Lasers, photo-electric effects and ionization of atoms

In summary, lasers typically cut metals by heating them up to melting point, using pure energy transfer rather than ionization. The photoelectric effect is not the most appropriate way to describe light interacting with solids. Reflection occurs due to a quantum mechanical effect where photons are absorbed and reemitted in any direction. Air has very little absorption due to its low density compared to solids. Diffusion of light occurs when it enters a denser material, but the direction of the wave does not change again after being reemitted from a second molecule. High power laser systems use dielectric mirrors rather than metalized mirrors.
  • #1
Nerd
17
0
I get the photo-electric effect but I am just wandering, is that the only principal on which lasers function? I mean do lasers "cut" materials by ionizing the atoms?
 
Physics news on Phys.org
  • #2
Lasers typically cut metals by heating them up to melting point. When applied, they typically vapourize metal at just that region, and heat is dissipated very fast which is why they're used - to give very decent cuts where CNC milling wouldn't be the greatest option.

No need for ionisation, just pure energy transfer. Also, I don't think the photoelectric effect is the most appropriate way to describe light interacting with solids. I don't know what is though, but plenty on here do.
 
  • #3
What you say makes sense, however if lasers did cut metal by heating it to melting point, I would think that this happens by exciting the electrons in the metal, which would lead to more kinetic energy and thus increase heat. If this is true than why do mirrors reflect lasers beams, afterall the same that happens to the metal's atoms should happen to the mirror's atoms?
 
  • #4
It depends vastly on the wavelength. Speaking from a generalised point of view, every material has a characteristic absorption spectrum - for each wavelength of electromagnetic radiation, we can say it'll absorb x% of it. For metal-cutting, you would use a laser tuned to maximum absorption (and hence maximum heating) which I think is in the infrared range. It doesn't matter if it's transparent to us or reflective to us as long as absorption takes place in whatever range of wavelengths that is viable.

I have no idea why reflection occurs to tell you the truth. It doesn't seem to be actually explained anywhere (at least pre-undergraduate level).
 
  • #5
Then we can assume that air (oxygen, nitrogen ect.) have practically no absorption, since the whole electro-magnetic spectrum can travel through air. This means that it would be impossible to cut solid oxygen with any type of laser, right? Do you know if a mirror can reflect X-rays?
 
  • #6
Nerd said:
What you say makes sense, however if lasers did cut metal by heating it to melting point, I would think that this happens by exciting the electrons in the metal, which would lead to more kinetic energy and thus increase heat. If this is true than why do mirrors reflect lasers beams, afterall the same that happens to the metal's atoms should happen to the mirror's atoms?

Fact is high power laser systems do not use metalized mirrors. They use what is called a dielectric mirror, it consists of multiple layers of a dielectric material, the thickness of a layer is determined by the wavelength of the laser and the angle of reflection needed. These mirrors are transparent to all but the wavelength designed for.
 
  • #7
Nerd said:
Then we can assume that air (oxygen, nitrogen ect.) have practically no absorption, since the whole electro-magnetic spectrum can travel through air. This means that it would be impossible to cut solid oxygen with any type of laser, right? Do you know if a mirror can reflect X-rays?

It depends on the absorption spectrum of oxygen. I would imagine that solid oxygen has a slightly different spectrum from gaseous oxygen (because these things mostly come from the bulk properties of a material and not the individual atoms).

As for x-ray reflecting mirrors, again, I have no idea. Considering that mirrors are simply just a very convenient way to execute a quantum mechanical trick, then surely an x-ray reflector should be possible. In fact there is research going on into antennas (the sort that you use to pick up TV and radio) being used to pick up and transmit visible light signals. This of course is not the same thing as an x-ray reflector but EM waves are just that, there is nothing special about any particular wavelength or range of wavelengths, other than the arbitrary significance we associate with them.

As I said, I honestly have zero idea as to why reflection occurs! Here's what Wikipedia says on the matter:

All interactions between photons and matter are described as series of absorptions and emissions of photons. When an arriving photon strikes a single molecule at the surface of a material, is absorbed and almost immediately reemitted, the ‘new’ photon may be emitted in any direction, thus causing diffuse reflection.[citation needed]

Specular reflection (following Hero's equi-angular reflection law) is a quantum mechanical effect explained as the sum of the most likely paths the photons can take. Light–matter interaction is a topic in quantum electrodynamics, and is described in detail by Richard Feynman in his book QED: The Strange Theory of Light and Matter.

The energy of the incoming photon may match the energy required to change the molecule from one state to another, causing a transition in kinetic, rotational, electronic or vibrational energy. When this occurs, the photon may not be reemitted or alternatively may be reemitted with a loss of energy. These effects are known as Raman, Brillouin and Compton scattering.

