Electro-Magnetic Electro-Magnetic Radiation Part: 4 of 4.

Electro-Magnetic

Electro-Magnetic Radiation

Part: 4 of 4

The Doppler Effect

Imagine that

Imagine that while on his route,…

…that a mailman passes

…that a mailman passes a stationary ambulance.

mailman

…that a mailman passes a stationary ambulance. mailman

stationary ambulance …that a mailman passes a stationary ambulance. mailman

…that a mailman passes a stationary ambulance. stationary ambulance

At that exact moment,

At that exact moment, the driver receives a call,…

…and begins to blast

…and begins to blast the ambulance’s siren.

Sound waves begin to disperse

Sound waves begin to disperse in all directions,…

…both behind the ambulance,

…both behind the ambulance, and in front of it.

behind …both behind the ambulance, and in front of it.

behindin front

Since the ambulance

Since the ambulance is stationary,…

…then the sound waves leave the ambulance

…then the sound waves leave the ambulance with equal spacing.

equal …then the sound waves leave the ambulance with equal spacing.

The pitch is determined

The pitch is determined by the wavelength,…

…and this pitch

…and this pitch has a medium tone.

(medium tone)

The sound waves

The sound waves pass the mailman.

The mailman knows

The mailman knows there is an emergency,…

…so he decides

…so he decides to get out of the way.

As the ambulance

As the ambulance begins to move forward,…

…its sound waves also

…its sound waves also begin to change their spacing.

From this…

equal spacing From this…

…to this.

unequal spacing …to this.

The waves in front

The waves in front of the ambulance…

…are now a lot

…are now a lot closer together.

closer together …are now a lot closer together.

And the waves

And the waves behind the ambulance…

…are now a lot

…are now a lot farther apart.

farther apart …are now a lot farther apart.

Since tone is determined

Since tone is determined by the wavelengths,…

…then the wavelengths on front side

…then the wavelengths on front side will now sound higher in pitch,…

higher in pitch …then the wavelengths on front side will now sound higher in pitch,…

…while the wavelengths on the back side

…while the wavelengths on the back side will now sound lower in pitch.

lower in pitch …while the wavelengths on the back side will now sound lower in pitch.

This concept is called

This concept is called the “Doppler Effect”.

The Doppler Effect is defined

The Doppler Effect is defined as the change in the wavelengths…

…due to the relative motion

…due to the relative motion between the observer and the source.

The key term here

The key term here is “relative motion”,…

…because it doesn’t matter which object

…because it doesn’t matter which object is doing the moving.

So whether the ambulance

So whether the ambulance is stationary…

(stationary)

…and the mailman (stationary)

…and the mailman runs towards the ambulance,… (stationary)

…and the mailman runs towards the ambulance,… (stationary) (forward motion)

…or whether the mailman

is stationary…

…or whether the mailman is stationary…

…or whether the mailman is stationary… (stationary)

…and the ambulance (stationary)

…and the ambulance moves towards the mailman,… (stationary)

…and the ambulance moves towards the mailman,… (stationary) (forward motion)

(stationary) (forward motion)

…is irrelevant. (stationary) (forward motion)

Because either way,

Because either way, relative to each other,…

…there is motion which exists

…there is motion which exists between the (2) objects.

And it’s this relative motion

And it’s this relative motion between the (2) objects…

…which causes the Doppler Effect

…which causes the Doppler Effect to be created and detected.

The shortening of the wavelengths

The shortening of the wavelengths on the front end of the ambulance…

…is called

…is called a “blue shift”.

blue shift …is called a “blue shift”.

And the lengthening of the wavelengths

And the lengthening of the wavelengths on the back end of the ambulance…

…is called

…is called a “red shift”.

red shift

Since the blue wavelengths

Since the blue wavelengths of visible light are shorter,…

shorter Since the blue wavelengths of visible light are shorter,…

shorter

…and since the red wavelengths

shorter …and since the red wavelengths of visible light are longer,…

shorterlonger …and since the red wavelengths of visible light are longer,…

shorterlonger

shorterlonger …then we use words

shorterlonger …then we use words like “blue” and “red”…

shorterlonger

shorterlonger …to denote the changes in wavelengths

shorterlonger …to denote the changes in wavelengths for all types of waves.

shorterlonger

shorterlonger And even though these waves

shorterlonger And even though these waves are sound waves,…

shorterlonger

shorterlonger …the Doppler Effect will also work

shorterlonger …the Doppler Effect will also work for all other types of waves as well.

