The formula for the refractive index of a prism is: $$\mu = \frac{\sin \left(\frac{A + D_m}{2}\right)}{\sin (A/2)}$$

However, this requires me to find out the angle of minimum deviation ($D_m$). This would require me to take several readings, plot a graph, and then find the minimum point of the graph.

If I have two readings of the angle of emergence and angle of incidence, is there a way I can find the refractive index of the prism?

  • $\begingroup$ If you know the the prism angle, you should be able to estimate by using Snell's law given the measurements that you have already. $\endgroup$
    – tmwilson26
    Commented Oct 28, 2015 at 17:31
  • $\begingroup$ @tmwilson26 That's what I'm asking, how? $\endgroup$ Commented Oct 29, 2015 at 12:51

2 Answers 2


The following diagram should help:

enter image description here

We know of course the relationship between $\alpha$ and $\beta$ because the angle $A$ is known: $$\alpha + \beta = A$$

Next, we have the relationship between $i$ and $\alpha$ from Snell's law:

$$\frac{\sin{i}}{n}=\sin\alpha$$ and similarly for $\beta$:


This gives you three equations with three unknowns: $\alpha,~\beta,~n$.

I would recommend solving this numerically, or graphically. Simple example of a graphical solution:

enter image description here

I calculated the expected exit angle given an input angle of 45° and a refractive index of 1.543 - then calculated what exit angle I would get if I didn't know the refractive index. The error goes to zero when you have the right index...

Python code used to generate this:

# prism simulation
from math import pi, asin, sin
import numpy as np
import matplotlib.pyplot as plt

A = 60*pi/180. # 60 degrees, in radians

# pick an angle of incidence
i = pi/4

# pick a refractive index
n = 1.543

# compute the exit angle
alpha = asin(sin(i)/n)
beta = A - alpha
e = asin(sin(beta)*n)

print 'exit angle is %.1f deg'%(e*180/pi)

# calculate n graphically from just i and e
na = np.arange(1.4,1.6,0.001) # possible values

# what would the intermediate values alpha, beta be?
aa = np.arcsin(np.sin(i)/na)
bb = A - aa
ee = np.arcsin(np.sin(bb)*na)

plt.plot(na, 180*(ee-e)/pi)
plt.xlabel('possible value of n')
plt.ylabel('error in exit angle')
plt.title('graphically estimating n')

Note that my answer exploits a loophole in your question (namely that you haven't said that the 2 known values of the required angles are random):

If at a particular angle of incidence $i$, the angle of emergence $e$ is equal to $i$, then we have the condition for minimum deviation. Then,

$$ \delta_{min}=2i-A$$, where $A$ is the angle of the prism. Then you can use the relation you mentioned to calculate the refractive index $\mu$ of the prism.

On a side note, and as an answer which does not exploit a loophole in the question, you would be better off using a standard source of light such as a sodium-D lamp (589 nm) and tilting the source until you get a condition for grazing emergence. Then, you can use geometry to find the angle of incidence inside the prism, which will be equal to its critical angle $i_c$, and, as we know,

$$\sin i_c= \frac {1}{\mu}$$

  • $\begingroup$ Aha... thanks. Finding loopholes won't help really. $\endgroup$ Commented Oct 29, 2015 at 12:51

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