PHYS 1407 – Conceptual Physics II
Rectilinear Propagation of Light
Leader:
_____________________________ Recorder:
___________________________
Skeptic:
_____________________________ Encourager: _________________________
Materials
Two light sockets with identical low voltage bulbs
4 x light weight alligator clips
Piece of cardboard with 1 cm circle cut out
Piece of cardboard with similar sized equilateral triangle
Sheet of white paper
Flashlight
Introduction
In this activity we will investigate the direction that light travels in air.
Procedure
In the following make all predictions honestly and completely before carrying out the task. Predictions are not graded right or wrong, but only that they were made. Honest, even if incorrect, predictions are a much more valuable learning tool.
You will hold the light bulb about a foot in front of the piece of cardboard with the bulb centered on the hole in the mask. The white sheet of paper will be used as a screen to view the light about 1 foot behind the piece of cardboard.
P1) Predict the appearance of the light on the screen.
P2) Predict what you would see on the screen of you moved the light bulb upwards.
P3) Predict what you will see on the screen if you move the light bulb downwards.
Connect a light bulb to the battery holder and test your predictions. Disconnect the light bulb after you’re done to preserve battery life.
Q4) Were your predictions correct? Resolve any discrepancies. Be sure to address all three predictions.
Now you will hold a second light bulb over the first one.
P5) Predict how the light on the screen will appear when you hold a second light bulb above the first.
P6) Predict how moving the upper light bulb slight upwards will affect what you see on the screen.
Connect the two light bulbs in series. Hold the first one centered on the middle of the mask and the second one a few inches above the first. Test both of your predictions.
Q7) Were your predictions correct? Resolve any discrepancies. Be sure to address both predictions.
Q8) You probably noticed that you obtained two bright spots. Why are there two bright spots in this case?
Q9) Describe in the space below a test so that you can determine which bright spot corresponds to which bulb.
Q10) Carry out your test and record your results.
Q11) What do your observations suggest about the path taken by the light?
D11) On the figure below, draw in the bright spots on the screen to the right of the mask.
D12) On the figure draw a line from the bulb to the bright spot it produces.
Q13) Does the line you drew in D12) match your answer to Q11).
P14) Predict what you will see on the screen when you hold a single bulb in front of the piece of cardboard with the triangle cut out of it.
Q15) Test your prediction. Were you correct? Resolve any inconsistencies.
P16) Predict what you will see if you hold two light bulbs in front of the triangular mask.
Q17) Test your prediction. Were you correct? Resolve any inconsistencies.
D18) On the figure below, draw three lines from the light bulb, one through the center of the triangle, one through the top and one through the base.
D19) Use the lines your drew in D18) to draw the triangle you should see on the screen.
Q20) Does the triangle you drew in D19) agree with the observation you made in Q15)?
Lines showing the direction that light travels are called rays. A small light bulb like the ones we are using is an example of what is referred to as a point source of light.
D21) Draw rays showing light coming from the point source shown in the following figure. Be sure to indicate all directions the light could travel.
S22) Rectilinear propagation means essentially that things travel in straight lines. Does light travel in straight lines? Support your answer with specific evidence drawn from this lab.