# Refraction at grazing incidence

1. Jun 6, 2013

### nilic1

1. The problem statement, all variables and given/known data

I have a problem with the question in the attachment below. I have no idea on how to proceed as I thought that the ray of light would continue to travel in a straight line with no refraction whatsoever.

2. Relevant equations

n1sinθ1=n2sin2

3. The attempt at a solution

I am lost on this one!
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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2. Jun 6, 2013

### ehild

The angles of incidence and refraction are defined as angles enclosed by the rays and the normal of the interface. Grazing incidence means θ1=90°. What angle does the refracted ray enclose with the normal of the face of cube?

ehild

3. Jun 6, 2013

### nilic1

My new attempt is:

n1 sinθ1 = n2sinθ2
1.55 x sin90 = 1.33 x sin θ2
1.55/1.33 = sin θ2
1.165 = sin θ2
∴ something is wrong!

4. Jun 6, 2013

### ehild

The light enters from air. What is n1 then?

ehild

5. Jun 6, 2013

### nilic1

n = 1 for air.

so the working changes to

n1 sinθ1 = n2 sinθ2
1 x sin 90 = 1.33 x sinθ2
1/1.33 = sinθ2

θ2 = 48.5 degrees

I am still puzzled .... so when angle i = 90, the medium is air and not glass. If this is so then is the glass cube is irrelevant for the question? If so wouldn't the ray of light go straight and not be bent at the water surface or in the water?

According to the answer in the book the angle in part b ii is 62.9 degrees. Still cannot figure out the path taken by the ray of light. I appreciate your help ehild.

6. Jun 6, 2013

### nilic1

ok finally I got it... but my question still is .. Why doesn't the ray continue to travel in a straight line?

Last edited: Jun 6, 2013
7. Jun 6, 2013

### ehild

The ray enters from air onto the surface of the glass cube. That grazing incidence means that it makes an angle a bit less than 90° with the normal, so it really strikes the air/glass surface and enters into the glass, instead going straight into the water. The refracted ray travels inside the glass cube and falls onto the bottom face at a certain angle. Here it is refracted again, from glass into water. Draw the path of the ray and calculate the angles.

ehild

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8. Jun 6, 2013

### nilic1

So grazing is slightly less than 90 degrees... That explains everything! Thank you so much ehild.

9. Jun 6, 2013

### ehild

You are welcome

ehild