# Geometric optics - thickness of acrylic ?

1. Mar 18, 2014

### carnivalcougar

1. The problem statement, all variables and given/known data
A ray is deflected by 2.37cm by a piece of acrylic. Find the thickness t of the acrylic if the incident angle is 50.5 degrees.
http://imgur.com/kx2VT5c

2. Relevant equations

n1sinΘ1 = n2sinΘ2

3. The attempt at a solution

n of acrylic is 1.5. Therefore, the refracted angle is 30.958 degrees. ( 1(sin(50.5)) = 1.5(sin(θ)) )

I'm not sure if I can make a triangle within the acrylic that is above the ray line where one angle is 50.042, one is 90, and the other is 30.958 and use 2.37cm as on side of the triangle.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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2. Mar 18, 2014

### SammyS

Staff Emeritus
Break this up into two triangles.

What is the distance that the ray travels through the acrylic ?
...

3. Mar 18, 2014

### carnivalcougar

The distance that the ray travels through the acrylic is the hypotenuse of a triangle I can make using 2.37cm as one side. However, I do not know how to find the angles of this triangle

4. Mar 18, 2014

### SammyS

Staff Emeritus
What angle does that hypotenuse make with the normal?

What angle does the exit ray make with the normal?

5. Mar 18, 2014

### carnivalcougar

The exit ray makes an angle of 50.5 with the normal while the refracted ray makes an angle of 30.958 with the normal.

I drew which triangle I am talking about on the diagram.

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6. Mar 18, 2014

### SammyS

Staff Emeritus
What is the complimentary angle to the 50.5° angle you have drawn ?

7. Mar 18, 2014

### carnivalcougar

That would be 39.5 degrees

8. Mar 18, 2014

### SammyS

Staff Emeritus
Rotate that angle 90° counter-clockwise.

Where does it now line up ?

9. Mar 18, 2014

### carnivalcougar

I think it would be south of the X axis. Which would give me the angle I need to find the third angle of the triangle?

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10. Mar 18, 2014

### SammyS

Staff Emeritus
Yes.

11. Mar 18, 2014

### carnivalcougar

Would I then use the law of sines to find the hypotenuse of this triangle?

Thanks!

12. Mar 18, 2014

### SammyS

Staff Emeritus
You have to add the 39.5° angle to the angle of refraction.

Then use the definition of the cosine to find the hypotenuse. Right?

13. Mar 18, 2014

### carnivalcougar

Oh yeah that would have been easier. Either way I got 7.0852 for the hypotenuse. Then I used the law of sines on the triangle that has the same hypotenuse but the opposite side is the normal line, and therefore, the thickness of the acrylic to find what the thickness of the acrylic is. This came out to be 6.07587cm.

14. Mar 18, 2014

### SammyS

Staff Emeritus
Good !