
Emission
13
Kirchhoffs Laws

• A hot solid, liquid or gas at high pressure
• has a continuous spectrum.
• 2. A gas at low pressure and high temperature
• will produce emission lines.
• 3. A gas at low pressure in front of a hot
  continuum
• causes absorption lines.

14
(No Transcript)
15
Measuring the Wavelengths of Helium
cross-meter scale
diffraction grating
B
A
30cm
16
Helium Data Table
A 30 cm
Chart
l - l
lnm
Chart
1541.9 x B/C
Error x100
l Chart
17
Spectra Checklist
Draw the spectra of a continuous spectrum
emission spectrum of hydrogen
emission spectrum of mercury emission spectrum
of neon emission spectrum of nitrogen and the
emission spectrum of helium Measure the
wavelengths of the helium emission spectrum! Do
not touch the emission bulbs! Use cardboard
mounted diffraction gratings to view spectra!