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Snell's Law
Moderators: kgudger, bfinio, MadelineB, Moderators
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kerri_widrick
- Posts: 3
- Joined: Sun Mar 01, 2015 2:49 pm
- Occupation: student: 11th grade
- Project Question: Snell's Law
- Project Due Date: May 30, 2015
- Project Status: I am conducting my experiment
Snell's Law
I am attempting to recreate the science fair project about using a laser to measure the speed of through different mediums (this website uses gelatin, however I am making it my own)... I am running into trouble when trying to use Snell's Law... so far I have measured the angle of incidence and angle of refraction, but when I plug them in I get number like 0.8660254038 for v1 (velocity through the first medium)... am I using Snell's Law correctly? If so, what does this number mean?
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bfinio
- Expert
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- Occupation: Lead Staff Scientist, Science Buddies
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Re: Snell's Law
Hi kerri,
Are you using the form of Snell's Law from the top of this page?
https://www.sciencebuddies.org/science- ... background
sin(theta_1) / sin(theta_2) = v1 / v2 = n2 / n1
Can you show what numbers you plugged in for what variables? If I understand the experiment's procedure correctly, your goal is to calculate v2 (the speed of light in gelatin). v1 is the speed of light in air, which is a quantity you should be able to look up and not have to calculate.
Are you using the form of Snell's Law from the top of this page?
https://www.sciencebuddies.org/science- ... background
sin(theta_1) / sin(theta_2) = v1 / v2 = n2 / n1
Can you show what numbers you plugged in for what variables? If I understand the experiment's procedure correctly, your goal is to calculate v2 (the speed of light in gelatin). v1 is the speed of light in air, which is a quantity you should be able to look up and not have to calculate.
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kerri_widrick
- Posts: 3
- Joined: Sun Mar 01, 2015 2:49 pm
- Occupation: student: 11th grade
- Project Question: Snell's Law
- Project Due Date: May 30, 2015
- Project Status: I am conducting my experiment
Re: Snell's Law
Yes this is the equation I am using... when I measured the angle of incidence it was 45 degrees and the angle of refraction was 30 degrees. I then plugged 45 in for theta_1 and 30 in for theta_2... I know that air has a refraction index of 1.0003, so should sine(theta_2)=1.0003??
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bfinio
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- Joined: Mon Aug 12, 2013 2:41 pm
- Occupation: Lead Staff Scientist, Science Buddies
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Re: Snell's Law
Hi Kerri,
It's kind of difficult to explain an algebra problem in a text-based forum like this (much easier to write it out on a piece of paper). I think you have the right variables, but you aren't plugging them into the equation correctly. You have:
theta_1 = 45 degrees (gelatin)
theta_2 = 30 degrees (air)
n1 = ??? (gelatin)
n2 = 1.003 (air)
Is that right? If so, then you would use this form of Snell's Law:
sin(theta_1) / sin(theta_2) = n2 / n1
and plug in the variables you know (both thetas and n2), then solve for n1. Then, you can use the definition of index of refraction to calculate the speed of light in the gelatin:
n1= c / v1
where c is the speed of light in a vacuum (note that that equation is not listed directly in the project on our site, you would need to read one of the references in the bibliography to find it).
The other option is to just use the other form of Snell's Law:
sin(theta_1) / sin(theta_2) = v1 / v2
And look up the speed of light in air (v2). Then you know all the variables except v1, so you can solve for it.
If that doesn't make sense, and you're having trouble solving an algebraic equation, then I'd recommend asking your science or math teacher for help. They might be able to explain it better in person.
Good luck,
Ben
It's kind of difficult to explain an algebra problem in a text-based forum like this (much easier to write it out on a piece of paper). I think you have the right variables, but you aren't plugging them into the equation correctly. You have:
theta_1 = 45 degrees (gelatin)
theta_2 = 30 degrees (air)
n1 = ??? (gelatin)
n2 = 1.003 (air)
Is that right? If so, then you would use this form of Snell's Law:
sin(theta_1) / sin(theta_2) = n2 / n1
and plug in the variables you know (both thetas and n2), then solve for n1. Then, you can use the definition of index of refraction to calculate the speed of light in the gelatin:
n1= c / v1
where c is the speed of light in a vacuum (note that that equation is not listed directly in the project on our site, you would need to read one of the references in the bibliography to find it).
The other option is to just use the other form of Snell's Law:
sin(theta_1) / sin(theta_2) = v1 / v2
And look up the speed of light in air (v2). Then you know all the variables except v1, so you can solve for it.
If that doesn't make sense, and you're having trouble solving an algebraic equation, then I'd recommend asking your science or math teacher for help. They might be able to explain it better in person.
Good luck,
Ben
Last edited by bfinio on Mon Mar 02, 2015 3:38 pm, edited 1 time in total.
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kerri_widrick
- Posts: 3
- Joined: Sun Mar 01, 2015 2:49 pm
- Occupation: student: 11th grade
- Project Question: Snell's Law
- Project Due Date: May 30, 2015
- Project Status: I am conducting my experiment
Re: Snell's Law
This is very helpful and I think I understand what I am doing wrong now!! Thanks for the help!
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bfinio
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Re: Snell's Law
No problem!
One more note - depending on the type of calculator you are using, make sure you know whether it requires degrees or radians for trigonometric functions (the "sine" function). Don't worry if you don't know what trigonometry and radians are, because that's high school level math. Radians are another type of unit used to measure angles. If your calculator is expecting radians and you plug in degrees (or vice versa), that will give incorrect results.
One more note - depending on the type of calculator you are using, make sure you know whether it requires degrees or radians for trigonometric functions (the "sine" function). Don't worry if you don't know what trigonometry and radians are, because that's high school level math. Radians are another type of unit used to measure angles. If your calculator is expecting radians and you plug in degrees (or vice versa), that will give incorrect results.