# Refraction - when can you see past the label?

1. Jun 16, 2012

refraction -- when can you see past the label?

1. The problem statement, all variables and given/known data
as shown in the figure.

2. Relevant equations
n1sin1 = n2sin2
n1 and n2 represent the refractive indices of media 1 and 2 respectively

3. The attempt at a solution
Critical angle for glass-air interface = arcsin(1/1.5) = 41.8 degree
so,a = 12tanC = 10.7cm?
I just guess...I am not sure about what the question is asking for...
Does it mean,assume the observer's position doesn't change,
he can see O before placing the circular label.And he couldn't see O after placing the circular label?

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Last edited: Jun 16, 2012
2. Jun 16, 2012

### ehild

Re: refraction

Show your attempt of solution, please. What equation is relevant? Draw the light ray from the spot towards the observer's eye.

ehild

3. Jun 16, 2012

### SammyS

Staff Emeritus
Re: refraction

You have posted threads on PF before, but apparently nobody has pointed out to you that the rules for posting a thread in the HomeWork section state that you must show some effort at arriving at a solution before we can help you.
"NOTE: You MUST show that you have attempted to answer your question in order to receive help. You MUST make use of the homework template, which automatically appears when a new topic is created in the homework help forums. Once your question or problem has been responded to, do not go back and delete (or edit) your original post."

What have you tried ?

Where are you stuck ?

4. Jun 16, 2012

Re: refraction

Sorry,I have just edited my question.

5. Jun 16, 2012

### ehild

Re: refraction

Your solution means that the light can not get out from the glass if that label is placed above the spot onto the surface. When a light ray strikes the surface at angle lees then the critical, the label is there. If it strikes the surface at angle greater than critical, it totally reflects. The observer can look from any direction, he can not see the spot. I think the problem means that solution. Otherwise the position of the observer should have been given.

ehild

6. Jun 16, 2012

### SammyS

Staff Emeritus
Re: refraction

Yes!

I agree with ehild regarding the interpretation of this problem !

7. Jun 16, 2012

Re: refraction

thx sooooooooo much :)