Science Buddies: "Ask an Expert"

Hello,

Well I have another question.
Do you think I should test 5g 10 g 15g 20g 25g 30g. and test each 3 times then test an unknow solution

or do you think I should do 10g 20g 30g testing each 3 times

or just 5g -15g and test each 6 times

do you have any other ways?
While adding sugar I guess i should decrease water, for example if I put 5g I should put about 95 mL and then for 10g do 90 mL of water and for 15g 85 mL and etc.

Any Advice?

Thank you,
Alaa

Alaa1991 wrote:Oh thanks so much

do you think a physic teacher might have a prism, instead of me building it, and buying bunch of things to make it.

Is it possible to buy one from any stores?

Thanks for your advice

You need a hollow prism to put the liquid inside, correct? The teacher will only have a solid prism. You cannot buy one.

Did you find a laser pointer? I would see if your parents or someone has one from work, as the ones at target looked expensive.

Louise

I have went to a physic teacher and asked him for a prism, but as you said he doesn't have it, but he gave me a semi-circle plastic thing, and said this also works for refraction, and I could direct a laser at it.

I explained that I needed a hallow prism where I can put water in it, and that is what he gave me, do you recommend me using that semi circle, which I can fill with water, and direct a laser at it, and do the exp. and I don't have to build that prism, buying a couple of materials to make it.

or would you recommend be making it. I don't know does it make a difference what method I use?

Alaa1991 wrote:Hello,

Well I have another question.
Do you think I should test 5g 10 g 15g 20g 25g 30g. and test each 3 times then test an unknow solution

or do you think I should do 10g 20g 30g testing each 3 times

or just 5g -15g and test each 6 times

do you have any other ways?
While adding sugar I guess i should decrease water, for example if I put 5g I should put about 95 mL and then for 10g do 90 mL of water and for 15g 85 mL and etc.

Any Advice?

Thank you,
Alaa

You should not decrease the water. You should make the solutions volumetrically if possible, using a volumetric flask which your chemistry teacher should have. If you cannot do this, you should use a constant amount of water.

The point of this experiment is to make a calibration curve using the known samples. You should do this several times to make sure you have good data and error bars. You want your calibration curve to have a wide range, since you want to make sure the unknown falls within the range. I would do the 5-30g range, and I would test as many times as possible. 3 is the minimum. The measurements should not take to long.

At some point you have to just _do_ the experiment. I cannot tell you the best way to do this- I understand how it works- but you won't know exactly what you need until you start the experiment. Suppose for example, your unknown contains 0.5 g/ 100 mL. Then this proposed range will not work, you would need 0.25 g, 0.5 g, .75 g, etc. Do the experiment. If the unknown is very far away from the calibration points, then you fix it.

Louise

Louise

Alaa1991 wrote:I have went to a physic teacher and asked him for a prism, but as you said he doesn't have it, but he gave me a semi-circle plastic thing, and said this also works for refraction, and I could direct a laser at it.

I explained that I needed a hallow prism where I can put water in it, and that is what he gave me, do you recommend me using that semi circle, which I can fill with water, and direct a laser at it, and do the exp. and I don't have to build that prism, buying a couple of materials to make it.

or would you recommend be making it. I don't know does it make a difference what method I use?

The prism should take a few minutes to make. You've been asking questions here about this project for weeks! You have has plenty of time to get the materials.

I think you should also be able to answer this question yourself. You are asking us about _every detail_, some of this you need to try to figure out yourself... I cannot tell if you have no understanding of the project or you are not wanting to think about the project, or you think your answer will be wrong and an "expert" will be right. Your introduction looked fine, so I think you understand the project... try to answer these questions yourself! If you are uncertain about something, feel free to ask, but show you have put some time in to thinking about it.

For this particular questions:

I believe the equations provided in the science buddies instructions are specific to the geometry of a prism. You will certainly get refraction through your semi-circular container, but I have no idea what the formulas would be to work up the data. I also think that the setup might also act as a (cylindrical) lens and distort the beam. You could certainly derive the equations for a circular object, but that will almost certainly be harder than just gluing together some microscope slides.

