Lab 3: REFLECTION, REFRACTION OF LIGHT AND TOTAL INTERNAL REFLECTION

Group #:________________

People (please print names)

1. Group Leader____________________________

2.____________________________

3.____________________________

4.____________________________

5.______________________________

Lab 3: REFLECTION, REFRACTION OF LIGHT AND TOTAL INTERNAL REFLECTION

	Warning: The laser beam is intense enough to burn the retina of your eye. Never look directly into a laser or let the laser beam shine directly or reflect into anyone’s eye.

Objectives:

1. To examine the reflection of light at a surface between two transparent materials quantitatively
2. To understand the law of reflection
3. To examine the refraction of light at a surface between two transparent materials quantitatively
4. To understand Snell’s law of refraction
5. To observe total internal reflection and discover under what circumstances it occurs.

Materials:

To complete this lab you will need the laser pointer and the semicircular transparent chamber provided in your lab kit. In addition, you will need a transparent plastic ruler.

You also need to fill the semicircular transparent chamber with a slightly cloudy liquid: fill it with water and add a few drops of coffee creamer. Slightly clouding the liquid will enable you to better observe the laser beam through the liquid.

PART 1: REFLECTION AND REFRACTION OF LIGHT

Place the semicircular chamber on a plane sheet of white paper, use the laser pointer to shine a laser beam as close to the paper as possible, and it should hit the chamber near the center of its flat surface.

You can use a paper clip to keep the button on the laser pointer pushed in and the laser beam on to make your measurements. However, do not keep the laser beam on for too long, otherwise you will drain the batteries and you will need to replace them to complete the experiment.

1. Determine the angle of reflection and the angle of refraction for seven different angles of incidence stating with the incident ray along the normal. The beam should hit the flat surface at the same point.

For each measurement, mark carefully and precisely the middle of the semicircular chamber, the point of incidence, the point of reflection and the point of refraction. Then you can remove the laser beam and trace the incident and the reflected ray and the refracted ray.

You should decide and agree within your group on the values of angles of incidence you want to do the measurements for. You will need to fill out a table with the best results obtained by your group.

The pictures show our data for an angle of incidence of 30⁰:

• Share your results with the other members of your group and agree on the values that you want to enter in the table below, θ₁ is the angle of incidence θ_1r is the angle of reflectionand θ₂ is the angle of refraction:

	θ1 (incidence)	Sin θ1	θ1r (reflection)	θ2 (refraction)	Sin θ2
1
2
3
4
5
6
7

• Based on your measurements, what do you conclude about the relationship between the angle of incidence and the angle of reflection?

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• We can define the plane of incidence as the plane defined by the incident beam and the normal to the surface of the transparent chamber. Based on your observations, is the reflected beam also in this plane? Explain how you reached your conclusion.

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• Insert sin θ₁ and sin θ₂ data from the table above into an excel spreadsheet. Create a data chart with the horizontal axis being sin θ₁ andthe vertical axis being sin θ₂. Use the fit routine in the excel software to find the relationship between sin θ₁ and sin θ₂.

• Using Snell’s law of refraction and given that the index of refraction of air is 1.00, calculate an average value for the index of refraction of the liquid in the transparent chamber.

Show the calculations that your group has decided to be correct here:

• Determine the index of refraction of the liquid in the transparent chamber with a second method, based on your graph.

• Each group member should make their own calculations and their own measurements then put together your results and fill out the following table:

	Name	naverage calculated	naverage determined from the graph
1
2
3
4
5

• Decide within your group which data are the best, upload that Excel file and the graph with the fit on the submission site along with the completed worksheet.

PART 2: TOTAL INTERNAL REFLECTION

In this part of the lab you will investigate total internal reflection, the phenomenon that allows light to travel long distances inside a light fiber.

This time the laser beam enters the chamber through the curved face. In this activity, be sure that the beam always travels along a radius, and, therefore, always strikes the center of the flat surface of the semicircular chamber for every incident angle.

1. Is there an angle of incidence for which no light is transmitted from the water into the air? If so, what is that angle? (Hint: Remember that θ₂ cannot be greater than 90°. Calculate the corresponding θ₁, given that n₁=1.33 and n₂ = 1.00.)

Show the calculations that your group has decided to be correct here:

• With the same setup and method as in the first part of the lab measure θ₁ and the corresponding θ₂ for several different incident angles.  Observe the refracted ray as you increase θ₁ up to about 60°. Record your values.

Carefully locate and measure the smallest incident angle for which there is no transmitted ray and record your value. That is your experimental value for the critical angle.

• Share your results with the other members of your group and agree on the values that you want to enter in the table below.

	θ1	θ2
1
2
3
4
5
6		No transmitted ray
7		No transmitted ray

• What was the experimental value you found for the critical angle? Fill out the following table with the experimental results obtained by each member of the group:

	Name	θcritical
1
2
3
4
5

• Does the experimental value for the critical angle agree with the one you calculated in Question 1 in this part of the lab? Explain.