What Happens When You Reflect a Laser Back Into Itself?

Summary

In 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

• 🔬 The experiment explores the effect of reflecting a laser beam back into its source.
• 🌌 A laser diode operates by applying current to a semiconductor, causing electrons to combine with holes and release photons.
• 🔄 Mirrors at the ends of the semiconductor stimulate emission, creating coherent light that matches the phase and direction of existing photons.
• 🚫 Reflecting the laser light back into the diode does not increase its intensity; instead, it can cause destructive interference, reducing the light output.
• 🔍 A Michelson interferometer setup is used to visualize interference patterns created by combining laser beams.
• 📉 When light beams are out of phase, they can destructively interfere, leading to dark spots where light intensity is reduced.
• 🔴 Red laser pointers are most affected by this interference, possibly due to their wavelength being more susceptible to the interference conditions.
• 🟢 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.
• 🟣 Violet lasers do not exhibit noticeable changes in brightness when their light is reflected back into the laser.
• 🌐 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 the赞助商 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

00:00

🌌 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.

05:01

🔬 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.

Keywords

💡Laser

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. In the video, the laser is used to explore what happens when light from a laser is reflected back into the laser source. The handheld red laser pointer used in the experiment is a common example of a laser diode, which operates on the principle of stimulated emission.

💡Laser Diode

A laser diode is a semiconductor device that emits coherent light when an electric current passes through it. The video explains how laser diodes work by discussing the interaction of electrons and holes in the semiconductor material, leading to the release of photons. The laser diode is central to the experiment, which investigates the effects of reflecting laser light back into the diode.

💡Stimulated Emission

Stimulated emission is a process where the interaction of light with matter causes the matter to emit light at the same phase and frequency as the incident light. This concept is crucial to the operation of lasers. In the video, stimulated emission is discussed in the context of how photons released by the laser diode are in phase, creating coherent light.

💡Coherent Light

Coherent light is light whose waves are in phase, meaning the peaks and troughs of the waves match up. This is a key property of laser light, which allows it to be focused into a tight beam. The video script explains how the mirrors at the ends of the laser diode help to create coherent light by reflecting photons that are in phase.

💡Mirror

In the context of the video, a mirror is used to reflect the laser light back towards the laser diode. The script mentions using a special type of mirror that is reflective on the front of the glass to facilitate the experiment. Mirrors play a critical role in laser diodes by reflecting light to stimulate further emission of photons.

💡Destructive Interference

Destructive interference occurs when two waves of the same frequency overlap and cancel each other out because they are out of phase. The video demonstrates this phenomenon by shining a laser back into itself, resulting in a dimmer light output. This concept is central to understanding why reflecting a laser into itself does not increase its brightness.

💡Phase

Phase refers to the position of a wave in its cycle. In the video, the phase of the reflected light is discussed in relation to the light already inside the laser. When the reflected light is out of phase with the internal light, it can lead to destructive interference, which is observed when the laser light is shone back into the laser.

💡Michelson Interferometer

A Michelson interferometer is an optical instrument that uses light interference to measure distances, refractive indices, and other properties. In the video, a simplified version of this instrument is used to demonstrate how two beams of light can interfere with each other, creating patterns of bright and dark fringes. This experiment helps to visualize the concept of light interference discussed in the script.

💡Frequency Doubler Crystal

A frequency doubler crystal is a device that converts light of one frequency to light of a higher frequency. The video mentions that green laser pointers use such a crystal to convert infrared light into visible green light. This is relevant to the experiment because when green light is reflected back into the laser, it does not affect the laser's operation as it does not convert back to infrared.

💡Infrared Light

Infrared light is a type of electromagnetic radiation with wavelengths longer than visible red light. The video script explains that green laser pointers emit infrared light, which is then converted to visible green light by a frequency doubler crystal. The experiment with the green laser pointer shows that reflecting it back into the laser does not affect the laser's operation, highlighting the importance of the type of light used in the experiment.

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]