Louise

I wonder does it matter what is I use a prism or a semi-circle plastic thing I don't know what it is called. But will that affect the exp. I guess now after just knowing the answer to that I will start the exp.

Thank you

Hi, please excuse my interjection here - Disposable cuvettes are very cheap, and if you shine the laser through two adjacent sides (joined at a 90 degree angle) of the cuvette, then it will act as a prism with theta_p (the variable in equation 1) = 90 degrees. Here is one place to order disposable cuvette samples:
http://www.testtubesonline.com/Cuvette_ ... 2-0008.htm
Some suppliers will send free samples.

Even so, it might be faster and more satisfying to build the prism yourself with microscope slides.

Good luck!

ChrisG wrote:Hi, please excuse my interjection here - Disposable cuvettes are very cheap, and if you shine the laser through two adjacent sides (joined at a 90 degree angle) of the cuvette, then it will act as a prism with theta_p (the variable in equation 1) = 90 degrees. Here is one place to order disposable cuvette samples:
http://www.testtubesonline.com/Cuvette_ ... 2-0008.htm
Some suppliers will send free samples.

Even so, it might be faster and more satisfying to build the prism yourself with microscope slides.

Good luck!

That is a good suggestion- just get the 'Cuvette Macro Four Sided' as the others do not have adjacent sides which are clear
Louise

I know I could have just got my materials way earlier, but the problem was that I never had a chance, and /or could not find micrscope slides anyway

I will be using a refraction box, semi circle , below is a picture of it.

http://www.hometrainingtools.com/tbimag ... 100.94.jpg

but that is not the problem anymore it works and its fine. I got everything, but I want to know is a laser with a straight line supposed to work, I mean I have tried it, and followed the steps, and on the paper these seems to be a line and not a dot going through the paper.

Alaa1991 wrote:I know I could have just got my materials way earlier, but the problem was that I never had a chance, and /or could not find micrscope slides anyway

I will be using a refraction box, semi circle , below is a picture of it.

http://www.hometrainingtools.com/tbimag ... 100.94.jpg

but that is not the problem anymore it works and its fine. I got everything, but I want to know is a laser with a straight line supposed to work, I mean I have tried it, and followed the steps, and on the paper these seems to be a line and not a dot going through the paper.

That is what I was saying in the other reply- did you not even read the answer to your question?
"For this particular question:
I believe the equations provided in the science buddies instructions are specific to the geometry of a prism. You will certainly get refraction through your semi-circular container, but I have no idea what the formulas would be to work up the data. I also think that the setup might also act as a (cylindrical) lens and distort the beam. You could certainly derive the equations for a circular object, but that will almost certainly be harder than just gluing together some microscope slides."

The light will not go through the semi circle the same as the prism- the instructions we provided will not work- the equations will not work- you will have to derive how to do this yourself using the the equations of geometric optics-

micrscope slides are sold at the site you quote-
http://www.hometrainingtools.com/

Louise

Louise wrote:

Alaa1991 wrote:I know I could have just got my materials way earlier, but the problem was that I never had a chance, and /or could not find micrscope slides anyway

I will be using a refraction box, semi circle , below is a picture of it.

http://www.hometrainingtools.com/tbimag ... 100.94.jpg

but that is not the problem anymore it works and its fine. I got everything, but I want to know is a laser with a straight line supposed to work, I mean I have tried it, and followed the steps, and on the paper these seems to be a line and not a dot going through the paper.

Reading this again, I think you are actually saying you expect to see a line of laser light all the way from the laser to the wall, and instead, you only see a dot on the wall and maybe on the wall of the cuvette?

The laser is a line. However, you cannot see it unless it interacts with matter. So the drawings show the line of the beam, but you will only see the beam in the air if it is very dusty or when it hits an object like a wall.

