Download Feb20_modified

Homework 4
Unit 21 Problem 17, 18, 19
Unit 23 Problem 9, 10, 13, 15, 17, 18, 19, 20
Units 21 and 22 are covered
The Nature of Light
• Light is radiant energy.
• Travels very fast –
300,000 km/sec!
• Can be described either
as a wave or as a
particle traveling
through space.
•
•
As a wave…
– A small disturbance in an electric field creates
a small magnetic field, which in turn creates a
small electric field, and so on…
• Light propagates itself “by its bootstraps!”
– Light waves can interfere with other light
waves, canceling or amplifying them!
– The color of light is determined by its
wavelength.
As a particle…
– Particles of light (photons) travel
through space.
– These photons have very specific
energies. that is, light is quantized.
– Photons strike your eye (or other
sensors) like a very small bullet, and
are detected.
The Effect of Distance on Light
• Light from distant objects
seems very dim
– Why? Is it because the photons
are losing energy?
– No – the light is simply
spreading out as it travels from
its source to its destination
– The farther from the source you
are, the dimmer the light seems
– We say that the object’s
brightness, or amount of light
received from a source, is
decreasing
Brightness =
Total Light Output
4pd 2
This is an inverse-square law –
the brightness decreases as the
square of the distance (d) from
the source
The Nature of Matter
• The atom has a nucleus at
its center containing protons
and neutrons
• Outside of the nucleus,
electrons whiz around in
clouds called orbitals
– Electrons can also be
described using wave or
particle models
– Electron orbitals are quantized
– that is, they exist only at
very particular energies
•
– The lowest energy orbital is
called the ground state, one
electron wave long
•
To move an electron from one orbital to the next higher
one, a specific amount of energy must be added.
Likewise, a specific amount of energy must be released
for an electron to move to a lower orbital
These are called electronic transitions
Measuring Temperature
•
It is useful to think of temperature
in a slightly different way than we
are accustomed to
– Temperature is a measure of the
motion of atoms in an object
– Objects with low temperatures have
atoms that are not moving much
– Objects with high temperatures have
atoms that are moving around very
rapidly
•
The Kelvin temperature scale was
designed to reflect this
– 0  K is absolute zero –the atoms in
an object are not moving at all!
Results of More Collisions
• Additional collisions mean
that more photons are
emitted, so the object gets
brighter
• Additional hard collisions
means that more photons of
higher energy are emitted, so
the object appears to shift in
color from red, to orange, to
yellow, and so on.
• Of course we have a Law to
describe this…
Wien’s Law and the Stefan-Boltzmann Law
• Wien’s Law:
– Hotter bodies emit more
strongly at shorter
wavelengths
• SB Law:
– The luminosity of a hot
body rises rapidly with
temperature
Taking the Temperature of Astronomical Objects
• Wien’s Law lets us estimate
the temperatures of stars
easily and fairly accurately
• We just need to measure the
wavelength (max) at which
the star emits the most
photons
• Then,
T=
2.9 ´10 6 K × nm
lmax
The Stefan-Boltzmann Law
• If we know an object’s
temperature (T), we can
calculate how much energy
the object is emitting using
the SB law
4
L = sT
•  is the Stefan-Boltzmann
constant, and is equal to
5.6710-8 Watts/m2/K4
• The Sun puts out 64 million
watts per square meter – lots
of energy!
Absorption
•
If a photon of exactly the
right energy (corresponding
to the energy difference
between orbitals) strikes an
electron, that electron will
absorb the photon and move
into the next higher orbital
– The atom is now in an
excited state
•
If the photon is of higher or
lower energies, it will not be
absorbed – it will pass
through as if the atom were
not there.
•
•
This process is called absorption
If the electron gains enough energy to leave the
atom entirely, we say the atom is now ionized, or
is an ion.
Emission
• If an atom drops
from one orbital
to the next lower
one, it must first
emit a photon
with the same
amount of energy
as the orbital
energy
difference.
• This is called
emission.
Frequency
•
•
•
•
Sometimes it is more convenient
to talk about light in terms of
frequency, or how fast
successive crests pass by a given
point
You can think of frequency as a
measure of how fast you bob up
and down as the waves pass.
Frequency has units of Hz
(Hertz), and is denoted by the
symbol 
Long wavelength light has a low
frequency, and short wavelength
light has a high frequency
•
Frequency and wavelength are related by:
   c
‘c’ is the speed of light.
The Electromagnetic Spectrum I
• There is more to light than just
the visible part of the spectrum
– Radio waves are very long
wavelength photons (not
sound!) with wavelengths
longer than a meter or so
– Microwaves (yes, the ones
we cook with) are at the
upper end of the radio part
of the spectrum
– Infrared wavelengths are
just longer in wavelength
than the visible spectrum