Science Buddies: "Ask an Expert"

Hi, I need help please, I need to do an experiment to determine the strength between 2 different laser pointers, I have no idea where to start as we have had no introduction to this in class. I would appreciate any help, thanx Jaco

Hi!
As an interesting twist to the project, you might want to look at the sciencebuddies physics project ideas which deal with lasers:
https://www.sciencebuddies.org/science- ... ?from=Home
Here are also some links dealing with laser pointers in general:
http://optical-components.globalspec.co ... r_Pointers
http://www.scif.com/safety/safetymeetin ... icleID=216
Hope this helps, good luck!

JacoMuller wrote:Hi, I need help please, I need to do an experiment to determine the strength between 2 different laser pointers, I have no idea where to start as we have had no introduction to this in class. I would appreciate any help, thanx Jaco

I'm not sure what you mean by 'strength'- as far as I know, it is an undefined term for lasers. Three characteristics of a laser that are commonly measured are the power (or energy), the beam size, and beam divergence (how fast it spreads out to a larger spot). To measure the power of a laser, you need a power meter or photodiode. To measure the beam divergence, you could measure the beam size at two distances and estimate the divergence from rules of geometry. [The other characteristics of a laser that are usually specified are the wavelength, and if the laser is 'continuous' (like a laser pointer) or pulsed (in which case the rate of pulsing is also specified).] Sometimes, but more rarely, the shape of the beam is also listed. ] All of these characteristics contribute to the perceived brightness and the amount of damage a laser can do. Here is an example (from a catalog selling laser pointers) describing the laser in terms of the parameters I mentioned:

...The 532nm wavelength is at the peak sensitivity of the human eye, yielding an exceptionally bright green beam...
Laser Type Class IIIa Laser Diode
Maximum Output Power <5 mW
Wavelength 532nm
Beam Diameter 1.1mm
Beam Divergence <1.2 mrad
Power Requirements 2 AAA Batteries (included)
Operating Time 3 hours continuous use
Dimensions 0.51" Dia. x 5.63" L
Weight 2.7 oz. (with batteries)

You should check with your teacher about what you are actually supposed to be measuring. Also- when working with lasers be sure to follow appropriate safety procedures. See:

http://felesmagus.com/pages/lasers-safety.html

While most laser pointers are _relatively_ safe, it is always a good idea to treat all lasers in a safe manner. You learn good habits, and you are extremely unlikely to hurt yourself when you 'play it safe'. Some laser pointers, particularly the green ones, are sufficiently high power to cause eye damage and/or damage other materials. Class III lasers should always be handled extremely carefully, as eye damage is possible.

The wikipedia has fairly good pages on lasers, if you need some background information.

Good luck.

Louise

Could a radiometer (the partially evacuated glass bulb with the spinning vanes) be used to determine, or compare various laser pointer powers? Would a 1 mW pointer cause the vanes to spin half as fast as a 2 mW pointer?

Could a radiometer (the partially evacuated glass bulb with the spinning vanes) be used to determine, or compare various laser pointer powers? Would a 1 mW pointer cause the vanes to spin half as fast as a 2 mW pointer?

I have no idea. This is something that you would have to run a controlled experiment on. To my knowledge, a full understanding of the physics involved in a Crookes Radiometer has not been deterimined (see http://en.wikipedia.org/wiki/Crookes_radiometer).

I strongly doubt that the rotational speed of a Crookes radiometer would be linearly related to the laser input power. You also have the problem of needing to spread the light from the laser into a big enough area to interact with enough of the surfaces to have an effect.

If the lasers are in the visible spectrum and have the same color spectrum, then you can compare them using the same techniques used in some of the early candle power light intensity experiments that involve shining two light sources onto a translucent screen and varying their distances until the intensities appear equal.

Craig- Thanks for your reply.
Having worked with high power lasers (up to 1,000 watts), I've seen laser power meters which consisted of a graphite block with a meat thermometer inserted into it with a scale on the thermometer that indicates laser power in watts. In short, it measures the heating effect of the laser.
If we assume the radiometer also functions via a heating effect, would it not be a relative indicator of laser power also?
The challenge is to determine that all the laser energy is transformed into heat, and the heating effect is accurately measured. That would require the entire beam hits the absorber, and is of a wavelength that will be completely absorbed by the absorber.
Regardless, a quick Google revealed that a standard Radio Shack silicon phototransistor can be easily made into a laser pointer relative power meter.
I was surprised that the Crooks radiometer was discovered (like so many other fascinating gadgets) by accident!
John

If we assume the radiometer also functions via a heating effect,

Unfortunately, that is a HUGE assumption that is known not to be the full explaination. My intuition based on my background in modeling various dynamics with respect to this device is that convection currents in a partial vaccum make this device extremely difficult to analyze accurately because the boundary conditions are dynamic and occur at the same dynamic locations where the wave equations also have changing boundaries. When neither Einstein nor Reynolds managed to figure it out, it is going to be one of those very interesting challenging things that is fun and frustrating to work with but unfortunately won't pay the bills.

Hi jwax,
Thanks for bringing up a very interesting topic of discussion. I agree with your hypothesis that higher power laser should (in general) produce a higher rate of rotation in a radiometer. This would be analogous to the commonly observed phenomenon that a radiometer spins faster in bright light versus weak light, or in the vicinity of a strong infrared radiation source versus a weak source. There are various videos online showing radiometers spinning under the influence of lasers (e.g http://ledmuseum.candlepower.us/ninth/radiom.htm). Craig also raises good points that the physics of the device are subtle and not well understood, boundary conditions are complex, and differences between a focused laser and dispersed natural light come into effect, so the relationship might not be simple. Also, these devices are very sensitive to the internal gas pressure, so the relationship might differ from one radiometer to another. If you ever do derive an empirical calibration curve between laser power and rotational speed in a radiometer, please let us know the results!
Thanks again for the interesting topic of discussion.
Chris

Thank you folks for the stimulating responses!
It's back to a darkened room for me, and perhaps with a calibrated power meter, and a handful of laser pointers, I'll be able to report some findings!