What Happens When You Reflect a Laser Back Into Itself?
Summary
TLDRIn this Action Lab video, the presenter explores the effects of reflecting a laser back into itself using a red laser pointer. They explain how laser diodes work, with photons being emitted through stimulated emission, creating coherent light. The experiment shows that reflecting the laser light back into the laser cavity results in destructive interference, causing the light to dim rather than intensify. The video also demonstrates this phenomenon using a Michelson interferometer, revealing how light waves can cancel each other out. The presenter notes that red lasers are most affected by this interference, while green and violet lasers show minimal change.
Takeaways
- đŹ The experiment explores the effect of reflecting a laser beam back into its source.
- đ A laser diode operates by applying current to a semiconductor, causing electrons to combine with holes and release photons.
- đ Mirrors at the ends of the semiconductor stimulate emission, creating coherent light that matches the phase and direction of existing photons.
- đ« Reflecting the laser light back into the diode does not increase its intensity; instead, it can cause destructive interference, reducing the light output.
- đ A Michelson interferometer setup is used to visualize interference patterns created by combining laser beams.
- đ When light beams are out of phase, they can destructively interfere, leading to dark spots where light intensity is reduced.
- đŽ Red laser pointers are most affected by this interference, possibly due to their wavelength being more susceptible to the interference conditions.
- đą Green laser pointers do not show significant interference because they use a frequency doubler crystal that does not convert reflected light back to the original wavelength.
- đŁ Violet lasers do not exhibit noticeable changes in brightness when their light is reflected back into the laser.
- đ The experiment demonstrates the wave nature of light, showing that light can interfere and even cancel each other out.
Q & A
What is a laser diode and how does it work?
-A laser diode is a semiconductor device that emits light when current is applied to it. Electrons in the N-type semiconductor combine with positive holes in the P-type semiconductor, releasing photons of light. These photons are initially incoherent, but mirrors at the ends of the semiconductor reflect light, and through a process called stimulated emission, the photons become coherent, resulting in laser light.
What is the role of the mirrors in a laser diode?
-The mirrors in a laser diode are crucial for reflecting light and enhancing the stimulated emission process. They help to create a coherent beam of light by ensuring that the photons released have matching phases and directions.
What happens when you shine a laser back into itself?
-When a laser is shone back into itself, it can cause the light to interfere with the internal photons, potentially leading to destructive interference. This can result in a dimmer light output rather than a brighter one, as the external light is out of phase with the internal photons.
Why does the laser light dim when it is reflected back into the laser diode?
-The laser light dims because the reflected light is out of phase with the internal photons, leading to destructive interference. The external light does not match the phase of the internally generated photons, which are specifically tuned for amplification within the laser cavity.
What is destructive interference and how does it relate to lasers?
-Destructive interference occurs when two waves of light are out of phase, causing their peaks and valleys to cancel each other out. In the context of lasers, shining a laser back into itself can create such interference patterns, leading to a reduction in the overall light intensity.
How does the color of the laser affect the interference pattern?
-The color of the laser can affect the interference pattern because different colors have different wavelengths. The script mentions that a red laser pointer is most affected, while a green laser pointer does not show significant changes. This is because green laser pointers often use a frequency doubler crystal that does not convert the reflected light back to the original frequency, thus not interfering with the laser's operation.
What is a Michelson interferometer and how is it used in the script?
-A Michelson interferometer is an optical instrument that uses a beam splitter to combine two light beams. In the script, it is used to demonstrate interference patterns by splitting a laser beam and reflecting each half off different mirrors, then recombining them to observe the interference effects.
Why is the red laser pointer most affected by the interference experiment?
-The red laser pointer is most affected because it has a longer wavelength compared to green or violet lasers. Longer wavelengths are more susceptible to interference effects due to the nature of wave interactions and the alignment of the laser's internal components with the wavelength of light.
What is the purpose of the fiber optic cable in the experiment?
-The fiber optic cable is used to focus the laser light into a precise point, allowing for a more accurate reflection of the laser back into its own cavity. This setup helps to control the direction and alignment of the laser light for the interference experiment.
How does theè”ć©ć betterhelp relate to the video content?
-Betterhelp is mentioned as the sponsor of the video, providing a service for online therapy. While it does not directly relate to the scientific content of the video, sponsorships like these help support the creation of educational content.
