The Speed of Light How to hold a hallway hostage for fun and intellectual profit. (starring: the equipment)

The team Anna Bezryadina Nathaniel Burd David Orenstein

Schematic (overview) Laser Beam Splitter Oscilloscope Control Box Mirror 2X this distance from the splitter to the mirror is the path difference between the two beams (we are assuming angles are negligible!). c=(Δd/Δt)

Zoom in on control box Photodetectors (light goes here) Detector outputs 12 Laser modulator (1 Mhz) Scope Modulator (1 Mhz) On Circuitry The control box detects the light beams by means of a photodiode circuit. It also modulates the laser and the oscilloscope so that the beam is pulsed at a frequency of one megahertz.

Zoom in on Circuitry 9V Incoming Light Diodes are one-way doors for current. When light hits the diode “reverse current” flows which is proportional to the intensity of the light. The increasing or decreasing reverse current is amplified by the circuit and measured by the oscilloscope Normal I can’t get through diode Reverse I gets through diode (this current is detected by scope) Scope

Why pulse at 1Mhz? To make measuring the time delay possible. Laser waves are basically identical and very fast (v~X10^14 Hz). The pulses make waves more distinguishable because we simply measure when each arrives. Box syncs up laser and scope. ?!?

Zoom in on Oscilloscope 1,A2,B Ch. A (from detector 1) Ch. B (from detector 2) External Trigger Input (from Control Box) B Start Delayed Main Start Stop.01 18

Results Circuits cause inherent delays! So what is Δt when Δd=0.05m (essentially 0)? Aha! Signal B delayed by -12.8ns (σ=1) compared to signal A Δt (ns)Measurer -11.3Anna -11.3Anna -11.7Nate -11.4Nate -17.2RWB, Ph.D RWB, Ph.D.

Results We took measurements for 4 spots in the hallway Average, error on bottom rows Nice, linear rise Δd (m)Δt (ns)Error (ns)

Results

The graph is a line t(ns)=aL(m)+b. So a has units of ns/m and c is therefore 1/a. (b has units of s). Accounting for errors in each time measurement the bottom line is that… a= (error.05082) ns/m b= (error 1.29) ns and c= X 10 8 (error.0416 X10 8 ) m/s 4.5% error from accepted value of 2.99X10 8 However, more than three errors off!

Bad Results? Error larger than statistics suggest. Neglecting angle Air not a vacuum Lenses & mirrors got in the way and slowed light down Measurement of distances was not very precise