Physic II Lab Experiment

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1. In this experiment, we are going to use the PHET Bending Light Simulation to verify Snell’s Law. Once we verify the law, we can use it with the simulation to demonstrate total internal reflection, a phenomenon important to fiber optic technology. In the last activity, we will work with the separation of colors by prisms and the creation of rainbows through a demonstration of the dependence of the index of refraction on the wavelength of light.

While completing the experiment Refraction, make sure to keep the following guiding questions in mind:

· Is the angle of refraction larger or smaller for a material with a higher index of refraction? 

· What are the applications of total internal reflection and what conditions do these applications place on the geometry of the devices.

· How the rainbows are created?

To complete the experiment you will need to:

1. Be prepared with a laboratory notebook to record your observations.

1. Click the image to open the simulation experiment.

1. Perform the experiment as described.

1. Transfer your data and results from your laboratory notebook into the lab report template provided at the end of this experiment description.

1. Submit your version of the laboratory experiment report.

In your laboratory notebook, you will collect data, make observations, and ponder the questions posed within the lab instructions.  Thus, the notebook should contain all the data collected and analysis performed, which will be invaluable to you as you write the results section of your laboratory report.  Furthermore, the notebook should contain your observations and thoughts, which will allow you to address the questions posed, both for the discussion section in the laboratory report and in helping you to participate in the online discussion included in the module.

 

M6A1 Experiment: Refraction

 

PART I – Snell’s Law

Snell’s Law contains four parameters: 2 indices of refraction and 2 ray angles. In the first portion of Activity 1, we are going to vary the index of refraction of material 2 in order to determine the effect that this parameter has on the refracted angle of the ray.

Start the PHET simulation “Bending Light”(if you haven’t done so already) by clicking on the image below.

http://phet.colorado.edu/sims/bending-light/bending-light_en.jnlp

http://phet.colorado.edu/sims/bending-light/bending-light_en.jnlp

· Select the protractor from the toolbox (see the illustration below).

· Use the red button to turn the laser on.

· Move the laser to set the angle of incidence to 50 degrees.

· Leave material 1 as air and the index of refraction as 1.00.

· Vary the index of refraction in material 2 between 1.00 to 1.60.

Record your results in your laboratory notebook. How did you break up the interval n2 = 1.00 to 1.60? Why did you choose this particular interval between data points? Use a table of indexes of refraction to find materials representative of various indexes of refraction. Look up at least 3 of their densities. In general, how does the index of refraction vary with the density of the material?

Now we are going to vary the angle.

· Select the reset button.

· Select the protractor from the toolbox.

· Use the red button to turn the laser on.

· Leave material 1 as air and the index of refraction as 1.00.

· Leave material 2 as water and the index of refraction as 1.33.

In your laboratory notebook, record the calculated angle of refraction for angles of incidence in 5 degree increments between 10 degrees and 80 degrees. Use the simulation to take data at these same angles. For each angle of incidence, find the difference between the calculated refracted angle and the measured refracted angle.

Part II – Total Internal Reflection

In your laboratory notebook, calculate, if possible, the critical angle (θcrit) for n1=1.60 and n2=1.00. Also, calculate the critical angle for n1 = 1.00 and n2 = 1.60.

· Select the protractor from the toolbox.

· Use the red button to turn the laser on.

· Set the angle of incidence to 50 degrees.

· Set the index of refraction of material 1 to 1.60.

· Set the index of refraction of material 2 to 1.00.

The light is now incident from a denser material to a less dense material. The light source is now inside a denser material than the surrounding material. As a result, the refracted ray will have a greater angle of refraction (in other words, it will be bent further from the surface normal) than the incident ray. Is there an incident angle for which the refracted ray will ever reach 90 degrees or essentially transmit no light into material 2?

Use the simulation to find this critical angle (θc) if it exists.

· Set the index of refraction of material 1 to 1.00.

· Set the index of refraction of material 2 to 1.60.

Use the simulation to find this critical angle (θc) if it exists.

In your laboratory notebook, compare your results from the simulation to the calculated predictions in your laboratory notebook.

Part III – Rainbows

In this activity, we are going to try to understand the structure of a rainbow. A rainbow is a multicolored arc in the sky caused by the reflection and refraction of light within water droplets in the atmosphere.

The most common form of a rainbow is the “primary rainbow,” or an arc that shows the familiar color spectrum, or rainbow pattern of red thorough indigo, with red being the outermost color. This rainbow is caused by light being:

1. Refracted while entering a water droplet,

2. Reflected inside on the back of the water droplet, and

3. Refracted again when leaving the water droplet.

In a “double rainbow,” a second arc is seen outside the “primary rainbow.” In the second arc, the order of colors is reversed. This second rainbow is caused by light reflecting twice inside water droplets.

· Select the tab labeled Prism Break.

· Select the circular prism.

· Use the red button to turn the laser on.

· Change the index of refraction to that of water.

· Select White Light on the laser controls.

· Select Show Reflections on the laser controls.

We are going to use the circular prism to represent the cross section of a water droplet. Position the laser and circular prism such that the laser refracts through the simulated water droplet like this.

Results and Analysis

In your laboratory notebook, write down which of the colors experience the greatest change in direction through refraction. Does this color order, indigo on top, match the order of colors seen when observing a rainbow?

Rolph, E. (unknown). Full featured double rainbow in wrangell-st. elias tational park, alaska. [Photograph]. Retrieved from http://commons.wikimedia.org/wiki/File:Double-alaskan-rainbow.jpg

How then can we account for the observed color order, red on top and indigo on the bottom? The diagram below can help.

 
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