In fact I did read the book cited and came out knowing less :rofl:.Oh, I might add - air has very little absorption mostly because it's far less dense than a solid. In my early education, that corresponds to a very very low decay/absorption constant (look up the exponential law of absorption). Whereas, with a solid there is a much higher chance of absorption.
 
Last edited:
  • #8
Thank you.
I have another question though. Diffusion occurs when a wave enters a optical denser material or visa versa, for instance, a light wave moving from air into water will change direction. Does the light wave's direction change at the first water molecule that reemits it or does it happen deeper into the water, and why doesn't the direction of the wave change again as it is reemited from the second molecule?
 
  • #9
Integral said:
Fact is high power laser systems do not use metalized mirrors. They use what is called a dielectric mirror, it consists of multiple layers of a dielectric material, the thickness of a layer is determined by the wavelength of the laser and the angle of reflection needed. These mirrors are transparent to all but the wavelength designed for.

I think he was talking about mirrors outside lasers :). But hey, I didn't actually know that snippet.

Nerd said:
Thank you.
I have another question though. Diffusion occurs when a wave enters a optical denser material or visa versa, for instance, a light wave moving from air into water will change direction. Does the light wave's direction change at the first water molecule that reemits it or does it happen deeper into the water, and why doesn't the direction of the wave change again as it is reemited from the second molecule?
That's refraction, not diffusion. I think it's covered in one of the FAQ threads here. Other than that, I honestly have no idea (I'm actually trying to find out why myself).
 
  • #10
dst said:
I think he was talking about mirrors outside lasers :). But hey, I didn't actually know that snippet.
So was I.
http://en.wikipedia.org/wiki/Dielectric_mirror" [Broken]
 
Last edited by a moderator:
  • #11
Nerd said:
Thank you.
I have another question though. Diffusion occurs when a wave enters a optical denser material or visa versa, for instance, a light wave moving from air into water will change direction. Does the light wave's direction change at the first water molecule that reemits it or does it happen deeper into the water, and why doesn't the direction of the wave change again as it is reemited from the second molecule?

Sorry, I meant refraction. It's a translation error on my part.
 
  • #12
Nerd said:
Then we can assume that air (oxygen, nitrogen ect.) have practically no absorption, since the whole electro-magnetic spectrum can travel through air. This means that it would be impossible to cut solid oxygen with any type of laser, right? Do you know if a mirror can reflect X-rays?

The whole electromagnetic spectrum is really really large (really infinite)

At any rate, both molecular nitrogen N2 and molecular oxygen O2 absorb very strongly in the ultraviolet. Almost 100% of the incident light from the sun below 190 nm is absorbed. Modtran is a wonderful program for simulating transmittance through the atmosphere

google Modtran and Hitran
 
  • #13
Nerd said:
Do you know if a mirror can reflect X-rays?

x-rays can be reflected at large (close to 90 degrees) angles of incidence. This principle has been used by astronomers to make x-ray telescopes.

See http://en.wikipedia.org/wiki/Wolter_telescope
 

1. What is a laser and how does it work?

A laser, or light amplification by stimulated emission of radiation, is a device that produces a narrow and intense beam of light. It works by stimulating atoms or molecules to emit photons, which then bounce back and forth between two mirrors to create a concentrated beam of light.

2. What is the photoelectric effect?

The photoelectric effect is the emission of electrons from a material when it is exposed to light. This phenomenon was first observed by Albert Einstein and is used in many technologies, such as solar panels and image sensors in cameras.

3. How does ionization of atoms occur?

Ionization of atoms occurs when an atom gains or loses electrons, resulting in a charged particle called an ion. This can happen through various processes, such as high temperatures, collisions with other atoms, or exposure to radiation.

4. What are some practical applications of lasers?

Lasers have many practical applications, including in scientific research, medical procedures, telecommunications, and manufacturing. They are used in technologies such as barcode scanners, DVD players, and laser printers.

5. How do lasers play a role in spectroscopy?

Lasers are often used in spectroscopy, which is the study of the interaction between matter and electromagnetic radiation. The intense and focused light beam of a laser allows for precise measurements and analysis of the absorption and emission of light by atoms and molecules.

Similar threads

  • Quantum Physics
Replies
2
Views
164
  • Quantum Physics
Replies
5
Views
2K
  • Quantum Physics
Replies
3
Views
1K
Replies
1
Views
923
  • Atomic and Condensed Matter
Replies
18
Views
1K
  • Classical Physics
Replies
1
Views
502
  • Quantum Physics
Replies
4
Views
825
  • Atomic and Condensed Matter
Replies
4
Views
945
  • Atomic and Condensed Matter
Replies
3
Views
1K
Replies
1
Views
634
Back
Top