Objectives

• Use a laser to observe reflection & refraction of light
• Use Snell's law to determine the index of refraction of various materials
• Observe total internal reflection

Introduction

On reflection, angle incident to the surface equals angle reflected from the surface. In optics, the angle of a light ray is determined from the normal (perpendicular or 90 degrees) to the surface.

On refraction, light bends in traveling from one medium to another. This is described by Snell's law;
                    n₁sinθ₁=n₂sinθ₂
The parameter n is a property of a medium called index of refraction (a material's index of refraction is related to the speed of light v  in the material by n=c/v ). Note, angles in Snell's law are determined from the normal to the surface. Of interest, by measuring the angle that light makes at the boundary, one can determine index of refraction for the medium.

In this lab a laser is used as a collimated beam of bright light that can be easily traced through its traveled path. In addition, a computer simulation is used to help understand total internal reflection.

This experiment is to be done separately by student partners (partners will have different data). Students are asked to complete all of one component and then move onto another component. Components can be done in any order.

For each component of this experiment (including the computer simulation), sketch the position of all light beams and refraction mediums (note, better results are obtained with larger incidence angles). Measure the incident and refracted angles from the sketch using a protractor (with the computer simulation, use the protractor tool). Determine the index of refraction for the material. Also do any additional requirements for each component.

Caution with laser:  Be very careful in this lab not point the laser at other students or in your own eyes. Although the laser used in this lab is of very low power and will not cause eye damage unless pointed into an eye for several seconds, the beam can still cause momentary blindness similar to briefly glancing at the sun. Be safe with your eyes, they are valuable.

Experiment Note

For this experiment, you are required to make your own sketches and measurements. These sketches and measurements are expected to be different than your partner's.

Prelab

Textbook reference: section on Snell's law

1.  What is the index of refraction for glass, water and plastic (the values)? 

2.  Describe what difference it makes to index of refraction with the color of light that is used?
        - Is the index for red light larger, smaller or the same as for blue light?
        - Is the angle of refraction (i.e. the angle calculated from Snell's law) of red light larger, smaller or the same as for blue light?

3.  Sunlight enters a lake of polluted water on an angle. Determine the index of refraction of the polluted water (which is different than pure water) from the angle that sunlight makes on going into a lake.
     Start by drawing a diagram of light entering the water on an angle and indicate what angles are measured. Then write down Snell's law with the symbols used on your diagram. Finally solve for water index of refraction using symbols and the known index of refraction of air.

Glass index of refraction

Experiment

1. Place the plate glass on paper and shine the laser through the plate glass at an angle. Large incidence angle (large to the normal) yields the best results.

2. Now record on the paper the laser beam positions and the plate glass position. Include also the reflected beam  as well as the beam that passes through the plate.

Analysis

• On the paper, sketch/label in details such as incident, reflected and refracted beams. Outline/label the plate glass. Make it obvious on the paper what was happening.
• Measure the angles of reflection and refraction with a protractor on the paper sketch
• Determine the index of refraction for the glass plate.

Questions

• Is there any difference between the exit angle compare to the entry angle.
• Is there any difference between the reflected angle with the incident angle
• Is it more accurate to measure angles to the normal or to the surface. Which do the equations use?

Plastic index of refraction

Experiment

1. Place the solid plastic semi-circle on paper and shine the laser through the plastic flat surface close to the center of that flat side. A large incidence angle (large to the normal) yields the best results.

2. Now record on the paper the laser beam positions and the semi-circle position. Include also the reflected beam  as well as the beam that passes through the plastic.

Analysis

• On the paper, sketch/label in details such as incident, reflected and refracted beams. Outline/label the plastic semi-circle. Make it obvious on the paper what was happening.
• Measure the angles of refraction with a protractor on the paper sketch
• Determine the index of refraction for the plastic.

Questions

• How exact can you measure angles in this experiment.
• How exact is your sketch in noting beam positions. Relate this to an uncertainty in angular measurements.

Water index of refraction

Experiment

1. Place the plastic semi-circle container, filled partly with water, on paper and shine the laser through the plastic at an angle, close to the center of the flat side.

2. Now record on the paper the laser beam positions and the semi-circle position. Include also the reflected beam  as well as the beam that passes through the water.

Analysis

• On the paper, sketch/label in details such as incident, reflected and refracted beams. Outline/label the plastic semi-circle container of water. Make it obvious on the paper what was happening.
• Measure the angles of refraction with a protractor on the paper sketch
• Determine the index of refraction for water.

Questions

• How does the plastic holder affect your determination of n?
• How does the semi-circle shape affect the exit angle of the light beam.

Lab Report

(Lab Reports must be submitted to the Physics lab at the end of your lab period.  The lab report may be submitted on a separate piece of paper or as a page in your notebook.  See Lab Point)

Refraction Computer simulation

Overview

Click this link to open the refraction simulation (a PhET simulation) in a new window
and then select Intro.

The simulation shows a ray (turn on by clicking red button) striking an interface.  
Change the angle by dragging the source.
Notice larger angles (as measured to the normal) change more on refraction.
Identify the incident, refracted and reflected rays.

Clicking the Wave mode shows refraction changes to the ray wave lengths.
Switch back to the Ray mode and move the protractor into position
for measuring the ray angles. 

For all parts be sure to show your work, which includes indicating the angles you used to calculate the results.

1. Index of Refraction

Set up the simulation for a ray to travel from water into Mystery A material.
Determine the incident and reflected angles.
1a.  Calculate the index of refraction of Mystery A material.

Set up the simulation for a ray to travel from air into Mystery B material.
Determine the incident and reflected angles.
1b.  Calculate the index of refraction of Mystery B material.

2. Total Internal Reflection

Set up the simulation for a ray to travel from glass into water.
Move the source around and notice at certain angles
the ray no longer bends into the water.
This is total internal reflection within the glass material
(ray does not exit the glass material)

2a.  Determine the incident angle where the refracted ray just disappears (actually, the refracted ray is skimming the surface at 90 degrees).  This incident angle is call the critical angle.

Total Internal Reflection

Total internal reflection can occur when light is in a material that is more dense than its surrounding. Light incident on the internal surface at certain larger angles will reflect back into the material with no light escaping (no light refracting out). This is the foundation to fiber optics (now often used in communication transmission). The incident angle at which light just escapes (refracted angle of 90 degrees) is the called the critical angle.

Experiment

• With the half circle solid plastic, see if you are able to observe total internal reflection by shining the laser into the plastic on the circular part (and observing the internal reflection from the planer part). Determine the critical angle for total internal reflection in the plastic.