Outlines
đ Laser Reflection Experiment
The script discusses an experiment where a laser is reflected back into itself to observe the effects. It begins with an explanation of how a laser diode works, involving the combination of electrons and holes in a semiconductor to release photons. These photons are initially incoherent but become coherent through stimulated emission, where light bouncing within the semiconductor stimulates the release of matching photons. The experiment involves using a special mirror and a device called a visual fault locator to reflect the laser light back into the laser pointer. Surprisingly, when the laser light is reflected back into the laser, it becomes dimmer instead of brighter, indicating a destructive interference where the reflected light interferes with the internal light production, causing it to cancel out.
đŹ Understanding Light Interference
This part of the script explores why reflecting light back into a laser causes it to dim, explaining the concept of destructive interference. It uses the example of a Michelson interferometer to demonstrate how light beams can interfere with each other, creating patterns of bright and dark areas. The script shows an experiment where two laser beams are combined, resulting in alternating bright and dark fringes. When both beams are on, certain areas appear darker due to the destructive interference where the light waves cancel each other out. The script also mentions that different colors of lasers, such as green and violet, do not show the same effect as red, suggesting that the interference is wavelength-dependent. The conclusion is that light, being an electromagnetic wave, can interfere and even cancel each other out, which is a surprising and counterintuitive result of the experiment.
Mindmap
Keywords
đĄLaser
đĄLaser Diode
đĄStimulated Emission
đĄCoherent Light
đĄMirror
đĄDestructive Interference
đĄPhase
đĄMichelson Interferometer
đĄFrequency Doubler Crystal
đĄInfrared Light
Highlights
Experiment to see what happens if a laser is reflected directly back into itself.
Laser diodes work by applying current to a semiconductor, releasing photons of light.
Mirrors at the ends of the semiconductor reflect light, leading to coherent laser light.
Stimulated emission causes light to be released that matches the phase and direction of existing photons.
A normal mirror's reflective surface causes many reflections, making it hard to observe laser light.
Using a front-reflective mirror to shine the laser directly back into the laser cavity.
A visual fault locator is used to focus the laser light into a fiber optic cable.
Shining the laser back into itself makes it dimmer due to destructive interference.
Laser light can almost completely cancel out when reflected back into the laser.
Destructive interference occurs when external light is out of phase with internal photons.
Demonstrating interference with a Michelson interferometer setup.
Interference patterns show up as bright and dark spots when two laser beams are combined.
Light beams can interfere and cancel each other out, making some areas darker.
Green laser pointers do not affect the laser when reflected back due to frequency doubling crystals.
Red laser pointers are most affected by the interference when reflected back into the laser.
The experiment did not destroy the world by shining a laser on itself.
Transcripts
today we're going to be seeing what
happens if you reflect a laser directly
back into itself will it make the laser
grow brighter and brighter in a positive
feedback loop that continually rises in
temperature growing hotter and brighter
until eventually either it destroys
itself or all life on
Earth or will something else happen
that's much more interesting so what I
have here is a regular handheld red
laser pointer inside of this there's
something called a laser diode laser
diodes work like this when you apply a
current to the semiconductor electrons
in the N type semiconductor combine with
positive holes in the ptype
semiconductor when they meet it releases
a photon of light these photons that are
released go in random directions and
they're incoherent meaning that the
Peaks and valleys of their wavelengths
don't match up exactly but on either end
of the semiconductor there are these
mirrors these mirrors reflect any light
that happen to hit them now photons have
an interesting property unlike electrons
they want to be in the same state at the
same time so if you have a photon near
electrons and holes combining they're
more likely to release a photon that
matches the phase and direction of other
photons flying around it this is called
stimulated emission just having light
bounce around the semiconductors makes
them release light that matches the
direction and phase so it becomes
coherent light there's a little hole
that some of the light can escape from
so the light that makes it out is mostly
coherent light where all the photons are
doing the same thing and hence you have
laser light so what happens if instead
of letting that light out we shine it
right back where it came from will it
just keep stimulating more and more
light Generation Well let's see but
before we do that I'd like to thank the
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actionlab or click the link in the
description and thanks to betterhelp for
sponsoring this video now let's get back
to our experiment a normal Mirror Has
the silvered surface or the reflective
surface on the back of the glass so you
get many Reflections and it's kind of
confusing and hard to look at so I'm
going to use a different type of mirror
that's reflective on the front of the
glass okay now we have to figure out a
way to shine this laser directly back
into the tiny hole that it came out of
in order to get the laser to go directly
