Explaining Why Lasers are Easy to See on Projector Screens

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In summary, the output of a laser pointer is easy to see on a screen illuminated by an overhead projector because the laser's energy levels are inversed on the lasing transition.
  • #1
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Q) Why is it easy to see the output of a laser pointer on a screen illuminated by an overhead projector. Assume 1mW laser and 500W quartz halogen bulb. Explain quantitatively, and state all other assumptions.


Im not sure whether it involves the coherence length/time of the two waves, visibility or some variation of the two. I am really desperate for help on this one.

Im going to ask a double question (still lasers though) since this is my first post :smile:

Q) second things is simply what controls whether a energy level scheme is suitable or unsuitable for CW operation

I believe it depends on the difference between the two decy rates, or is that slightly wrong? or totally?
 
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  • #2
HINT: Compare the intensity of the "images" on the screen from each of the sources.
 
  • #3
In the simplest 3 level laser system you have a ground state [tex] | 0 \rangle [/tex], and two excited states [tex] | 1 \rangle [/tex] and [tex] | 2 \rangle [/tex]. The ground state [tex] | 0 \rangle [/tex] is pumped to the upper excited state [tex] | 2 \rangle [/tex] by the driving field. The simplest useful systems have a very small rate of spontaneous decay from the upper excited state to the ground state, while the rate of spontaneous decay from the first excited state to the ground state is usually very large, much bigger than all the other rates in the problem. You can easily see that this leads to a nearly empty [tex] |1 \rangle [/tex] state as compared with the [tex] | 2 \rangle [/tex] state, thus achieving the needed population inversion on the lasing transition.

Just as a note, you can imagine other three level laser schemes which are slightly rearranged, but nothing is essentially different.
 
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  • #4
Thanks to both of you, physics monkey I am going to assume that t2-1=20ns and t1-0=1000ns would give a large enough inversion for CW operation

May I also extend my question a bit more...If the resonator of a laser has one curved mirror and the other a flat grating will this make any difference to calculations of the beam parameters, or will it be the same as the curved-flat mirror geometry?
 
  • #5
I understand that resonators are only there for logitudinal mode control purposes. If they support the same modes, I don't see why they would be different. They also introduce losses in the path unfortunately, but this normally has an effect on efficiency rather than the output beam itself.
 

1. How do lasers work?

Lasers work by producing a beam of monochromatic, coherent light through a process called stimulated emission. In this process, atoms are excited to a higher energy state and then emit photons of light when they return to their ground state. The photons are all in phase and have the same wavelength, resulting in a concentrated beam of light.

2. Why are lasers easy to see on projector screens?

Lasers are easy to see on projector screens because they produce a concentrated beam of light that is directed onto the screen. This beam of light is also highly directional and does not spread out like traditional light sources, making it appear brighter and more defined on the screen.

3. How do lasers produce different colors?

Lasers can produce different colors by using different materials or techniques to create different wavelengths of light. For example, red lasers typically use a semiconductor material called gallium arsenide, while green lasers use a crystal called potassium titanyl phosphate. By adjusting the materials and techniques used, lasers can produce a wide range of colors.

4. Can lasers be harmful to the eyes?

Yes, lasers can be harmful to the eyes if proper precautions are not taken. The intensity of the laser beam and the duration of exposure can determine the level of harm. The eye's natural reflex to blink and look away also helps protect it from potential damage. It is important to always handle lasers with caution and wear appropriate eye protection.

5. How are lasers used in projectors?

Lasers are used in projectors as a light source to create images on a screen. In traditional projectors, a lamp is used to produce light, but in laser projectors, the light is produced by lasers. This allows for higher contrast and brighter images, as well as a longer lifespan for the projector. Laser projectors are also more energy-efficient and can produce a wider range of colors.

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