Light and Matter: Reading Messages from the Cosmos
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93 Terms
The warmth of sunlight tells us that light
is a form of energy
We can measure the flow of energy in light in units
watts
1 watt =
1 joule / s
White light is made up of
all the colors of the rainbow
How Do We Experience Light?
Emission, Absorption, Transmission, Reflection/scattering
Transparent objects
transmit light
Opaque objects
block (absorb) light
The angle at which the light strikes the mirror is
the same angle at which it is reflected
Interactions between light and matter determine
the appearance of everything around us
Why is a rose red?
The rose reflects red light.
Light is a form of
energy
Visible light comes in
many colors that combine to form
white light.
Matter can
emit light, absorb light, transmit light, and
reflect (or scatter) light
Light can act either like
Light can act either like
Particles of light are called
photons
A wave is
a pattern of motion that can carry energy without carrying matter along with it
The vibrations of the electric field determine
the wavelength and frequency of a light wave
Wavelength is
the distance between two wave peaks
Frequency is
the number of times per second that a wave
vibrates up and down
Wave speed =
wavelength * frequency
A light wave is
a vibration of electric and magnetic fields
Light interacts with charged particles through
electric and magnetic fields
Longer wavelength means
lower frequency
Shorter wavelength means
higher frequency
wavelength * frequency =
speed of light = constant
Particles of light are called
photons
Each photon has
a wavelength and a frequency
the energy of a photon depends
on its frequency
λ × f
= c
λ =
wavelength
f =
frequency
c = 3.00 × 108m/s =
speed of light
E = h × f =
photon energy!
h =
6.626 × 10-34
joule × s =
Planck's constant
Polarization describes
the direction in which a light wave
is vibrating
Reflection can change
the polarization of light
Polarized sunglasses block
light that reflects off of horizontal surfaces
The higher the photon energy,
the shorter its wavelength
Light can behave like
either a wave or a particle
A light wave is
a vibration of electric and magnetic fields
Light waves have
a wavelength and a frequency
Photons are
particles of light
Human eyes cannot see
most forms of light
The entire range of wavelengths of light is known as
the electromagnetic spectrum
Atomic number =
# of protons in nucleus
Atomic mass number =
# of protons + neutrons
Molecules consist of
two or more atoms (H2O, CO2)
Isotope:
same # of protons but different # of neutrons (4He, 3He)
Different isotopes of a given element contain
the same number of protons, but different numbers of neutrons
Phases of same material behave
differently because of differences in chemical bonds
Ionization:
stripping of electrons, changing atoms into plasma
Dissociation:
breaking of molecules into atoms
Evaporation:
breaking of flexible chemical bonds, changing liquid into solid
Melting:
breaking of rigid chemical bonds, changing solid into liquid
Phase of a substance depends on
both temperature and pressure; Often more than one phase is present
Electrons in atoms are
restricted to particular energy levels
The only allowed changes in energy are those
corresponding to a transition between energy levels
Matter is made of
atoms, which consist of a nucleus of protons and neutrons surrounded by a cloud of electrons
Adding heat to a substance
changes its phase by breaking chemical bonds
As temperature rises, a substance transforms
from a solid to a liquid to a gas, then the molecules can dissociate into atoms
Stripping of electrons from atoms (ionization) turns
the substance into a plasma
The energies of electrons in atoms correspond
to particular energy levels
Atoms gain and lose energy only
in amounts corresponding to particular changes in energy levels
What are the three basic type of spectra?
– Continuous
– Emission line
– Absorption line
The spectrum of a common (incandescent) lightbulb spans
all visible wavelengths, without interruption
A thin or low-density cloud of gas emits light only at
specific wavelengths that depend on its composition and
temperature, producing a spectrum with bright emission
lines
A cloud of gas between us and a lightbulb can
absorb light of specific wavelengths, leaving dark absorption lines in the spectrum
Each type of atom has a
unique set of energy levels
Each transition corresponds to a
unique photon energy, frequency, and wavelength
Downward transitions produce
a unique pattern of emission lines
upward transitions produce
a pattern of absorption lines at the same wavelengths
Each type of atom has a
unique spectral fingerprint
Observing the fingerprints in a spectrum tells us
which kinds of atoms are present
Molecules have additional energy levels because
they can vibrate and rotate
Nearly all large or dense objects emit
thermal radiation, including stars, planets, you
An object's thermal radiation spectrum depends on only
one property:
its temperature
Hotter objects emit more
light at all frequencies per unit area
Hotter objects emit
photons with a higher average energy
Which is hottest?
a blue star
Why don't we glow in the dark?
People only emit light that is invisible to our eyes
The Doppler shift can tell you about
the relative motion of distant objects because photons behave as waves
If an object is moving toward an observer, the observed
wavelength of the light decreases
Light appears blueshifted
If an object is moving away from an observer, the observed
wavelength of the light increases
Light appears redshifted
We generally measure the Doppler effect from
shifts in the wavelengths of spectral lines
The amount of blueshift or redshift tells us
an object's speed toward or away from us
Doppler shift tells us only about
the part of an object's motion toward or away from us
I measure a line in the lab at 500.7 nanometer. The same line in a
star has wavelength 502.8 nanometer. What can I say about this
star?
It is moving away from me
Different Doppler shifts from different sides of a rotating
object
spread out its spectral lines
Spectral lines are wider when
an object rotates faster
We can determine which atoms something is made of
by
looking for their fingerprints in the spectrum
Nearly all large or dense objects emit a
continuous spectrum that depends on temperature
The spectrum of that thermal radiation tells us
the object's temperature