So as the ambulance

So as the ambulance begins to race forward,…

(forward) So as the ambulance begins to race forward,…

(forward)

…its sound waves start heading (forward)

…its sound waves start heading for the mailman. (forward)

As they pass the mailman,

As they pass the mailman, he hears a higher pitch,…

higher pitch As they pass the mailman, he hears a higher pitch,…

…and he knows for certain

…and he knows for certain that the ambulance is approaching.

So as he goes

So as he goes to take a look,…

…he realizes that today

…he realizes that today just wasn’t his day.

Houston, …he realizes that today just wasn’t his day.

Houston, we have a problem! …he realizes that today just wasn’t his day.

The Doppler Effect

The Doppler Effect and Stars in Motion

Imagine that a star

Imagine that a star were (5) LY away from the Earth.

5 LY Imagine that a star were (5) LY away from the Earth.

How could we know

How could we know whether or not…

…that the star were moving either towards

…that the star were moving either towards or away from the Earth?

The Doppler Effect

The Doppler Effect can be useful…

…in determining the motions

…in determining the motions of any moving star.

shorterlonger

shorterlonger Imagine that the star

shorterlonger Imagine that the star were not moving…

shorterlonger

shorterlonger …and reveals the absorption line pattern

shorterlonger …and reveals the absorption line pattern shown above.

shorterlonger

shorterlonger But if the star begins to move slowly towards the Earth…

slowly shorterlonger But if the star begins to move slowly towards the Earth…

slowly shorterlonger

slowly shorterlonger …then its absorption line pattern

slowly shorterlonger …then its absorption line pattern will be Doppler shifted…

slowly shorterlonger …towards the blue end

slowly shorterlonger …towards the blue end of the spectrum.

shorterlonger

shorterlonger And if the star begins to move

shorterlonger And if the star begins to move even faster towards the Earth…

faster shorterlonger And if the star begins to move even faster towards the Earth…

faster shorterlonger

…then its absorption line pattern faster shorterlonger

…then its absorption line pattern will be even more Doppler shifted… faster shorterlonger

faster shorterlonger …towards the blue end

…towards the blue end of the spectrum. faster shorterlonger

faster shorterlonger …towards the blue end of the spectrum.

Compare all three.

no movement towards Earth Compare all three.

no movement towards Earth slow movement towards Earth Compare all three.

no movement towards Earth slow movement towards Earth fast movement towards Earth Compare all three.

no movement towards Earth slow movement towards Earth fast movement towards Earth

no movement towards Earth slow movement towards Earth fast movement towards Earth It’s the same star

no movement towards Earth slow movement towards Earth fast movement towards Earth It’s the same star traveling towards the Earth,…

no movement towards Earth slow movement towards Earth fast movement towards Earth …but just moving

…but just moving at different speeds. no movement towards Earth slow movement towards Earth fast movement towards Earth

no movement towards Earth slow movement towards Earth fast movement towards Earth We know for sure

no movement towards Earth slow movement towards Earth fast movement towards Earth We know for sure that it’s the same star,…

no movement towards Earth slow movement towards Earth fast movement towards Earth …because it’s the same fingerprint pattern

no movement towards Earth slow movement towards Earth fast movement towards Earth …because it’s the same fingerprint pattern found in all (3) cases.

no movement towards Earth slow movement towards Earth fast movement towards Earth The fact that the lines

The fact that the lines are blue shifted at all,... no movement towards Earth slow movement towards Earth fast movement towards Earth

no movement towards Earth slow movement towards Earth fast movement towards Earth …tells us that the star

…tells us that the star is moving towards the Earth. no movement towards Earth slow movement towards Earth fast movement towards Earth

no movement towards Earth slow movement towards Earth fast movement towards Earth But it’s the amount of blue shift,

no movement towards Earth slow movement towards Earth fast movement towards Earth But it’s the amount of blue shift, however,…

no movement towards Earth slow movement towards Earth fast movement towards Earth …which reveals the star’s speed

no movement towards Earth slow movement towards Earth fast movement towards Earth …which reveals the star’s speed towards the Earth.

no movement towards Earth slow movement towards Earth fast movement towards Earth faster …which reveals the star’s speed towards the Earth.

no movement towards Earth slow movement towards Earth fast movement towards Earth slower faster …which reveals the star’s speed towards the Earth.

no movement towards Earth slow movement towards Earth fast movement towards Earth slower no motion faster …which reveals the star’s speed towards the Earth.

shorterlonger

shorterlonger Now imagine

shorterlonger Now imagine that the same star…

shorterlonger

shorterlonger …begins to move slowly

shorterlonger …begins to move slowly away from the Earth.

slowly shorterlonger …begins to move slowly away from the Earth.

slowly shorterlonger The absorption line pattern

slowly The absorption line pattern will now be Doppler shifted… shorterlonger

slowly shorterlonger …towards the red end

slowly shorterlonger …towards the red end of the spectrum.

shorterlonger

shorterlonger And if the star begins to move

shorterlonger And if the star begins to move even faster away from the Earth,…

faster shorterlonger And if the star begins to move even faster away from the Earth,…

faster shorterlonger …then its absorption line pattern

faster …then its absorption line pattern will be even more Doppler shifted… shorterlonger

faster shorterlonger …towards the red end

faster shorterlonger …towards the red end of the spectrum.

shorterlonger faster …towards the red end of the spectrum.

