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**HarryChiling** · 05-14-2008, 11:40 AM

Originally posted by Whitwoo View Post

Hi there,

I am having a few problems with a couple of questions in one of my assignments. Can someone please point me in the right direction?
I was okay, until they asked me to start mixing and matching equations, and now I am having loads of troubles!
As I am studying correspondence, there is no one to help me. :idea:

Questions as follows:

-Light rays are deviated through 18 degrees when they strike a block of glass at 39 degrees. What is the refractive index of the glass?

-A light ray approaches the surface of a medium at an angle of 39.24 degrees, and at a speed of 1.92 x 10 to the power of 8 m/s. Whe nthe ray strikes the surface will it reflect or refract, and at what angle?

-Light strikes the surface of a liquid at 24 degrees and is deviated through 5.25 degrees. At what speed does it travel inside the liquid?

-Calculate the critial angle for interferance between flint glass (n=1.654) and water

Thank you!! :)

First lets layout our formulas for use:

Speed of Light in Vacuum / Speed of Light in Medium = Index of Refraction

Index_incidence * sin(angle_incidence) = Index_{refractive material} * sin(angle_refraction)

sin (critical angle) = Index_{incident material} / Index_{refractive material}

Deviation = Incident - Refracted

Now all our ducks are in order,

So question #1: Light rays are deviated through 18 degrees when they strike a block of glass at 39 degrees. What is the refractive index of the glass?

OK, so our givens are:

deviation = 18
incident = 39
index of incidence (assumed to be air) = 1.0

1 * sin(39) = n? * sin(r?)

we don't have the angle of refraction, but

18 = 39 - r?
r? = 39 - 18
r? = 21

so now we got:

1 * sin(39) = n? * sin(21)
n? = sin(39) / sin(21)
n? = 1.76

So the index of the material is 1.76

Question #2: A light ray approaches the surface of a medium at an angle of 39.24 degrees, and at a speed of 1.92 x 10 to the power of 8 m/s. When the ray strikes the surface will it reflect or refract, and at what angle?

First we need to find the index of the medium

n = 3.00 x 10⁸ / 1.92 x 10⁸ = 3 / 1.92 = 1.56

so now our givens are:

incident angle = 39.24
incident index (assumed to be air) = 1.0
refractive material = 1.56

sin(39.24) = 1.56 * sin(R)
sin(R) = sin(39.24) / 1.56
sin(R) = 0.41
R = 23.9

The critical angle is:

sin(critical) = 1 / 1.56
critical = 39.9

so the ray is refracted by 23.9 degrees

Question #3: Light strikes the surface of a liquid at 24 degrees and is deviated through 5.25 degrees. At what speed does it travel inside the liquid?

Alright givens:

incident = 24
deviation = 5.25
incident index (assumed air) = 1.00

refracted = 24 - 5.25 = 18.75

sin(24) = n? * sin(18.75)
n? = sin(24) / sin(18.75)
n? = 1.27

Speed of light in medium = 3.00 x 10⁸ / 1.27 = 2.36 x 10⁸

Question #4: Calculate the critial angle for interferance between flint glass (n=1.654) and water

sin(critical) = 1.33 / 1.654 = 0.80
critical angle = 53.5

I don't think I made any mistakes but double check it all.

**sharonm516** · 05-14-2008, 01:40 PM

:drop:Whoa! You are smart!:cheers:

Harry, you are the true guru!

**Whitwoo** · 05-14-2008, 02:13 PM

Wow, thank you so much.
I've been searching for weeks to try and find someone in my practice that will help me - and I find someone on the internet that spells it out for me!
I really appreciate it. :)

**HarryChiling** · 05-14-2008, 03:15 PM

Originally posted by Whitwoo View Post

Wow, thank you so much.
I've been searching for weeks to try and find someone in my practice that will help me - and I find someone on the internet that spells it out for me!
I really appreciate it. :)

Check out www.technicalopticians.org and also www.opticampus.com as well as this site right here, if you have a little extra cash lieing around then join the forum and you will have access to a private area of the forum that will allow you more access. Plus I answer every question I can int he private forums. Remember to look at what they are asking for in the question and lay out you givens. If you do this you will often find a formula that would be usefull as well as find out what you are missing in that formula to further search for another formula. In the above examples you gave the questions require that you have the basics down, and it looks like in all the question you were combineing formulas to find the missing parts. So focus on what the question is asking for and you should be fine. Good luck in your course.

PS - If you post it in the Ophthalmic Optics Forum I know that Darryl, Lensgrinder, and a few others will try to beat me to answering it so you often get the question faster as I am trying to type out a response between the generator cycles and edging.