# Deviation and dispersion in a prism

1. Sep 15, 2011

### Misr

Hello,world
[PLAIN]http://img718.imageshack.us/img718/1433/unledkzg.jpg [Broken]
Could you explain the reason for this?This would help me so much

Last edited by a moderator: May 5, 2017
2. Sep 15, 2011

### Staff: Mentor

The reason for what, specifically?

3. Sep 16, 2011

### Misr

it seems that no body can see the picture
Sorry I will reupload it again

4. Sep 16, 2011

### Misr

[PLAIN]http://img718.imageshack.us/img718/1433/unledkzg.jpg [Broken]
[/PLAIN]

Last edited by a moderator: May 5, 2017
5. Sep 16, 2011

### Staff: Mentor

No, it was just me at work. I can see the picture fine here at home. I don't think we understand what you are asking. You're going to have to type out a question, otherwise we have no idea what you are confused with.

6. Sep 16, 2011

### Misr

Oh ok
Small deviation means large dispersion and vice versa,so why?
Is that the reason why the prism should be in the minimum deviation position to disperse light?

7. Sep 17, 2011

### sankalpmittal

Angle of deviation + Angle of prism = Angle of incidence + Angle of emergence

Let Angle of prism be constant ie k

Angle of deviation =$\partial$
Angle of incidence=i
Angle of emergence=e

Now

$\partial$ = k - i+e

Now realize that

$\partial$ $\propto$ i

Forget about e now because each colour has different e

In your first image of large deviation , and small i the e will be very small tally the relation.
In your second image of small deviation , and large i the e will be very large tally the relation.

If e will be large so more dispersion and vice versa.

If you want to know the proof of relation then feel free to ask.

In case of minimum deviation i=e , so ∂ = k - 2i ,
there is maximum dispersion also.

Claculate dispersion like this :
ev+ei+eb+eg+ey+eo+er where v,i,b,g,y,o,r are different colours : 7

Last edited by a moderator: May 5, 2017
8. Sep 17, 2011

### Misr

I don't understand those strange equations you are writing Could you use simpler equations?What kind of equations are they?
I just understand that Angle of deviation + Angle of prism = Angle of incidence + Angle of emergence
Thanks very much!

9. Sep 17, 2011

### Misr

I think this script doesn't work on my computer
Could you use images or words please?

10. Sep 18, 2011

### Misr

ok I can see the equations now but I have some questions:
We studied that alpha=i+e-k not k-i+e so how do u explain that?
how do u consider that alpha is directly proportional to i?This is not a direct relation
I haven't known that
I can't imagine how I can forget about e , doesn't e depend on i not on the color?
which relation?Anyways I certainly want to know the proof
Thanks very much for your help

11. Sep 18, 2011

### sankalpmittal

Sorry , my fault , its ∂ = -k + i+e
I mistyped it !

I think the following sites may crystal clear your concepts better than I can :

The proof of relation ∂ = -k + i+e : http://www.askiitians.com/iit_jee-Ray_Optics/Prism
The factors affecting deviation in prism :http://en.wikipedia.org/wiki/Minimum_deviation
Relation between dispersion and deviation in a prism :http://www.physicsclassroom.com/class/refrn/u14l4a.cfm

Minimum deviation : http://www.mtholyoke.edu/~mpeterso/classes/phys103/geomopti/MinDev.html

Hope this helps :)

12. Sep 18, 2011

### Misr

Ok,I'll check them and complete this discussion
Thanks very much

13. Sep 18, 2011

### Misr

Still confused Can't anyone in the world solve my problem :(

14. Sep 18, 2011

### Misr

Can A prism disperse light when it is not the in the positsion of minimum deviation?

15. Sep 18, 2011

### YPelletier

I don't have a formal proof or a conceptual explanation, but sometimes I understand things while working on a numerical problem. Here's what I would do in this case:

1) Using Snell's law, I compute the angle of deviation when it is minimum (when the ray of light is horizontal inside the prism). I use a typical refractive index: 1.50 .

2) For a different color, I repeat step 1 with a slightly different refractive index (let's say 1.45).

3) Subtracting the values obtained in steps 1 and 2, I find the angle of dispersion.

4) I repeat step 1, 2 and 3 for a random case where the deviation is not minimum (angle of incidence = 45 degrees, for example). As a result, I should find a smaller angle of dispersion.

16. Sep 18, 2011

### sankalpmittal

Why ? Have you gone through those sites which I gave in post #14? Your question is not refined and hence it is quite hard for me to explain it to you.

Yes , why not? I recommend that you should go through those sites again and read them carefully one by one !

[Post by YPelletier]
This is what I typed in my earlier post. I am not sure what OP is searching for ? I gave answer and even gave reference sites?

What is the question Misr ? Please type your question again so that I can be clear what answer do you want exactly ?

17. Sep 18, 2011

### Misr

You mean I should use numbers?I don't really understand what you are trying to say
could you use an example?

I want to prove that small deviation means large dispersion and vice versa

18. Sep 19, 2011

### Misr

Ok ,how toi do that?
I tried this on another problem
I wanted to show that the thin prism is always in the position of minimum deviation whatever the angle of incidence
but I failed , so could you help with this idea ?

19. Sep 20, 2011

### Misr

When white light falls on a prism do all the wavelength of white light have the same angle of incidence?

20. Sep 23, 2011

### sankalpmittal

When white light falls on prism , it is dispersed after the refraction not at that flick of second. At the line of separation between two media it is after all a monochromatic beams of light. It will have same angle of incidence though.

Prove it experimentally , your question :small deviation means large dispersion and vice versa

Take angle of incidence = 40o

Compute by Snell's law :

sin 40o/sin r = 1.52

Make use of trigonometric tables.
Find for r.
Now find angle of deviation using this formula :

d=i-r or d=r-i

Whatever comes greater.

Now use this formula :
e= d+A-i

Where A is angle of prism. Make it constant ie 60o.
Here consider e as angle of dispersion.
Find e or angle of dispersion.

Repeat it again and again using different angle of incidence. 50o , 60o , 70o etc.