Refraction - when can you see past the label?

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refraction -- when can you see past the label?

Homework Statement


as shown in the figure.


Homework Equations


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

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


jadelamlam said:

Homework Statement


as shown in the figure.

Homework Equations



The Attempt at a Solution

attachment.php?attachmentid=48388&d=1339823659.jpg



Hello jadelamlam.

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 ?
 


Sorry,I have just edited my question.
 


jadelamlam said:

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?

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
 


ehild said:
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
Yes!

I agree with ehild regarding the interpretation of this problem !
 


thx sooooooooo much :)