# Homework Help: Find the refractive index of different objects using Snell’s law

1. Nov 5, 2006

### australianschoolkid

Hi
I have just conducted the experimental stage of a practical 'to find the refractive index of different objects using Snell’s law' (which states sine i/sine r =N). The objects chosen were Ice, Glass, and Perspex. As 3 trials were taken for all 3 objects, if these points were graphed, and a line of best fit was applied, I believe the gradient of this line would be the refractive index? Is this correct? Also does the I and R angles of the light leaving the prism hold any significance?

attached are my results.

RESULTS / DATA:

Perspex
I1 R1 N1 I2 R2 N2
40 25 -5.63 25 38.5 -0.18
10 8.5 -0.68 8.5 11.5 -0.91
29 20.5 -0.67 20.5 30 -1.01

Glass
I1 R1 N1 I2 R2 N2
22.5 13.5 -0.61 12.5 22.5 0.14
30.5 30 0.80 20 32.5 1.03
20 14 0.92 14 25 -7.48

Ice
I1 R1 N1 I2 R2 N2
22 31 0.02 30 34 -1.87
23 19 -5.65 19 24 -0.17
2 1.5 0.91 1.5 2 1.10

Note - The 1 results were those angles entering the prism, while the 2 results were those angles of the light leaving the prism.

cheers
matt

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2. Nov 5, 2006

### andrevdh

You can obtain the index of refraction of the material of the prisma from the gradient of a graph of your data.

To do this relabel your data: $$I_1,\ R_2\ =\ \theta _a$$ and $$R_1,\ I_2\ =\ \theta _p$$, where the a refers to angles measured in air and the p refers to angles measured inside of the prisma.

The gradient of a graph of the sine of the angles in air against the sine of the angles in the prisma will then give the refractive index of the material. This follows from Snell's law.

Last edited: Nov 5, 2006
3. Nov 5, 2006

### australianschoolkid

Thanks

Thanks heaps for replying, when you tell me to relabel my data as $$I_1,\ R_2\ =\ \theta _a$$, what does the "," represent? is this some sort of function between the two?

Matt

4. Nov 5, 2006

5. Nov 5, 2006

### bilkul_king

no its just like that there is not funtction in that

6. Nov 5, 2006

### australianschoolkid

so could you please explain what hes saying?

7. Nov 5, 2006

### australianschoolkid

how ive been graphing so far

this is what ive done, is this correct? so if i take the gradient this shall be the refactive index?

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8. Nov 5, 2006

### australianschoolkid

hi i was wondering if you could please help me with the graphing of snell's law, as ive been told you can graph somthing over somthing and the gradient of this line is the refactive index, are you able to assist as im quite confused as the current results i am getting are quite wrong!

cheers matt

9. Nov 5, 2006

### andrevdh

Sorry my explanations are clear only to me. Lets try again.

Instead of labelling your data values as $$I_1\ and\ R_2$$ label them as $$\theta _a$$, that is these angles are the angles measured in air.

Change your labelling for the $$R_1\ and\ I_2$$ angles to $$\theta _p$$, that is these are the angles measured inside the material of the prisma.

Then plot a graph of $$\sin(\theta _a)$$ as a function of $$\sin(\theta _p)$$.

The gradient of such graph will be the index of refraction of the prisma material. Hope this makes it clearer.

Last edited: Nov 5, 2006