1. Refraction Lab
Use ray tracing and Snell’s Law to determine the refractive index of glass and water

2. Equipment Cardboard Straight pins Graph paper Glass block
Semi-circular glass Petri dish.

3. Refraction Lab data collection for glass block
Place the graph paper on top of the cardboard.
Place the glass block on top of the paper, aligned with the grid.
Trace the glass block with a pencil.
Then, while peering through the block, arrange four straight pins so that they appear to be aligned when viewed through the block. Two must be in front of the block, and two behind.
Mark the position of the pins, and label the positions with the trial number.
Repeat for at least three more trials.
Remove the glass block from the paper.
Using a ruler, connect the pins for each trial with a straight line to trace the path the light rays took as they traveled through the block. Make sure you bend the rays only at the surface of the block.

4. pin
normal θ1 θ2
pin
pin θ2
normal
This figure shows one ray of light as it passes through the glass block, as viewed from above. When viewed through the block, the pins appear to be aligned, but when viewed from above, they clearly are not. θ1
n2=?
pin
n1=1.0
Example glass block ray tracing.

5. Refraction Lab data analysis for glass block
Use a protractor to measure q1 and q2 as the light enters and leaves the glass. Make sure your measurements are relative to the normal, and not relative to the surface.
Use Snell’s law to determine the refractive index for the glass. Snell’s law is n1sin(q1)=n2sin(q2). The refractive index of air is 1.0.
For four trials, you should get eight sets of angles, and eight estimates of the refractive index of glass.
Average your eight values of the refractive index of glass.
Advanced lab: Figure out how to graph your Snell’s Law data such that the index of refraction appears as the slope of a best-fit straight line.

6. Refraction Lab data collection for semi-circular water dish
Place the graph paper on top of the cardboard.
Fill the water dish about half full, and place the water dish on top of the paper, aligned with the grid.
Trace the water dish with a pencil.
Then, while peering through the water, arrange three straight pins so that they appear to be aligned when viewed through the block. One pin must be placed in at the very center of the flat side of the dish, right up against the dish. One of the other pins must be placed in front of the dish, and the other behind the dish.
Mark the position of the pins, and label the positions with the trial number.
Repeat for at least three more trials.
Remove the water dish from the paper.
Using a ruler, connect the pins for each trial with a straight line to trace the path the light rays took as they traveled through the water. Make sure you bend the rays only at the flat surface of the dish.

7. Example ray tracing through water disk. pin θ2 θ1
Example ray tracing through water disk.
pin
normal
This figure shows one ray of light as it passes through the water dish, as viewed from above. When viewed through the water, the pins appear to be aligned, but when viewed from above, they clearly are not. Why does the ray not bend on the curved side of the dish, and only bends on the flat side?
n2=?

8. Refraction Lab data analysis for water dish
Use a protractor to measure q1 and q2 as the light enters the water dish on the flat side. Make sure your measurements are relative to the normal, and not relative to the surface.
Use Snell’s law to determine the refractive index for the water. Snell’s law is n1sin(q1)=n2sin(q2). The refractive index of air is 1.0.
For four trials, you should get four sets of angles, and four estimates of the refractive index of water.
Average your four values of the refractive index of water.
Advanced lab: Figure out how to graph your Snell’s Law data such that the index of refraction appears as the slope of a best-fit straight line.