# Request for Advice on a Physics Lab

#### fishspawned

Hello
I'm trying to come up with a solid Lab to use for my chapter on Waves and Optics. I am considering having it center on refraction. Here is the problem:

All labs I create for my class must avoid being a set of instructions for the student to perform. The must always be set up as a discovery where the experiment itself is not known and must be constructed by the student themselves. As a result, the questions they are given to start with are usually open ended (some labs end up being constructions, such as bridge building or Rube Goldberg devices, but that is not what i want to do here). The issue is that Snell's law is not something one can simply happen upon, so to simply ask them to try and construct and experiment to determine the relationships involved in refracted light would be not within their scope.

I'm wondering if anybody had any ideas as to how to approach a good lab that still requires the students to devise their own experiment to solve a particular problem.

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#### brainpushups

Do they need to actually discover the law? That seems unreasonable without guidance. What if it was just an open ended problem of figuring out the relationship between the angle of incidence and angle of reflection for different materials? Could you restrict them to small angles? If so they could perhaps discover that the angle of incidence is approximately proportional to the angle of refraction – that's as close as the ancients came. After students experiment with it could you guide them to the actual law with questions?

#### fishspawned

i would never expect a high school student to come up with Snell's Law, no.
I can't think of how it would suddenly occur to them to see a trig relationship. So I am hoping to change the nature of the question in a way where there is still something to discover without it being a straightforward paint by numbers follow the instructions thing. I find that with every other unit in Physics, the Lab Experiments are fairly easy to come up with where the experiment they create leads to new discoveries, but optics, waves, sound still has me stumped.

actually sound is fun - i get them to figure out what the temperature is outside by measuring the speed of sound. That one works out well, but it is set up more as a group challenge then an actual lab. I really want a good one to do with light. refraction seems to be the best option.

#### berkeman

Mentor
What equipment and supplies do you have available in your lab? Do you have low-power lasers that can be used to make measurements to show that Snell's law is correct (and to find the index of refraction of clear plastic blocks, etc.)?

#### fishspawned

we have laser pointers and can likely order any other items we need. Of course, mind you, I would like to avoid the "find the n value for this substance" type of experiment which still is simply a paint by numbers type of lab. although maybe there is something there with "proving the law is correct". An earlier "prove that acceleration due to g on earth is 9.8m.s^2" so this may work.

#### vela

Staff Emeritus
Homework Helper
Have you considered a lab on polarization instead of refraction? You could do a lab where students have to come up with a model to explain how polarization works. If they measure the intensity of the light that passes through two filters as a function of the angle between their axes, they'll get a nice sinusoidal curve, which is a starting point for developing a model. Once they think they have an explanation, have them test their model by making a prediction and then verifying their model's prediction is correct.

#### Nidum

Gold Member
What about a modern twist on the ripple tank experiments ?

#### ZapperZ

Staff Emeritus
2018 Award
we have laser pointers and can likely order any other items we need. Of course, mind you, I would like to avoid the "find the n value for this substance" type of experiment which still is simply a paint by numbers type of lab. although maybe there is something there with "proving the law is correct". An earlier "prove that acceleration due to g on earth is 9.8m.s^2" so this may work.
What you can do is ask the students to investigate how much the direction of a path of light changes as you change the angle of entry into another medium.

1. Get glass blocks - rectangular in shape is fine, but if you can get half-circle glass blocks, those will be perfect.

2. Place the glass block on a piece of paper, and trace the straight edge of the glass block onto the paper.

3. Shine your laser into the flat surface of the glass block at an angle. Mark the entry point and the angle of entry. This angle is the angle of incident.

4. Find the exit point of the laser on the other side. Mark this point.

5. Assuming that light travels in a straight line inside this block (which isn't an outrageous assumption), you can draw a straight line between the two points that you have made. This is the direction that light travels in the medium.

6. Find the angle of refraction using the line drawn in #5.

7. Repeat using a different angle of incident.

What I'm describing is exactly what you can do with this PhET simulation:

Zz.

#### Andy Resnick

Hello
I'm trying to come up with a solid Lab to use for my chapter on Waves and Optics. I am considering having it center on refraction. Here is the problem:

All labs I create for my class must avoid being a set of instructions for the student to perform. The must always be set up as a discovery where the experiment itself is not known and must be constructed by the student themselves. As a result, the questions they are given to start with are usually open ended (some labs end up being constructions, such as bridge building or Rube Goldberg devices, but that is not what i want to do here). The issue is that Snell's law is not something one can simply happen upon, so to simply ask them to try and construct and experiment to determine the relationships involved in refracted light would be not within their scope.

I'm wondering if anybody had any ideas as to how to approach a good lab that still requires the students to devise their own experiment to solve a particular problem.

How much time do students spend on a particular lab? I assume it's a 1-week long activity, maybe 3 hours total...

There are some good suggestions so far, mine is that you haven't really decided what you want the students to learn. That is, setting up a 'discovery' type lab, and wanting the topic to be 'waves and optics' is entirely too vague- when the activity is done, what do you want students to (ideally) have learned?

This is not a simple task for the educator- and it's certainly not something that can quickly slapped together. It requires that you (the teacher) have a clear idea about the specific learning objective(s). One activity that will help is to have the students present their results to each other- a kind of 'peer review' process.

I would say that it's perfectly acceptable to give students a pile of stuff and ask them to prove (or disprove!) Snell's law. Only, don't give them just glass/plastic- give some of them calcite (don't tell them what you are giving them!). Then ask the students to present their findings to their peers as part of the lab. Similarly, you could ask them to provide evidence that light possesses wavelike properties- don't tell them how, let them figure it out- and have them present their findings to their peers.

Regardless of whatever activity you choose, *you* have to first have a clear idea about what the ideal student will learn. It doesn't have to be a formula! It can also involve learning how to measure something and how experimental data is used to support or refute a hypothesis.

Good luck!

#### RogueOne

At which level is this class? General physics, or advanced placement (typically college credit course).

My high school physics teacher is responsible for the first paradigm shifts that lead me into pursuing sciences as a career. Here is how he introduced me to wave motion.

If we're connecting trig into it, then this is how I would do it. These visuals will help them understand what they are quantifying. After they understand the relationships, some students will be able to come to the equations without you forcing them to memorize soh-cah-toa. It will also help them get past the confusion that sets in when they find out that "soh-cah-toa" isn't JUST for triangles!!

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"Request for Advice on a Physics Lab"

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