• Support PF! Buy your school textbooks, materials and every day products via PF Here!

Angle of Incidence with partial reflection and partial refraction

  • Thread starter skibum143
  • Start date
112
0
1. Homework Statement
A beam of light in air strikes a piece of glass (n=1.51) and is partially reflected and partially refracted. Find the angle of incidence if the angle of reflection is twice the angle of refraction.


2. Homework Equations
n1sintheta1 = n2sintheta2


3. The Attempt at a Solution
The angle of reflection equals the angle of incidence, which is twice the angle of refraction.
Therefore, I set up the equation as:
1 * sin (2theta) = 1.51 * sin (theta)
But when I try to solve for theta I get that they cancel out (sin 2theta / sin theta) just equals 2? I think I'm doing the trig wrong but I don't know why?
 
129
0
Yeah you are doing the trig wrong. You cannot just take the 2 out of the sin function and cancel.

Use this trig identity: sin(2x) = 2 sin x cos x
 
112
0
yep, i'm awful at trig so that isn't surprising. haha.

i'm still not getting it right though: if i use that identity, i get:

1 * 2 sin theta cos theta = 1.51 * sin theta

If I divide the left by sin theta, the sin theta cancels out (right???), leaving me:
2 cos theta = 1.51
which gives me a theta of 40.97 deg, which is wrong...

Not sure what I'm doing wrong now...
 
129
0
You need to double that angle, they ask for the angle of incidence not the angle of refraction.
 
112
0
right. i'm an idiot. thanks for your help!
 
129
0
Yep no problem.
 

Related Threads for: Angle of Incidence with partial reflection and partial refraction

Replies
3
Views
6K
Replies
1
Views
6K
  • Posted
Replies
6
Views
3K
Replies
3
Views
6K
Replies
1
Views
405
Replies
2
Views
2K
Replies
7
Views
9K

Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving
Top