PhysicsLAB Lab
Target Practice - Revised

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The purpose of this experiment is to demonstrate your knowledge of refraction and reflection by directing a laser beam to selected targets. You may work in groups of two or three. Although you may work alone, it is not recommended. No groups of four or more are allowed.
Each group will be provided with the following equipment:
  • 45-45-90º isosceles right prism model
  • rulers and/or meter sticks
  • protractors
  • table diagram with selected targets
  • graph paper (if requested)
Part I:
As an preliminary exercise, you are to complete drawing the path of the ray through the following prototype using the fact that the prism has a 1.45 index of refraction. Carefully draw in each dotted normal and clearly label all angles. When you are done with you diagram, have your teacher initial and date it before proceeding to part II.
Refer to the following information for the next question.

Completion of practice triangular prism diagram
Teacher Certification 

Part II:
You will be provided with a table diagram showing the position and height of the laser as well as your circled target position.
Using Snell's Law, the Law of Reflection, and the knowledge gained from your diagram in Part I, you are to calculate and diagram the path of the laser's beam through the prism to its target letter (D through G).
Once you have done your calculations, each group will set up their prism on the testing table and I will turn on the laser one time. We will record where your beam strikes the target wall. If you are not successful, you will be allowed to recalculate your prism's orientation and position and try a second time.
Refer to the following information for the next four questions.

Using the prism to send the beam to its pre-selected target letter.
Our group's pre-selected target letter was ____ 

First attempt: our beam missed the target by ____ cm. 

Second attempt: our beam missed the target by ____ cm. 

Third attempt: our beam missed the target by ____ cm. 

Part III:
You will now be requested to use a plane mirror to return the beam to its original destination, X marks the spot!
To assist your investigations with this phase of the lab, I am going to give you a hypothetical situation for you to demonstrate to me that you could numerically calculate how many degrees the mirror would need to be pivoted to accomplish the requested task.
Suppose that your laser beam (red line - not drawn to scale) reaches the mirror at an angle of 49º after it exits the prism as shown in the following diagram.
Further suppose that the center of the mirror is 15 cm from the edge of the table and 112 cm from the laser's final destination.
Show how you would pivot the mirror to get the incoming beam to be correctly reflected (remember you cannot pivot the red beam, only the mirror). Show your calculations as well as a supporting diagram. Be sure to carefully construct the dotted normal and to measure the angles of incidence and reflection.
Refer to the following information for the next question.

Completion of practice plane mirror diagram
Teacher Certification: 

Your grade is based on your supporting calculations, your ray diagrams, and the number of trials required to successfully reach your pre-selected target letter.
Each team's work must be completed within the testing period and turned in to the one-way box before leaving the classroom.

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Catharine H. Colwell
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