Once again,

Once again, let’s compare the (3) spectra.

no movement away from Earth Once again, let’s compare the (3) spectra.

slow movement away from Earth no movement away from Earth Once again, let’s compare the (3) spectra.

slow movement away from Earth fast movement away from Earth no movement away from Earth Once again, let’s compare the (3) spectra.

slow movement away from Earth fast movement away from Earth no movement away from Earth

slow movement away from Earth fast movement away from Earth no movement away from Earth It’s again the same star

slow movement away from Earth fast movement away from Earth no movement away from Earth It’s again the same star traveling away from the Earth,…

slow movement away from Earth fast movement away from Earth no movement away from Earth …it’s just now traveling

slow movement away from Earth fast movement away from Earth no movement away from Earth …it’s just now traveling at different speeds.

slow movement away from Earth fast movement away from Earth We know for certain

no movement away from Earth slow movement away from Earth fast movement away from Earth We know for certain that it’s the same star,…

no movement away from Earth slow movement away from Earth fast movement away from Earth

no movement away from Earth slow movement away from Earth fast movement away from Earth …because all (3) spectra

no movement away from Earth slow movement away from Earth fast movement away from Earth …because all (3) spectra have the exact same fingerprint pattern.

The fact that the lines slow movement away from Earth fast movement away from Earth no movement away from Earth

The fact that the lines are red shifted at all… slow movement away from Earth fast movement away from Earth no movement away from Earth

slow movement away from Earth fast movement away from Earth no movement away from Earth …tells us that the star

…tells us that the star is moving away from Earth. slow movement away from Earth fast movement away from Earth no movement away from Earth

slow movement away from Earth fast movement away from Earth But it’s the amount of red shift,

no movement away from Earth slow movement away from Earth fast movement away from Earth But it’s the amount of red shift, however,…

no movement away from Earth slow movement away from Earth fast movement away from Earth

no movement away from Earth slow movement away from Earth fast movement away from Earth …which reveals the star’s speed

no movement away from Earth slow movement away from Earth fast movement away from Earth …which reveals the star’s speed away from the Earth.

no movement away from Earth slow movement away from Earth fast movement away from Earth faster …which reveals the star’s speed away from the Earth.

no movement away from Earth slow movement away from Earth fast movement away from Earth slower faster …which reveals the star’s speed away from the Earth.

no movement away from Earth slow movement away from Earth fast movement away from Earth slower faster no motion …which reveals the star’s speed away from the Earth.

The Doppler Effect

The Doppler Effect and Rotating Stars

The Doppler Effect

The Doppler Effect is also useful…

…in determining the rotations

…in determining the rotations of stars.

star’s north pole …in determining the rotations of stars.

star’s equatorstar’s north pole …in determining the rotations of stars.

Imagine that the Earth lies

Imagine that the Earth lies is in this direction.

This star’s motion

This star’s motion is stationary in space.

The star is not approaching

The star is not approaching the Earth,…

…but neither is it receding away

…but neither is it receding away from the Earth either.

But the star does spin on its axis

But the star does spin on its axis in a counter-clockwise direction.

counter-clockwise

But the star does spin on its axis in a counter-clockwise direction. counter-clockwise

The far left side

The far left side of the star…

…approaches the Earth

…approaches the Earth with the greatest speed.

