Dppler and photoelectric effect

In summary, when using the doppler effect equations to determine the speed of celestial objects, the energy of the photons changes depending on the reference system. The normal doppler effect is distinguished by comparing the energy of the photon to the actual reception frequency, while the relativistic doppler effect is distinguished by comparing the frequency to the expected frequency based on the materials present. The actual frequency is determined by comparing spectral lines and looking for matches to expected materials and energy levels. The universal expansion also affects the frequency of photons traveling from a distant source, and must be accounted for in calculations and analyses.
  • #1

Will

Doppler and photoelectric effect

When using the doppler effect equations to determine the speed of celestial objects, what happens to the energy of the photons? If a certain device required a 600nm wavelenth of light and frequency 5E14 Hz to induce current, would photons from a celestial body moving away from Earth and emitting this wavelength have sufficient energy to induce current, since observed f is less than 5E14 Hz and E=hf ?
I was curious as to how astrophysicists can distinguish between one lightsource traveling at v1 with frequency f1 and another with v2,f2 if the observed f is the same for both.
 
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  • #2


Greetings !
Originally posted by Will
I was curious as to how astrophysicists can
distinguish between one lightsource traveling
at v1 with frequency f1 and another with v2,f2
if the observed f is the same for both.
There are two types of shifts for light.
One is normal doppler and the other is
relativistic. The relativistic can only
be distinguished by comparing the
frequency to the one we would expect in
this case (based on expected materials,
compared atomic/molecular emmision lines
and so on). The normal doppler effect
is distinguished by comparing the energy
of the photon according to E = h * frequency
(the signs/smilies menu doesn't work for me,
sorry) to the actual reception frequency.

Live long and prosper.
 
  • #3


Originally posted by drag
Greetings !

The normal doppler effect
is distinguished by comparing the energy
of the photon according to E = h * frequency
(the signs/smilies menu doesn't work for me,
sorry) to the actual reception frequency.

Live long and prosper.


So the energy equation uses what frequency, actual or observed?
 
  • #4


Originally posted by Will
So the energy equation uses what frequency, actual or observed?

The actual frequency is the observed frequency.
 
  • #5


Originally posted by Will
When using the doppler effect equations to determine the speed of celestial objects, what happens to the energy of the photons? If a certain device required a 600nm wavelenth of light and frequency 5E14 Hz to induce current, would photons from a celestial body moving away from Earth and emitting this wavelength have sufficient energy to induce current, since observed f is less than 5E14 Hz and E=hf ?
They would not. Energy depends on reference system.

Say, water will turn turbine falling from 100 m elevation, but it's energy vanishes if you place turbine at zero elevation and becomes negative if you move turbine higher. Bullet energy becomes zero in the co-moving with the bullet reference system.



I was curious as to how astrophysicists can distinguish between one lightsource traveling at v1 with frequency f1 and another with v2,f2 if the observed f is the same for both.

They can't. There is no way of figuring out "original" frequency of passing photon. Thus, they have to use additional information (say, about nature of source). For example, scientists are not exactly sure what was original temperature of CMBR when it was emitted. All they know is that at temperatures about T<(3-5)x1000 K hydrogen becomes transparent to radiation (with temperature T), so thery assume that this was the temperature of radiation (thus, of universe) at time of decoupling of CMBR from matter (hydrogen).
 
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  • #6
Greetings !

Will, I'm terribly sorry !
I was somewhat confused myself and now
I apparently transferred my confusion to you.
But, as long as I admit my mistake and
fix it now, all is not lost ! :smile:

Anyway, The normal Doppler effect for light
is the same as for sound. As the source
moves towards/away from you the frequency of
the photons (EM waves) increases/decreases.

Now, when you "add" relativity you must
also use the Lorentz transformation.
The energy of a photon from such a source
will be higher for you than it is
for the source (in terms of the Lorentz
transformation at least).

Now, the Lorenz transformation only really
comes into play when the source is moving at
relativistic velocities. So, the normal
doppler shift is more significant before
you get to velocities close to the speed
of light (0.8 c +). What's important is that
the relativistic energy and hence frequency
increase of the photon occurs in any case -
no matter whether the source moves towards or
away from you.

The doppler shift is v/c.
The Lorentz transformation is 1/ sqrt(1 - sqr(v / c)).

How is the actual frequency discovered ?

Like I mentioned above it is done by comparing
spectral lines and looking for matches for the
type of materials and energy of reactions you
expect to be present at the source.
Fopr example, one well known frequency is
1.420 Gigahertz (hydrogen - which is abundant
in space - 90% of all normal matter) and it's
frequency shift from a the source can be used
to set the whole spectrum "right".

Another issue worth mentioning is the Universal
expansion. Basicly if you consider that
the Universe expands by about 65 km per Mega Parsec
per second then the photons traveling from the
source to you will also expand according to
the amount of time it took'em to reach you
(their frequency will decrease).
So, you have to account for that in your
calculations and spectral analyses of distant,
in astronomical terms, sources. Alternatively, this
can also be used to make a general estimate of
distance to the source.

Hope I helped this time. :wink:

Live long and prosper.
 

What is the Doppler effect?

The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. It is commonly observed in sound waves, such as the change in pitch of an ambulance siren as it passes by.

How does the Doppler effect apply to light waves?

The Doppler effect also applies to light waves, causing a shift in the perceived color of a light source if either the source or observer is moving. This is known as the redshift or blueshift.

What is the photoelectric effect?

The photoelectric effect is the emission of electrons from a material when it is exposed to light or other electromagnetic radiation. This phenomenon was first observed by Heinrich Hertz in 1887 and was further explained by Albert Einstein in 1905.

How does the photoelectric effect support the wave-particle duality theory?

The photoelectric effect supports the wave-particle duality theory, as it demonstrates that light behaves as both a wave and a particle. The emission of electrons from a material can only be explained by the particle-like behavior of light, while the interference patterns of light can only be explained by the wave-like behavior.

What are the practical applications of the photoelectric effect?

The photoelectric effect has many practical applications, including in photovoltaic cells for solar energy conversion, in photoelectric cells for light detection, and in electron microscopes for imaging. It also paved the way for the development of quantum mechanics, which has had a significant impact on modern technology.

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