Science Buddies: "Ask an Expert"

Hi:

For each different CD, and each trial, you should measure the incident angle, and use that value for the calculations across the data row in the table. You don't calculate 'D' for the incident angle, as the sin θi - sin θi will always be zero. In your actual experiment, you probably do not change the placement of the laser, so your incident angle will probably always be the same. However, a good scientist will make sure and measure it each time anyway! Good luck.

Keith

thank you for your help, if i need anything ill post again...

ok, i did the experiment and it worked fine. But the results that i got are a bit unbelievable. i did 3 different trials. One trial on a dvd that had information, another one was on an empty dvd, and the other was on a CD that had information. I got 1560nm for the empty dvd and 1703nm for the CD with information, and i got 1434nm for the dvd that had information. Are these results ok???

and by the way, can you have more than 5 reflected angles.??

Hi

I'm doing my science project on this topic. When I use a CD, I get 4 clearly defined refraction beams, as shown on this site. However, when i use a DVD I can only get two refraction beams no matter what angle I put the incident beam at. Is this normal? How do you know which two beams they are? (m=1,m=0,m=-1,etc.)

Thanks!!!!

Hello and welcome to the forum.

I haven't done this experiment myself, so I don't know if DVDs normally give you only 2 refracted beams. You say you've tried several different incident angles, which would be the most logical thing to try. How dark is the room you're working in? For the DVD, with closer together tracks, you might need to have the room darker to see the beams. You also might try changing the distance from the DVD to the laser.

In terms of which 'm' the beam is, in the experiment it shows that beams on the other side of the perpendicular from the incident beam are positive, with the beam with the same angle as the incident beam = m=0. As you move away from the perpendicular on the same side as the incident beam, the first beam is m=-1 and so on. HTH.

Keith

WIth regards to this question:

I got 1560nm for the empty dvd and 1703nm for the CD with information, and i got 1434nm for the dvd that had information.

I searched the web for an answer, and it looks like the CD number is reasonable. However, DVD tracks are less than half the distance as CDs apart.

With regards to this question:

and by the way, can you have more than 5 reflected angles.??

It would seem that you've done a great job getting the experiment to work! Congratulations!

Keith

I have made sure the room i'm working in is dark, however i can still get only 3 refraction beams. My problem is that while i know one of them is m=0, i can;t be sure whether the one on the left is m=1 and the one on the right is m=-1. What do you think?

Also, a quick question regarding the formula to calculate this. Say you get a formula like 2X650\(75-5)....you get about 18.57, not 1857 which is an "in the ballpark" answer. Are you supposed to move the decimal like that? why?

elykl33t wrote:Say you get a formula like 2X650\(75-5)....you get about 18.57, not 1857 which is an "in the ballpark" answer. Are you supposed to move the decimal like that? why?

Values of the sin function are always in the range 0-1, so I'm not sure where you got (75-5) from. Did you forget to take the sin of the angles?

You first need to identify the m=0 beam. It should be on the other side of vertical with the same deviation. In the project photo, the incident angle is 70 degrees and the m=0 is at 110 degrees (70 + 110 = 180). The m=+1 will be the beam with the next higher angle, example shows 138 degrees. m=-1 will be at the next lower angle, example shows 84 degrees. m=-2 will be at the next lower angle, example shows 56 degrees.

As far as the formula, you are doing something wrong in the denominator. The range of the Sin function is -1 <= (Sin x) <= +1 for all angles x so the domain of the denominator is -2 <= denominator <= 2. Did you forget to use the Sin function and simply try to use the angles directly?

If you aren't getting m=+1, m=0, m=-1, and m=-2 beams, have you tried using different incident angles? Is your incident beam on a "radial" (co-planar with a radius line of the CD/DVD)? You should be able to get these to show up.

Oh thanks guys! sorry that was just stupid of me. I forgot how sin and all that works, and i just assumed "sin1" or w/e meant the angle of the beam. Yah i get it now.

And so basically, since i see one beam left of m=0 and one right of it, those are m=1 and m=-1 respectively?

since i see one beam left of m=0 and one right of it, those are m=1 and m=-1 respectively?

That would be my expectation; however, make sure you aren't being confused by the incident beam. There is a possibility that you chose an incident angle so the incident beam and the m=-1 or m=-2 are at the same angle which could cause considerable confusion.

PLEASE HELP!!

the formula on the site, d = m × λ / (sin θm − sin θi ), does not make sense to me!!!!

So, the "m" is the value of m (1, -1, -2, etc.) i understand that

the weird A shapedish thing is wavelength of the laser pointer i understand that

now, "sin θm" refers to the sin of the angle of your given beam correct? so if you did m=1, and its angle was +55 degrees, itd be sin(55) right?

and then sin θi refers to the sin of your incident beam, correct? so if the incident beam is 20 degrees, its the sin(20)?

my problem is that, as an example problem, if you do 1 X 650/(sin(50)-sin(20)), its comes out negative!!!!!!!!! i've done this myself multiple times, and it wont work! also, this is nearly identical to what it would be using the picture on this site correct????? PLEASE HELP!!!!!!!

I don't understand how you get a negative. Sin 0 = 0, Sin 90 = 1, between 0 and 90 degrees, Sin is a monotonic increasing function. So Sin 55 > Sin 20 so the denominator is positive. Could you be using a calculator set up for calculating in Radians instead of Degrees?

Caution: The angles in the equation are supposed to be measured from "normal" or perpendicular to the CD/DVD surface.

Note: For the case of m=-1, the denominator is expected to be negative to cancel out the negative in the numerator.

Yes! that was the problem, thank you. I didn't even think of that.

i learn a lot from this post.thanks.