However, everything I said above about the geometries is correct, and I think you will have problems working up the data since you cannot use the formulas given in the instructions.

A cylindrical lens (which I think your cell will act as) will turn a circular beam spot in to a line (oval) beam spot, which is what I thought you were saying.

If neither of these cases are true, please post back with more information, as I do not understand what you are asking.

Louise

Yes I know that the laser is supossed to dierct a striaght line, I was just saying when it hits the wall, isn't supposed to be a dot?
That is basically what I was asking.

Anyway I will try to come up with an equation. I guess what I am going to do is, measure the distance from a to b, and d to e on the paper, record it in the data, and then after finsihing all that I guess I could just do the equatin problem later.
Maybe I will try asking the physic teacher that gave me the semi cirlce to help me come up with an equation to find the index of refraction.

I would like to thank you very much for all the effort. Once again thank you Loiuse. I guess I am done just have to do my experiment and finsih the report. and I am done.

Alaa1991 wrote:Yes I know that the laser is supossed to dierct a striaght line, I was just saying when it hits the wall, isn't supposed to be a dot?
That is basically what I was asking.

Anyway I will try to come up with an equation. I guess what I am going to do is, measure the distance from a to b, and d to e on the paper, record it in the data, and then after finsihing all that I guess I could just do the equatin problem later.
Maybe I will try asking the physic teacher that gave me the semi cirlce to help me come up with an equation to find the index of refraction.

I would like to thank you very much for all the effort. Once again thank you Loiuse. I guess I am done just have to do my experiment and finsih the report. and I am done.

Yes. It is supposed to be a dot. Is it a dot or is it a line? I really don't understand what you are asking. I think I answered this question the first time. I will answer again and try to be more clear.

Look at the wall or the paper where you observe the beam after it goes through the refraction box.

If it is a dot- everything is fine.
If it is a line- then the refraction box is acting as a cylindrical lens. This will change a dot to a line. It may get more line shaped with higher salt content and more dot like with less salt. The lens properties change with refractive index.

With a prism, it will always be a dot. This is why the instructions say "dot".

Does this answer your question now?

For the equations, you should google "refraction box". I found several sites that explain how the semi-circular refraction box is different than the prism and might provide you information about how to get the equations.

You should not use the equations for the prism on the science buddies page. They are wrong for a refraction box.

Yes ! That was very very helpful. Thanks!

I've been trying to follow along to get enough clues as to what you were describing and what might be happening. I assume you have the semi-circlular refraction box flat on a perfectly level table, otherwise lots of other things can be going on.

1) If the laser beam is not alligned level with the table, you will get an elongation in the vertical dimension because you have 4 interface surfaces that are oblique to the beam in the vertical axis.

2) If the laser beam intersection with the flat side is not centered on the focal point of the semi-circle, you will again have 2 interface surfaces that are oblique to the beam in the horizontal axis which will elongate the beam in the horizontal dimension.

To understand what I'm saying, try drawing the sides of the refraction box as much thicker on a piece of paper and the beam not as a line but as the extremes of a cone that gets bigger.

In case 1 above, the top and bottom of the cone have slightly different incident angles and the differences in the ray traces will be larger the more oblique the cone axis is to the vertical surfaces. This can be seen by drawing a vertical cross section of your refraction box (what the edges would look like if you cut through the box vertically only thicker - this takes a bit of imagination to visualize without actually cutting a refraction box).

In case 2 above, the left and right side of the cone have slightly different incident anbles and theh differences in the ray traces will be larger the more oblique the cone axis is to the horizontal surface, but the larger effect comes at the curved surface because it is concave convex so it has a positive cylinder lens effect which magnifies the spread. This is much easier to draw as it looks just like the top edges of the refraction box just thicker to exagerate the effects.

Hope this helps. I haven't figured out how to post a drawing in the reply to make the explaination simpler

I just want to make sure I will be doing right, is the following procedure ok.