So a spectral line

So a spectral line from this point on the star’s surface…

…would show the greatest amount

…would show the greatest amount of blue shift.

shorterlonger …would show the greatest amount of blue shift.

shorterlonger

But other places shorterlonger

But other places on left side of the star… shorterlonger

shorterlonger

shorterlonger …also approach the Earth

…also approach the Earth but at different speeds. shorterlonger

shorterlonger

For example, shorterlonger

For example, consider this position. shorterlonger

shorterlonger For example, consider this position.

shorterlonger

At this location shorterlonger

At this location on the star’s surface,… shorterlonger

shorterlonger

shorterlonger …the motion is more complicated

…the motion is more complicated than the previous position’s motion. shorterlonger

shorterlonger …the motion is more complicated than the previous position’s motion.

shorterlonger

Only some of its motion shorterlonger

Only some of its motion is towards the Earth,… shorterlonger

shorterlonger Only some of its motion is towards the Earth,…

shorterlonger

shorterlonger …while the rest of it’s motion

…while the rest of it’s motion is lateral to our line of sight. shorterlonger

shorterlonger …while the rest of it’s motion is lateral to our line of sight.

shorterlonger

Therefore, the spectral line shorterlonger

Therefore, the spectral line which is located at this position… shorterlonger

shorterlonger

shorterlonger …is not nearly as blue shifted

shorterlonger …is not nearly as blue shifted as much as the other one.

shorterlonger

shorterlonger Now consider a point

shorterlonger Now consider a point on the front side of the star.

shorterlonger

Its motion at this location shorterlonger

Its motion at this location is lateral to our line of sight. shorterlonger

shorterlonger

shorterlonger So neither it is red shifted,

So neither it is red shifted, nor is it blue shifted. shorterlonger

shorterlonger

shorterlonger A spectral line from this location

shorterlonger A spectral line from this location will therefore be centered…

shorterlonger

shorterlonger …and show no Doppler shift

…and show no Doppler shift at all. shorterlonger

shorterlonger …and show no Doppler shift at all.

shorterlonger

A point which is located shorterlonger

A point which is located on the star’s right side… shorterlonger

shorterlonger A point which is located on the star’s right side…

shorterlonger

shorterlonger …will also demonstrate

shorterlonger …will also demonstrate complicated motion.

shorterlonger

Only some of its motion shorterlonger

shorterlonger Only some of its motion recedes away from the Earth,…

shorterlonger

…while the rest of it moves laterally shorterlonger

…while the rest of it moves laterally to our line of sight. shorterlonger

shorterlonger …while the rest of it moves laterally to our line of sight.

shorterlonger

A spectral line from this point shorterlonger

A spectral line from this point on the star’s surface… shorterlonger

shorterlonger

…will therefore be shorterlonger

…will therefore be only partially red shifted. shorterlonger

shorterlonger …will therefore be only partially red shifted.

shorterlonger

Now consider shorterlonger

Now consider this final position. shorterlonger

shorterlonger Now consider this final position.

shorterlonger

shorterlonger The far right side

The far right side of the star… shorterlonger

shorterlonger

…recedes away from the Earth shorterlonger

…recedes away from the Earth with the greatest speed. shorterlonger

shorterlonger …recedes away from the Earth with the greatest speed.

shorterlonger

shorterlonger Therefore, a spectral line

Therefore, a spectral line from this point on the star’s surface… shorterlonger

shorterlonger

shorterlonger …shows the greatest amount

…shows the greatest amount of red shift of all. shorterlonger

shorterlonger …shows the greatest amount of red shift of all.

shorterlonger

shorterlonger So for this

So for this rotating star,… shorterlonger

shorterlonger

shorterlonger …should we expect to see

…should we expect to see only these (5) spectral lines? shorterlonger

shorterlonger …should we expect to see only these (5) spectral lines?

shorterlonger

shorterlonger No, because there are more

No, because there are more than (5) individual points… shorterlonger

shorterlonger

shorterlonger …on the star’s surface

…on the star’s surface which are rotating. shorterlonger

shorterlonger

shorterlonger All of the other points,

All of the other points, which we did not select,… shorterlonger

shorterlonger

shorterlonger …would fill in the rest of the spaces

…would fill in the rest of the spaces between the (2) extremes. shorterlonger

shorterlonger

shorterlonger So rather than having

So rather than having only (5) spectral lines in total,… shorterlonger

shorterlonger

shorterlonger …all of the points

…all of the points would now just blend together. shorterlonger

shorterlonger

shorterlonger So this would instead result

So this would instead result in one single broad bar. shorterlonger

shorterlonger So this would instead result in one single broad bar.

shorterlonger

shorterlonger So the faster that the star rotates

So the faster that the star rotates on its axis,… shorterlonger

shorterlonger

shorterlonger …then the more red and blue shifted

…then the more red and blue shifted the points on its surface will become. shorterlonger

shorterlonger

And the more red and blue shifted shorterlonger

And the more red and blue shifted those points become,… shorterlonger

shorterlonger

shorterlonger …then the broader

…then the broader the bar also becomes. shorterlonger

shorterlonger …then the broader the bar also becomes.

shorterlonger

shorterlonger So a broad bar

shorterlonger So a broad bar indicates a fast rotation.

