Physics Laboratory 10

Refraction

Objective:

In this lab students will trace the path of a light ray through a block of glass. They will determine the angle of incidence and the angle of refraction at two air-glass boundaries and use these angles to determine the index of refraction of crown glass.

Refraction is the change in direction of propagation of a wave when the wave passes from one medium into another, and changes its speed. Light waves are refracted when crossing the boundary from one transparent medium into another because the speed of light is different in different media. The speed of light in a medium is v=c/n, where n is the index of refraction of the medium. If q₁ is the angle the incident ray makes with the normal to the boundary between the two media, and q₂ is the angle the transmitted ray makes with the normal to this boundary, then Snell's law gives the relationship between these two angles.

n₁sinq₁=n₂sinq₂

For an air-glass boundary we can set the index of refraction of air equal to one. Measuring the angles a light ray make with the normal to the interface both in the air and in the glass, we can solve Snell's law for the index of refraction of the glass.

n_glass=sinq_air/sinq_glass

Equipment needed:

	Millimeter ruler
	Protractor

Part I

Procedure:

The images below show a laser beam passing through a square block of glass. The angle of incidence is different in each of the images. Click on each thumbnails to obtain a larger image and print out that larger image.

For each larger image carefully complete the diagram as shown in the figure below.

Measure the angles q_air and q_glass with an uncertainty of less than 1^o. Use a protractor, or count squares and use trigonometric relations. Measure the width w of the block and the displacement d of the ray with an uncertainty of less than 1 mm. The sides of each small square on the paper in the images are 2 mm long, the sides of each bigger square are 1 cm long. Scale your measured w and d appropriately (or count squares) and record your measurements in table 1.

Table 1

image#	q_air	q_glass	n_measured	w	d_measured	d_calc	difference in d's (%)

Data Analysis:

Use the results you obtained from each of the images to determine the index of refraction n of crown glass. Find the average value. Find the percent difference between this average measured value and the nominal index of refraction for crown glass, n=1.52.

The expected displacement of a ray passing through the glass block is

d=wsin(q_air-q_glass)/cos(q_glass).

From the figure below we see that

d/L=sin(q_air-q_glass),
w/L=cos(q_glass),

and therefore

d=wsin(q_air-q_glass)/cos(q_glass).

Image5160.gif (3579 bytes)

Use your measured values of the width of the block w and of the angles q_air and q_glass to calculate d. Compare this calculated value with your measured value of d and find the percent difference.

Part II

Procedure:

The images below show a laser beam passing through a triangular block of glass. The angle of incidence is different in each of the pictures. Click on each thumbnails to obtain a larger image and print out that image.

For the first of the two images carefully complete a diagram as shown in the figure below.

Image5161.gif (4245 bytes)

Measure the angles q_air , q_glass and the angle of deviation q_d with an uncertainty of less than 1^o and record these angles in table 2

Use your measured q_air and q_glass to determine the index of refraction n of crown glass and record it in table 2.

For the second image measure the angles q_air and the angle of deviation q_d with an uncertainty of less than 1^o. Note: q_d is the angle of deviation from the incident direction.

Explain what is happening in the second image.

Table 2

Image#	q_air	q_glass	q_d	n_measured

			X	X

Open Microsoft Word and prepare a report using the template shown below.

Name:
E-mail address:

Laboratory 10 Report

	Summarize the experiment.
	For part I, insert table 1. Report your average value of the index of refraction of crown glass and the percent difference between this average value and the accepted value. Comment on your three diagrams. How does the deviation d vary with q_air?
	For Part II, insert table 2. Comment on the your two diagrams. Do you observe refraction, reflection, or both?

Save your Word document (your name_lab10.doc) and attach it to an e-mail message to mbreinig@utk.edu.

Refraction

Objective:

Equipment needed:

Name: E-mail address:

Laboratory 10 Report

Name:
E-mail address: