critical angle

[globy321]

If a question gives you index of refractions as n=1 and n=1.5. How do you calculate the critical angle?

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[ebasappa]

If a question gives you index of refractions as n=1 and n=1.5. How do you calculate the critical angle?

So basically, you use Snell's Law. You know that the critical angle is when the angle of refraction is 90 degrees:

1.5sin(theta)=1sin(90)

1.5sin(theta)=1

sin(theta)=1/1.5 -----> sin(theta)=.66

and then solve for theta using the answer choices

 

[3838]

I believe you would take the antisine of (1/1.5) to get theta. which is antisine (2/3) so. antisine(.6 )ends up at an angle between 30 and 45 right?

Basically n1sin(theta1)= n2sin(theta2) assuming that n1 is the less dense material and n2 is the more dense material. Set theta 1= 90deg. because the critical angle is the angle at which light is refracted along the surface of a medium. The angle of refraction is going to be 90 degrees (reflected perpendicularly to the normal) sin90 is 1. Then solve for theta2, to get the critical angle the angle at which the incident light ray mus strike to cause a 90degree angle of refraction.

If that explanation doesn't make sense (and it wouldn't to me. I think you have to see it) look at this: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/totint.html

 

[sticky1]

I believe you would take the antisine of (1/1.5) to get theta. which is antisine (2/3) so. antisine(.6 )ends up at an angle between 30 and 45 right?

Basically n1sin(theta1)= n2sin(theta2) assuming that n1 is the less dense material and n2 is the more dense material. Set theta 1= 90deg. because the critical angle is the angle at which light is refracted along the surface of a medium. The angle of refraction is going to be 90 degrees (reflected perpendicularly to the normal) sin90 is 1. Then solve for theta2, to get the critical angle the angle at which the incident light ray mus strike to cause a 90degree angle of refraction.

If that explanation doesn't make sense (and it wouldn't to me. I think you have to see it) look at this: http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/totint.html

How do you calculate antisine 0.6 without a calculator?

 

[Zoidburg]

If a question gives you index of refractions as n=1 and n=1.5. How do you calculate the critical angle?

Use:

n1*sin(Θcritical)=n2sin(90)

Now to decide which one is n1 and which is n2. To understand which to pick you need to recognize that when going from a medium with a higher index of refraction to a lower one, the ray bends away from the normal. When a ray goes from a low index to a high index medium, the ray bends toward the normal.

At some critical angle a ray travelling in a medium of a higher index will enter a medium of a lower index and bend so it is 90 degrees away from the normal. n1 is associated with the higher index and n2 is the lower one.

If you still aren't comfortable with this concept, try reading this:

http://hyperphysics.phy-astr.gsu.edu/hbase/phyopt/totint.html

 

[Zoidburg]

How do you calculate antisine 0.6 without a calculator?

I'd like to know how to calculate anti trig functions as well without a calculator if anyone knows.

For the MCAT, just knowing that 0,1/2,sqrt(2)/2,sqrt(3)/2,and 1 correspond to angles 0,30,45,60,and 90 respectively for antisin and seeing where 0.6 falls between will be enough to get the right answer on a multiple choice test like this. It has never failed me.