Physics Marking Scheme Mistake?

jumpguy

Was doing a physics question tonight and I came to the marking scheme. I think there might be a mistake, and if not, can someone please clarify?

I was doing Paper 2005, Section A, question 3 (To Verify Snell's Law experiment). I drew my graph with sine r on the x-axis and sine i on the y-axis. But in the marking scheme for this question, sine i is on the x-axis and sine r is on the y-axis. Even my book agrees with me. Hence, the refractive index (slope of the graph) I got was also incorrect, according to the marking scheme.

http://www.examinations.ie/archive/markingschemes/2005/LC021ALP1EV.pdf
This is the marking scheme in question.

http://www.teachnet.ie/tbrophy/pdfs/physics/snell.html

Quick google search agrees with the marking scheme, but on my first instinct I agreed with you, so must be an easy enough mistake to make.

jumpguy

I've an odd feeling that might be just read out of the marking scheme.

Look here, a YouTube video agrees with me:

http://www.youtube.com/watch?v=Ub4lpPRIGxs

Go to 1:15. You might also want to mute your speakers to avoid listening to the crappy music they put in it.


I'm now thoroughly confused. Can someone please clarify?

Well I just asked my dad (he works in the Physics dept in DIT) and he said that sin i would be on the X axis because the variable that you determine is generally put on the X axis, with the variable you can't control on the Y axis.

Which rings some bells with me. Also, the answer they give of 1.5 is the refractive index of glass, so that makes sense.

jumpguy

Thank you for asking.

That's normally the case (for example with measuring the focal lenght of mirrors and lenses, u goes on the x-axis and v goes on the y-axis.) However, we were told for this experiment sine i goes on the y-axis and sine r goes on the x-axis because of the slope formula.

Sine i y2-y1

---

---

Sine r x2-x1

Hence sine i is on the y-axis, sine r on the x-axis. I'd accept the answers if it wasn't that I have conflicting information.

^ yeah, that's what I immediately thought too, that because of the slope formula that sin i should be on the Y axis.

What answer did you get? Because I do know the answer should be 1.5.

jumpguy

1.5, which I knew immediately was strange. It's only a few months ago I did the experiment myself and got 1.5, using the sine r on x-axis, and sine i on y-axis graph.

You're right. I just did some calculations using the graph and data they have on the marking scheme, using the 1st 2 points. With sin i on the X axis I got an answer of 0.667, and with sin i on the Y axis I got 1.5.

You just outsmarted the SEC, well done!

jumpguy

Deleted User wrote: »

You're right. I just did some calculations using the graph and data they have on the marking scheme, using the 1st 2 points. With sin i on the X axis I got an answer of 0.667, and with sin i on the Y axis I got 1.5.

You just outsmarted the SEC, well done!

Thanks a million! I went back to my graph to check my slope aswell and I had it wrong. I must've pushed the wrong button on a calculator, I actually got 1.47 (it's within the error threshold). Bloody SEC!

Haha can't do anything right can they? Someone should probably notify them, if they don't already know.

You're applying to med aren't you? I think I've seen you around the same threads as myself!

spurious

Is that the final marking scheme or the original?

Marking schemes occasionally have errors in them as they are generally drawn up by one person. Contact the SEC and let them know.

jumpguy

Deleted User wrote: »

Haha can't do anything right can they? Someone should probably notify them, if they don't already know.

You're applying to med aren't you? I think I've seen you around the same threads as myself!

I'll be applying for med in 2011 (in 5th year now).

spurious wrote: »

Is that the final marking scheme or the original?

Marking schemes occasionally have errors in them as they are generally drawn up by one person. Contact the SEC and let them know.

I'm not sure. I got it off the SEC Website (www.examinations.ie). That's the one they have up...which I'd assume would be the final one. However, I'd be shocked if out of how many 100's of examiners they'd have, not one that year would spot the error and an errata would be made out.

The SEC's clarity on which email address to contact is poor, to say the least. :rolleyes: http://www.examinations.ie/index.php?l=en&mc=co&sc=ct

Fringe

Both aren't necessarily wrong. The convention is to put what you can control on the x-axis and your result on the y-axis. However, in this situation, if you want the equation of the line to be sin i/sin r = n, you would do it the other way around. The equation of the line in the marking scheme is sin r/sin i = 1/n. Both are actually correct but you just have to remember that the slope in the marking scheme is the inverse of the refractive index. Someone got 0.667 for the marking scheme. Try dividing that into 1 and see what you get.

jumpguy

Ahh I see, thanks. We actually did this. So there's really no "wrong axis"? Aslong as you get the inverse of your slope, you'll be grand.

Dirac

See: http://www.boards.ie/vbulletin/showpost.php?p=62409784&postcount=4