back into the laser cavity I have this
device here it's a visual fault locator
for fiber optic cables so you can plug
it onto a fiber optic cable and it
exactly focuses the light into the cable
so out of the other end is my laser
light now so this has the advantage that
at the tip of it here it's kind of white
so you can actually see how bright the
laser light is at the tip even when it's
completely on the surface whereas with
our normal laser pointer you put it on
the surface and you can't even see
anything so you don't even know what's
happening so let's try it with our fiber
optic cable exactly focused off of our
silvered mirror here so I'm just going
to aim it at itself I'm going to put it
right on the surface to direct it
exactly right back in okay so I'm
putting on there it's off center but
when I find the exact right reflection C
you can see it goes completely dim it's
so dim you can barely see the red light
at
all but then when I get it off center
it's bright again so shining the laser
back into itself actually made it dimmer
I can get it to almost completely cancel
out look how dim that
is and it's like twitching I'm not even
moving it right now but it's flickering
so that's when it's off center then get
it on and it just goes
away off center it's super
bright right in shining it back in and
it goes
dim let's see if we can actually see
this effect with just the normal laser
and not the fiber optic cable so you're
going to be looking at the surface of
the mirror diffusing some light right
off the surface of the mirror so we're
going to be looking at that diffuse
light coming off the mirror right on the
surface okay it's off center on my
finger right there let's move it
on it actually went
dim look at that it's off then put it
on it gets dimmer off center on off
center bright on Center Dim
look at that it's barely
shining so I'm barely getting any light
out of it it basically just starts
destroying the production of laser light
but why would this happen well when we
Shine the Light back inside the laser
it's no longer in Phase with the photons
bouncing around inside the laser since
this light has traveled some distance
and come back the laser production area
is specifically tuned to amplify
internal photons not external ones so we
could actually be stimulating a of
photons that are no longer in Phase with
the ones being produced in the laser
this can lead to destructive
interference meaning we actually get
less light but how could that be
possible we're shining more light in and
we're getting less light well it's true
let me show you another way to visualize
this you can see this interference
problem even better if we combine beams
of laser light that are outside of the
laser itself this is called a Michelson
interferometer in this setup I take my
laser light and split it through a beam
splitter so half of the light hits a
mirror behind it and half hits a mirror
at a right angle then each of these
beams combined together at one single
point on the back here okay so here's my
actual setup so here are my two light
sources they're actually mirrors in this
case and then what I have here is just a
lens that's going to spread out the
light so we can see it better cuz this
is just a laser pointer so basically I
would just get a tiny little dot see how
that's just a tiny little dot so I'm
going to spread it out with a lens here
so we can see it better okay so here's
what shows up on the back stop when I
have one light kind of this dim red
patch but when I turn on the other light
it gets brighter but there's dark spots
so you can see that there's this
interference patterns showing up now let
me turn off the other
light then turn it on off on off on off
on okay so now look what it looks like
like when I place the camera in one of
the dark fringes so I have a hole poked
here and I'm going to put it in the dark
Fringe and put a camera behind that hole
so you can see what it looks like when I
cover one light versus when both are on
so you can see here that it's actually
darker with both lights on and when I
cover one light then it becomes brighter
that's because the camera is now in the
dark area so when both lights are on
it's darker in that Fringe and then when
we take away one of the lights that
Fringe pattern doesn't show up up and so
it's actually brighter even with just
one light this is happening because the
beam paths are not exactly the same so
the light beams get out of phase from
each other in the dark spots the Peaks
and valleys of both beams are cancelling
each other out this is crazy it reminds
us that light is literally
electromagnetic waves that have the same
properties as normal waves they can
interfere with each other and even
cancel each other out so that shining
two beams of light can actually make
less light in some areas rather than
more light one interesting note here is
that I found it works best with a red
laser pointer if I try it with my green
laser pointer nothing really
happens even when I get it right
on but that's because green laser
pointers actually put out infrared light
and then they put it through a frequency
doubler Crystal like this and so when
you're shining the green light back in
it's just hitting the Crystal and it
doesn't convert it back to infrared
light so it doesn't mess with the laser
at all and then this one's really hard
to see on camera but this one's a violet
laser and I don't really see any
difference in the brightness of light
when I shine it back into the laser so
the red light seems to be most affected
by it so in the end we didn't end up
destroying the world by shining a laser
on itself and thanks for watching
another episode of the action lab and
we'll see you next time
[Music]
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