Date: Tue, 15 Feb 2011 09:39:00

Author: Doug Johnson

Subject: Re: Question Speed of Light Experiment


Steve Wonnell and Others who responded,
Thank you so much for your help! I really appreciate the time and effort
you have put into your responses on the Speed of Light Experiment.
I will passed on this information to those on our lab committee who are
working on this. I am not sure which route we will go. Our college has set
aside $50k for the purchase of 12 setups. Many faculty feel it is too
expensive for what it teaches, yet we have others willing to bump it up to a
full Newport Optical Bench System with support equipment. Hopefully we will
find something that works and doesn't create a lot of headaches down the
I do have one more question on this subject. Someone suggested this
setup below.

Any comments on this? It looks like it would work and with their diode
laser it would cost much less than the Pasco setup.

Thanks again for your comments!

....Doug J.

PS. Steve, thanks again for your last comment below on the Pasco setup!

On 2/15/11 6:09 AM, "Steve Wonnell" wrote:

> Hi Doug,
> I'd like to speak up in support of the rotating mirror approach.
> We used this when I taught the modern physics lab about 15 years ago,
> but the apparatus sold by Pasco today appears to be identical to that
> which I used then.
> The approach is beautifully elegant, and relies almost completely on
> classical geometry and optics.
> One point to note is that the reflecting mirror is not flat; it's slightly
> concave and designed to re-focus the laser beam back to a point at the
> point of observation and to be located a specific distance from rotating
> mirror. If you put the mirror closer or further away, the focus is not
> quite so sharp. Also, as Jerry notes, the shift in beam spot location is
> proportional to the distance from the reflecting mirror, so in general you
> do want the mirror as far away as is feasible. Given the curvature of the
> reflecting mirror, it is best to work with the designed distance, which I
> think may have been something like 15m--I don't remember the specific
> distance.
> You can always add additional flat mirrors to the table to extend the beam
> length. No need for a long hallway or large classroom. This can be done
> on a small (3' x 7' table) with flat mirrors to extend the length of the
> laser beam.
> It is true that a modulated laser beam, time-of-flight measurement
> detected by an oscillscope, is simpler, more direct, more robust, and
> maybe less expensive. But I think that the rotating mirror approach
> has an elegance to it, not to mention an historical interest, that the
> more direct approach lacks and that it is well worth exposing students to
> this approach.
> I don't know how well the rotating mirror unit holds up, but it is
> certainly delicate and the instructions regarding its use must be
> followed. This is true of most optical apparatus, though.
> -- Steve W.
> On Fri, 11 Feb 2011, Jerry DiMarco wrote:
>> Date: Fri, 11 Feb 2011 12:24:56 -0700
>> From: Jerry DiMarco
>> Reply-To:
>> To:
>> Subject: Re: [tap-l] Question Speed of Light Experiment
>> We have been using this apparatus for several years, and I would not
>> recommend it. At that price, one would expect many years of service. But
>> all units have needed work, most often the mirror bearings and motor speed
>> control. Though the mirror rotates at ~90,000 RPM (with 1 min duty cycle),
>> you still need 10 - 15 m between mirrors to get the desired accuracy. We
>> unfortunately have only 6 - 7 m between mirrors, which makes the reading
>> difficult to take.
>> As a lab activity, it is difficult for many students to get through
>> in the 2 hours we alot. Some instructors have chosen to assist the
>> students in some way so they can complete the lab on time. Mostly I hear
>> grumbles about the lab. I'm not sure how much longer it will be used here...
>> Jerry
>> D
>> At 2/11/2011 09:44 AM, you wrote:
>>> Does anyone have a Speed of Light Experiment that they would
>>> recommend?? We are seriously looking at the Pasco setup below. Your
>>> comments would be
>>> appreciated. ...Doug J
>>> Basic Speed of Light Apparatus OS-9262
>>> ID=635&Detail=1
>> <><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
>> Jerry DiMarco
>> Manager of Lecture Demonstrations and Instructional Labs
>> Montana State Univ., Physics Dept.
>> Bozeman, MT
>> Our Motto: "A demo a day helps learning to stay"

From Tue Feb 15 14:04:28 2011