II. Semi-Circular Water Dish

1-Place the semi-circular water dish on the center of your paper. Trace around it. Carefully fill to near the top with water.

2-Place two pins on one side of the dish, one snugly against the side slightly off the center of the dish and the other directly in line with it. (Pins A and B in the diagram)
3- Look through the dish until these two pins line up in your eyesight.
4-Place another pin on the other side of the dish snugly against the side so that all three pins that are lined up in sight. (Pin C in the diagram)
5-Place a fourth pin (Pin D) behind the dish in line with the other three. You should see all four pins lined up when you look through the dish.
Repeat Steps 2-5 to locate a ray passing through the lower portion of the semi-circle.
6-Remove the water dish and pins.
7-Connect the pin holes to show the paths of light traveling through the dish.
8-With your ruler, carefully draw a dotted line “normal� to the circular surface where the light ray enters/leaves the dish. The normal will be a line through the center of the circular dish.
9-Measure the angles of incidence and refraction.
10-Use Snell’s Law to calculate the index of refraction of the water. You will make this calculation for each set of angles.
11-Average the two values together. This is the water’s index of refraction.


and then I will do it again for sugar concentration .

OR


1- place semi circle on a sheet of paper over the carkboard and trace its outline. remove the plastic temporiarly. mark the center of the flat side of the semi circle , draw a normal pependicular line through, through the center, put the plastic back.
2- Aim the laser from the curved side, and try to hit the center of the flat side.
Note that, even before you reach the critical angle, there will be a reflected ray
from the flat side and from the curved side. This is normal; light always reflects a
little bit when it hits a piece of plastic or glass. This is NOT TIR though as long as
the refracted ray is still coming out on the other side.

3-Increase the angle of incidence θ by rotating the paper and the semi-circle
together until the refracted ray is parallel to the flat side of the semi-circle.
Make sure the laser is still hitting the center of the flat side as you rotate! The incident ray reflected rays refracted ray θ 14 point at which the refracted ray starts to disappear (when it is parallel to the
flat side) is the onset of TIR. At this point, stop rotating, mark the incident ray with pins, and remove the semi-circle.
4. Connect the incident ray to the normal on the flat side and measure the angle that the incident ray makes with this normal (the angle marked “θ� in the drawing above). This is the critical angle θc. Use it in the equation
sinθc = 1/n2
to calculate the index of refraction (n2) of the plastic semi-circle.

I know that I am taking so much of your time, forgive, I am running out of time! and I just get to get things done before the end of feb.

Thanks for your effort

Well thanks for the information, but I still don't get you are asking me to do?
Do you mean I should draw it on a piece of paper, and then measure the index of refraction?

Please I want to know How can I do the experiement, what procedure can I follow to find the concentration.

I believe you are missing an important detail. The semi-circular dish has a focus. The focus is the center of the radius of the semi-circle. The Pin against the flat side of this kind of refraction box must be positioned precisely at the focus and be perpendicular to the table and as you rotate the dish, the focus of the dish must remain lined up with that pin.

Well I want to know can't I just do the following procedure:

I guess that I am to put the semi-circle on a table, direct the laser from the curved side, to go directly to the wall. I would put a big piece of paper on the wall, where I will mark the places where the laser hits. I will direct the laser at the semi-circle when it is empty, and mark where it hits, mark it with a, then fill it with water, and obviously it is supposed to refract, and where ever it hits I mark that as b. and then measure the distance between a and b. and that distance will be labeled x. after finding that, I will put a piece of paper under the semi cirlce, with the semi circle empty. Direct the laser, and mark where it enters as d, and where it exits as f. then measure the distance, as L. And do the same when filled with water.
Next I will get a piece of string, from point a to f. and mark the place where it crosses between d and f, as e. that will give the angle

can I follow that, or do you think that the first procedure is better, but instead of putting the pins on the side, I should put it in the middle, which is the focus, and it should be prependicular. Is that what you are saying?

IF you don't mind, would you write me the steps, I am so confused.

