Why Does the Angle of Refraction Change in Borosilicate Glass Experiments?

[FeDeX_LaTeX]

Hello;

For my physics investigation I'm investigating a relationship between wavelength and index of refraction for a medium exhibiting normal dispersion. The medium is borosilicate glass (BK7). However, I'm encountering some problems that I didn't expect.

For example, directing a red ray of light with wavelength equal to 750 nm with an incident angle of 20° gives me an angle of refraction of 18°. This was measured five times and 18° was the result each time.

However, on another day, performing the same experiment, with the same filter, the same ray box, still using an incident angle of 20°, I get an angle of refraction of 16° (2° away from my previous result). This was also measured five times to ensure that the measurement was accurate. The positioning of the protractors were identical.

Can anyone suggest why this is the case?

I assume that I accidentally had the glass block orientated the other way round (rotated 180°), which could have played a part in this difference in the angle of refraction. If this were the case, is it possible that different parts of the glass block exhibit normal and anomalous dispersion, causing this change in the angle of refraction to be observed?

Is this a correct explanation or is there an error in my reasoning/can anyone offer a better explanation?

Also: I don't understand why, for example, having an incident angle of 32° gives me an angle of refraction of 24°, yet having an incident angle of 36° gives me an angle of refraction of 22°, and an incident angle of 40° yields an angle of refraction of 27°. Is the variation of dispersion through my medium the cause for this? If my medium exhibited normal dispersion, shouldn't the angle of incidence and angle of refraction be proportional for the glass medium?

(I am using standard Oxford TEXET protractors with graded measurements of 1°, so there is an inaccuracy of +/- 0.5°, I think. But a difference of 2° is quite large.)

Thanks.

 

[FeDeX_LaTeX]

Is it also possible that these are normal results and I should not have expected an inversely proportional relationship?

 

[Redbelly98]

For an angle of incidence of 20°, the angle of refraction should be 13°. This follows from Snell's law and BK7's refractive index of 1.51 at 750 nm. Looks like something is awry with the way you are measuring angles.

I assume that I accidentally had the glass block orientated the other way round (rotated 180°), which could have played a part in this difference in the angle of refraction.

That is easily tested. Make a measurement, rotate the block, and redo the measurement.

 

[FeDeX_LaTeX]

Hello;

My teacher thinks that the glass block is BK7, but he is 'not sure'. I'm unable to confirm the exact type of glass we are using. He also says that I am using the right method to measure the angles... so is there anything else that could be wrong?

I will re-do the measurements. Thanks!