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Transcript
Radiation:
Visible Light is only one part of the electromagnetic spectrum,
if we were to only use optical telescopes we would limit our
view of the universe. So we use other types of electromagnetic
radiation to gather information from the universe.
Electromagnetic radiation can be described in terms of a stream of
photons, which are massless particles each traveling in a wave-like
pattern and moving at the speed of light. Each photon contains a
certain amount (or bundle) of energy, and all electromagnetic
radiation consists of these photons. The only difference between
the various types of electromagnetic radiation is the amount of
energy found in the photons.
Electromagnetic Waves have different wavelengths.
Waves in the electromagnetic spectrum vary in size from very long
radio waves the size of buildings, to very short gamma-rays smaller
than the size of the nucleus of an atom.
Electromagnetic waves can be described by their wavelength,
energy, and frequency
The electromagnetic spectrum includes, from longest wavelength to
shortest: radio waves, microwaves, infrared, optical, ultraviolet, X-rays,
and gamma-rays.
Parts of a wave
T = 1/f
f = 1/T
lxf=v
All electromagnetic waves move at a very specific speed – the
speed of light which is 3 x 108 m/s
In the time it takes to snap your fingers light will have traveled ¾
away around the earth.
The light from the Andromeda galaxy is 2.5 million years old!
The components of visible light:
Absorption spectra
If you look more closely at the Sun's
spectrum, you will notice the presence of
dark lines. These lines are caused by the
Sun's atmosphere absorbing light at
certain wavelengths, causing the intensity
of the light at this wavelength to drop
and appear dark. The atoms and molecules
in a gas will absorb only certain
wavelengths of light. The pattern of
these lines is unique to each element and
tells us what elements make up the
atmosphere of the Sun. We usually see
absorption spectra from regions in space
where a cooler gas lies between us and a
hotter source. We usually see absorption
spectra from stars, planets with
atmospheres, and galaxies.
Emission spectra
An emission spectra occurs when
the atoms and molecules in a hot
gas emit extra light at certain
wavelengths, causing bright lines
to appear in a spectra. As with
absorption spectra, the pattern
of these lines are unique for
each element. We can see
emission spectra from comets,
nebul and certain types of stars.
This image of the Pleiades star cluster shows haloes
around the stars due to the wave nature of light.
If light is a particle…
If light is a particle, then only the couple of rays of light that
hit exactly where the slits are will be able to pass through.
If light is a wave…
There are still only two light rays that actually go through
the slits, but as soon as they pass through they start to
diffract.
Notice that at some points the two sets of waves will
meet crest to crest, at other spots crest meets trough.
Where crest meets
crest, there will be
constructive
interference and the
waves will make it to
the viewing screen as
a bright spot.
Occurs when
the path of the
two rays differ
by one
wavelength
Where crest meets
trough there will be
destructive
interference that
cancel each other
out… a black spot will
appear on the screen.
Occurs when
one ray travels
an extra
distance of
one-half
wavelength
and are out of
phase
All macroscopic objects – fire, ice cubes, people, stars – emit
radiation at all times, regardless of their size, shape or chemical
composition.
The temperature of an object is a direct measure of the amount
of microscopic motion within it – the hotter the object – the
faster its molecules move and the more energy they radiate.
The Blackbody Spectrum:
Intensity is a term often used to specify the amount or strength
of radiation at any point in space. No natural object emits all its
radiation at just one frequency. Instead, the energy is spread out
over a range of frequencies with some frequency demonstrating a
more significant intensity.
The blackbody curve, describes
the distribution of the reemitted
radiation.
As temperature increases the curve
shifts towards the higher frequency
Consider how the color of metal
changes as it is heated
Wein’s Law:
Relates the temperature of an object to the wavelength at which it
emits the most radiation.
l= 0.29cm/T
Stefan’s Law: with T measured in Kelvins, the total amount of
energy emitted per square meter of its surface persecond can be
calculated F = s T4 , where sigma is known as the StefanBoltzman constant – 5.67 x 10-8 W/m2K4
Astronomers often use blackbody curves as thermometers to
determine the temperatures of distant objects, even the surface
temperature of the sun.
The Doppler Effect:
Stars that you are moving towards appear more blue and those
you are moving away from appear to be more red. The motion
induced change in the observed frequency of a wave is called the
Doppler effect.