THANK YOU so so much for your help!

Alaa1991 wrote:Well I want to know can't I just do the following procedure:

I guess that I am to put the semi-circle on a table, direct the laser from the curved side, to go directly to the wall. I would put a big piece of paper on the wall, where I will mark the places where the laser hits. I will direct the laser at the semi-circle when it is empty, and mark where it hits, mark it with a, then fill it with water, and obviously it is supposed to refract, and where ever it hits I mark that as b. and then measure the distance between a and b. and that distance will be labeled x. after finding that, I will put a piece of paper under the semi cirlce, with the semi circle empty. Direct the laser, and mark where it enters as d, and where it exits as f. then measure the distance, as L. And do the same when filled with water.
Next I will get a piece of string, from point a to f. and mark the place where it crosses between d and f, as e. that will give the angle

can I follow that, or do you think that the first procedure is better, but instead of putting the pins on the side, I should put it in the middle, which is the focus, and it should be prependicular. Is that what you are saying?

IF you don't mind, would you write me the steps, I am so confused.

THANK YOU so so much for your help!

Your pin procedure is fine. The one detail, and the one Craig and I have been trying to convey, is that this refraction box is a lens. The positioning of the axis is very much more important than with the prism. If you don't understand this, I really think you should try to build the prism. This is more complicated than the prism setup (which I am not entirely sure you understand either).

Look at:
http://www.batesville.k12.in.us/Physics ... n_Lab.html
http://www.glenbrook.k12.il.us/gbssci/p ... u14l2.html
{exercise 4}


Louise

yes now I realized how hard it is, and its giving me a headache. I guess I should build, I want to know but where can I get these mcroscope slides the ones that are 1 by 3. I can go to the store tomorrow but where?

Thank you
Alaa A.

Alaa1991 wrote:yes now I realized how hard it is, and its giving me a headache. I guess I should build, I want to know but where can I get these mcroscope slides the ones that are 1 by 3. I can go to the store tomorrow but where?

Thank you
Alaa A.

Most slides are a standard size. Does your biology teacher not have any?

The website you gave me with the picture of the refraction box also sells them.

I have always bought my microscope slides from the internet. I do not know what store might sell them. I really believe your teacher should have them.

Do you live in a big city? Do you have a lot of stores around?

Louise

I live in FlInt MICHIGAN

OK I will see if my biology teacher has 1 by 3
I have already asked and she gave me a small size
All right I will talk to all biology teacher and see who has

Thanks

Alaa1991 wrote:I live in FlInt MICHIGAN

OK I will see if my biology teacher has 1 by 3
I have already asked and she gave me a small size
All right I will talk to all biology teacher and see who has

Thanks

You can look in the yellow pages for "laboratory supplies" and see if there is anything near.

How small is the slide your teacher gave you?

Louise

Louise wrote:

Alaa1991 wrote:I live in FlInt MICHIGAN

OK I will see if my biology teacher has 1 by 3
I have already asked and she gave me a small size
All right I will talk to all biology teacher and see who has

Thanks

You can look in the yellow pages for "laboratory supplies" and see if there is anything near.

How small is the slide your teacher gave you?

Louise

The reason the procedure calls for 1 x 3 is you want to make an equilateral triangle. So, if you cut the three inch long slide in to 1/3, 1/3, and 1/3, you will have this. You can use smaller slides. If your teacher gave you 1" x 1" you could glue 3 of them together without cutting them. Also, you could just glue 3 slides of any size together... this part isn't very important. You just don't want it too large because then it will take a lot of water to fill.


Louise

Before you give up on the semicircular dish, which is actually easier to use once you master it, see if you understand Figure 1 of
http://phyastweb.la.asu.edu/phy114-tsen ... ht_rev.pdf
along with the following steps / explanation.