shorterlonger fast rotation So a broad bar indicates a fast rotation.

shorterlonger

shorterlonger But the slower the star rotates,

But the slower the star rotates, on the other hand,… shorterlonger

shorterlonger

shorterlonger …then the less red and blue shifted

…then the less red and blue shifted the points on its surface become. shorterlonger

shorterlonger

And the less red and blue shifted shorterlonger

And the less red and blue shifted those points become… shorterlonger

shorterlonger

shorterlonger …then the less broad

…then the less broad the bar also becomes. shorterlonger

shorterlonger …then the less broad the bar also becomes.

shorterlonger

shorterlonger So a narrow bar

shorterlonger So a narrow bar indicates a slow rotation.

shorterlonger slow rotation So a narrow bar indicates a slow rotation.

shorterlonger

shorterlonger And if the star

And if the star were to cease rotating all together,… shorterlonger

shorterlonger

shorterlonger …then the bar would reduce

…then the bar would reduce to one single spectral line. shorterlonger

shorterlonger …then the bar would reduce to one single spectral line.

shorterlonger

shorterlonger So the fact that the star’s spectral lines

shorterlonger So the fact that the star’s spectral lines are broad in the first place…

shorterlonger

shorterlonger …reveals that the star

shorterlonger …reveals that the star is spinning on its axis.

shorterlonger shorterlonger …reveals that the star is spinning on its axis.

shorterlonger …reveals that the star is spinning on its axis.

shorterlonger spinning …reveals that the star is spinning on its axis.

shorterlonger

shorterlonger But it’s the amount of the spectral line’s

But it’s the amount of the spectral line’s red or blue shift,… shorterlonger

shorterlonger

…which reveals the speed shorterlonger

…which reveals the speed at which the star rotates. shorterlonger

shorterlonger …which reveals the speed at which the star rotates.

shorterlonger …which reveals the speed at which the star rotates. faster

shorterlonger …which reveals the speed at which the star rotates. slower

shorterlonger

…which reveals the speed at which the star rotates. shorterlonger

shorterlonger

shorterlonger However, if the star rotated

However, if the star rotated while it also was approaching the Earth,… shorterlonger

shorterlonger

shorterlonger …then not only

…then not only would it’s bar be broad,… shorterlonger

shorterlonger …then not only would it’s bar be broad,…

shorterlonger

shorterlonger …but it would also be shifted

…but it would also be shifted towards the blue end of the spectrum. shorterlonger

shorterlonger …but it would also be shifted towards the blue end of the spectrum.

shorterlonger

But if the same star rotated shorterlonger

But if the same star rotated while it also was receding away from Earth,… shorterlonger

shorterlonger

shorterlonger …then not only

…then not only would it’s bar be broad,… shorterlonger

shorterlonger …then not only would it’s bar be broad,…

shorterlonger

shorterlonger …but it would also be shifted

…but it would also be shifted towards the red end of the spectrum. shorterlonger

shorterlonger …but it would also be shifted towards the red end of the spectrum.

shorterlonger

And the faster the star’s speed shorterlonger

And the faster the star’s speed either towards or away from Earth… shorterlonger

shorterlonger

…then the greater shorterlonger

…then the greater its red or blue shift would be. shorterlonger

shorterlonger

This would create an almost infinite shorterlonger

This would create an almost infinite number of combinations… shorterlonger

shorterlonger

shorterlonger …of possible bar broadnesses

…of possible bar broadnesses and red and blue shifts. shorterlonger

Review

shorterlonger

shorterlonger So not only can we determine

So not only can we determine that the star is rotating on its axis,… shorterlonger

shorterlonger

shorterlonger …but also thanks

…but also thanks to the Doppler Effect,… shorterlonger

shorterlonger

shorterlonger …we can also determine

…we can also determine how fast the star is rotating. shorterlonger

shorterlonger …we can also determine how fast the star is rotating.

shorterlonger

shorterlonger And not only can we tell a star’s motion

shorterlonger And not only can we tell a star’s motion either towards or away from Earth,…

shorterlonger

shorterlonger …but we can also determine

shorterlonger …but we can also determine the speed of that motion as well.

shorterlonger

And all of these motions shorterlonger

And all of these motions are detectable… shorterlonger

shorterlonger

…simply by using shorterlonger

…simply by using the Doppler Effect. shorterlonger

shorterlonger …simply by using the Doppler Effect.

End of Part 4

Electro-Magnetic Electro-Magnetic Radiation Part: 4 of 4.

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Electro-Magnetic Electro-Magnetic Radiation Part: 4 of 4.

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