Spot the error
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Author:  Doos [ Tue Jul 24, 2007 12:06 pm ]
Post subject:  Spot the error


I was making a picture to illustrate total internal reflection for the gemology project.
When I reviewed it I noticed a slight but noticable error.

The light is traveling from air into a hemicylinder (made of lets say glass) to air again.

Can you spot the error?
Winner gets everlasting fame.


Author:  RAW rocks [ Tue Jul 24, 2007 12:15 pm ]
Post subject: 

This is just a guess, but if it is total internal reflection then shouldn't the green line in the center not be going all the way through.

Author:  Doos [ Tue Jul 24, 2007 12:22 pm ]
Post subject: 


You are warm and cold.
The green line will both refract out and reflect back, so if your answer is that it doesn't reflect, then .. no, not the correct answer.

Author:  valeria102 [ Tue Jul 24, 2007 1:20 pm ]
Post subject: 

Sheesh! Can't tell.. Would have two Qs

1. No refraction?

2. The partial reflection below the CR... isn't that usually assumed away from the bare definition? I.e. if combining more than one effect...

Author:  Doos [ Tue Jul 24, 2007 1:40 pm ]
Post subject: 

Hi Valeria,

The laws of refraction are the key here.
Could you rephrase you questions?

Author:  sticky11 [ Tue Jul 24, 2007 2:31 pm ]
Post subject:  stab in the dark

absolutely no idea about these things, but if the green line is partially reflected, shouldnt the line that does not reflect but travels through also be a partial intensity?represented as a dotted line like the reflected line?
Just a stab in the dark from a newbie :wink:

Author:  Bill Hanneman [ Tue Jul 24, 2007 2:38 pm ]
Post subject: 

The exiting green line should be bent away from the normal.

Author:  Doos [ Tue Jul 24, 2007 2:40 pm ]
Post subject: 

Hi sticky,

Very well spotted, true. But not the answer I was looking for.
So I guess that makes two errors. One solved.

Eternal fame to sticky for logical thinking.

Author:  Doos [ Tue Jul 24, 2007 2:42 pm ]
Post subject: 

Hi Bill,

Good eye you have, all solved!

When light travels from an optically denser to an optically rarer medium, the light bends away from the normal .. not towards it as I mistakenly illustrated.

Another hall of fame spot for Dr. Hanneman.

Author:  RAW rocks [ Tue Jul 24, 2007 3:40 pm ]
Post subject: 


Will you post the corrected drawing or let me know where to find it so that I can understand better.

Author:  Doos [ Tue Jul 24, 2007 3:49 pm ]
Post subject: 


I will create an updated one tomorrow.

Author:  hatari [ Tue Jul 24, 2007 4:55 pm ]
Post subject: 

Hi Raw Rocks,

Here is a link to help you understand refraction. ... ction.html

This law is what helps bring out the "luster" in gem stones thus dimensions are important when faceting.

Here is a great website related to gems and refractive index (plus more): ... index.html

Hope it helps.


Author:  RAW rocks [ Tue Jul 24, 2007 6:06 pm ]
Post subject: 

thanks hatari,

The second link was especially helpful as it used gems as examples and was not overly technical.

Author:  Bill Hanneman [ Tue Jul 24, 2007 8:49 pm ]
Post subject: 

Can anyone explain those refractive index values for metals?

Author:  Brian [ Tue Jul 24, 2007 10:35 pm ]
Post subject: 

ooo, I'm so mad I missed this one.

I ask qualitative questions just like these on my refraction exam (shhh, don't tell my students). Only in my questions, I usually include a phrase similar to the following: The diagram below may not exhibit any errors, may exhibit a single error, or may exhibit multiple errors. How many errors can you spot in the diagram?

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