# Ray tracing a thin prism

by Misr
Tags: prism, tracing
P: 391
I finally found something about thin prisms on the web
 a thin prism, which is generally a prism with an apical angle of less than 15°. For simplicity, we will assume that the initial object ray is striking the first surface normally (or perpendicularly). Since the ray is normal—or at a 90° angle—to the first surface, no refraction occurs. However, once the ray strikes the second surface, it reaches the surface at an angle i and is therefore refracted (or bent) in the direction of the base. For thin prisms, which include most prisms in ophthalmic optics, the refraction at the second surface depends primarily on the apical angle (a) of the prism and the refractive index (n) of the material. In practice, the angle of incidence at the first surface will affect the extent to which light is deviated by the prism. The amount of deviation, in degrees, is given by the angle (d), while d = i' - i. Moreover, the angles of incidence (i) and refraction (i') are related by the refractive index (n) of the prism material according to Snell's law. For a thin prism, with a relatively small apical angle (a), it can be shown that the approximate deviation (d), in degrees, is given by: d = (n - 1) × a For small amounts of deviation, 1 degree of deviation is roughly equal to 1.75 prism diopters. For prisms made from hard resin, the refractive index (n) is 1.500. This simplifies our formula even further, since 1.500 - 1 = 1/2. Consequently, for prisms made from hard resin, the deviation in degrees is roughly equal to half the apical angle. For example, consider a ray of light from an object point passing through a hard resin prism (n = 1.500) with an apical angle of 10°. The deviation (d) of this ray is equal to (n - 1) × a = (1.500 - 1) × 10 = 5°. This is approximately 8.75 prism diopters.
I don't understand whether the angle of deviation depends on a the angle of incidence in a thin prism or not?
Is the thin prism always in the position of minimum deviation?
 P: 882 If you want to get understanding (rather than get ready answer to simplified yes/no question) - compute it yourself. Compute what is deviation angle as a function of angle of incidence and apical angle, then make several plots (for several apical angles, e.g. 10°, 20°, 30°, 45°): deviation angle as a function of angle of incidence. And compare the plots. That's pretty easy calculation. You know Snell's law. You can do it yourself! Yes, you can!
 P: 391 You are right,I already tried something like that,but I failed .I always get different angles of deviation for different angles of incidence,and this should not happen,because we study that the angle of deviation in a thin prism isn't affected by the angle of incidence.
 P: 882 Ray tracing a thin prism Please, post the plots you made and formulae you got - I'll try to point out what you did wrong.
 P: 391 That's my work-very sorry for the bad resolution I drew two diagrams At A=45 degrees ----- At A=10 degrees The first graph looks logical but there's a math error the second one looks very strange I don't know any laws about thin prism ,so I drew the angles manually,and at every angle of incidence,the angle of deviation has a different value.It's not supposed to be so according to the relation alpha=A(n-1) so the angle of deviation is only dependent on the angle of the prism and the refractive index Hope you could find the problem with the second graph Thanks
 P: 391 I've put my work as you told me,so could you tell me what's wrong with it?
 P: 882 First graph looks reasonably, the second is totally wrong - check calculations (the calculator may help) You should find first the formula describing the deflection angle as a function of incidence angle and apical angle, then make several graphs of this formula for various fixed values of apical angle. You may want to use any computer program for drawing graphs of functions (you'd probably been taught one of such programs at school - but even excel can do this), rather than computing all that trigonometry by hand...
P: 391
 You should find first the formula describing the deflection angle as a function of incidence angle and apical angle, then make several graphs of this formula for various fixed values of apical angle. You may want to use any computer program for drawing graphs of functions (you'd probably been taught one of such programs at school - but even excel can do this), rather than computing all that trigonometry by hand...
which formula?I don't know how to do all of this :(
 P: 882 Derive a general formula, describing the deflection angle as the function of two parameters: apical angle and incidence angle. Derivation is pretty easy. You must just draw a picture, give names (symbols, rather than actual values in degrees) to all angles, then combine and reduce several simple trigonometrical formulae.
 P: 391 do u mean that Apical angle=second Angle of incidence-Angle of refraction? This is just for thin prism I guess. How about math errors in both graphs
P: 882
 Quote by Misr do u mean that Apical angle=second Angle of incidence-Angle of refraction?
Not quite that, but that is an idea: to express the deviation angle as a function of apical and incidence angles. You must apply Snell's law twice, and express the incidence/refracted ray angles as sums of other angles.

 How about math errors in both graphs
First one seems ok (although I did not check it very carefully), the second is wrong.
P: 391
 Not quite that, but that is an idea: to express the deviation angle as a function of apical and incidence angles. You must apply Snell's law twice, and express the incidence/refracted ray angles as sums of other angles.
first:Is the equation I wrote correct?
Could you explain more:(
I still can't imagine what I have to do
 P: 391 I checked the numbers in the second graph,there's nothing wrong with them Could you give me some laws about thin prism to use them instead of drawing the prism manually?
P: 882
 Quote by Misr Could you give me some laws about thin prism to use them instead of drawing the prism manually?
There are no more laws applicable here than Snell's law and trigonometric laws for sinus of sum of angles, nor there is no need for special laws so closely related to Snell's one.
You may easily derve Misr's law: the law describing deflection angle of the prism...

first:Is the equation I wrote correct? - you haven't wrote the equation, except of sime side-notesyou used in calculations, but as you haven't presented it as a part of ordered reasoning, I can't judge them.

I still can't imagine what I have to do
Make a drawing how the ray passes through a prism. Something like in your first post. Mark all angles and give them symbols. Write down Snell's relations between incidence/refraction angles on both surfaces, using n as refraction index. Express all angles as sums/differences of: apical angle, incident angle on first surface, deflection angle.Transform those equations to have it in form:
deflection_angle = some_function_of(incident_angle, apical_angle, n)
Make graphs of that function for several fixed values of apical_angle and single example value of n
Compare those graphs visually and find some regularity of the graph shapes - it will be obvious as you look at the graphs.
 P: 391 you mean by "deflection angle" the angle of deviation?
P: 882
 Quote by Misr you mean by "deflection angle" the angle of deviation?
Yes. Sorry for wrong terminology. The angle between final ray and incoming one.
 P: 391 I've been working on this for hours I derived two equations but there's something wrong with them deviation angle=angle of emergence- angle of incidence+Apical angle This works on some values of angle of incidence but while making some calculations I found that's not true-it gives me a negative value for the angle And it is the same for Apical angle=Angle of refraction-second angle of incidence Could you help me with signs because I'm getting very bored
P: 882
 Quote by Misr two equations but there's something wrong with them deviation angle=angle of emergence- angle of incidence+Apical angle
You may chose any convention of signs you like.
I see you got lost in it. A little help from my side - I spent 10mins to make a readable drawing:

In this convention your equation is:$$\delta=\theta-\beta-\alpha$$

 but while making some calculations I found that's not true-it gives me a negative value for the angle
Please stay with naming and sign convention as on this drawing (angles are positive if they are like on the drawing, negative if they lay on opposide side of the normal to the surface), if you prefer other names and other sign convention - it is ok, but post a readable drawing.

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