Setup:
Find the focus point on the top inner edge of the flat side of the dish. The easiest way I know is to
1) trace the dish on a piece of paper by turning the dish upside down on the paper after aligning the flat side to the about the middle of a ruled piece of notebook paper. Align the flat edge with one of the ruled lines. Your trace paper would go sideways to the “paper� in the document I referenced.
2) fold the paper so that the top and bottom points of the “D� in figure on come together and the radius align and then crease the paper.
3) unfold the paper. The crease is now the “Normal� centerline (see Figure 1).
4) Place the dish right side up on the paper aligned with the drawing.
5) The focus on the inner surface of the flat side corresponds to a vertical perpendicular line that would goes through the crease.

Measurements: Please read the measurement steps and the error reduction steps before doing.
The experiment can be done by
6) fill the dish with the test liquid and adding a the light dispersal agent recommended in the original prism experiment procedure to scatter the laser beam and make it visible.
7) place the laser beam at an angle (see ray marked “Incident light� in Figure 1) such that the beam enters the liquid aligned with the crease.
8) Measure theta 1 and theta 2 for a several different angles (theta 1) similar to the original experimental prism procedure recording these angles. The ratio between theta 1 and theta 2 goes into Snelling’s equation for determining the refraction index.
Error reduction:
9) If you close one eye when sighting the angle, you eliminate what is called parallax.
10) If you place yourself on the laser side and from behind the laser pointer, sight the dispersal light in the liquid with your eye just above the laser path, just like a carpenter of brick mason sights a string or level to project where it is headed, you will reduce the observation error in determining the “Refracted light� angle.
11) If theta 1 and theta 2 are close to the same value, the precision of the measurement will be lower. Better to choose another theta 1.

Scientific methods applied to analyzing data:
12) Calculate the index of refraction for each set of measurements and average them.
13) Calculate a standard deviation. The higher the standard deviation, the more inaccurate one or more the measurements or calculations are.
14) On any that have a high standard deviation, calculate the variance of each individual measurement to determine which measurements were troublesome.

Craig_Bridge wrote:Before you give up on the semicircular dish, which is actually easier to use once you master it, see if you understand Figure 1 of
http://phyastweb.la.asu.edu/phy114-tsen ... ht_rev.pdf
along with the following steps / explanation.

Setup:
Find the focus point on the top inner edge of the flat side of the dish. The easiest way I know is to
1) trace the dish on a piece of paper by turning the dish upside down on the paper after aligning the flat side to the about the middle of a ruled piece of notebook paper. Align the flat edge with one of the ruled lines. Your trace paper would go sideways to the “paper� in the document I referenced.
2) fold the paper so that the top and bottom points of the “D� in figure on come together and the radius align and then crease the paper.
3) unfold the paper. The crease is now the “Normal� centerline (see Figure 1).
4) Place the dish right side up on the paper aligned with the drawing.
5) The focus on the inner surface of the flat side corresponds to a vertical perpendicular line that would goes through the crease.

Measurements: Please read the measurement steps and the error reduction steps before doing.
The experiment can be done by
6) fill the dish with the test liquid and adding a the light dispersal agent recommended in the original prism experiment procedure to scatter the laser beam and make it visible.
7) place the laser beam at an angle (see ray marked “Incident light� in Figure 1) such that the beam enters the liquid aligned with the crease.
Measure theta 1 and theta 2 for a several different angles (theta 1) similar to the original experimental prism procedure recording these angles. The ratio between theta 1 and theta 2 goes into Snelling’s equation for determining the refraction index.
Error reduction:
9) If you close one eye when sighting the angle, you eliminate what is called parallax.
10) If you place yourself on the laser side and from behind the laser pointer, sight the dispersal light in the liquid with your eye just above the laser path, just like a carpenter of brick mason sights a string or level to project where it is headed, you will reduce the observation error in determining the “Refracted light� angle.
11) If theta 1 and theta 2 are close to the same value, the precision of the measurement will be lower. Better to choose another theta 1.

Scientific methods applied to analyzing data:
12) Calculate the index of refraction for each set of measurements and average them.
13) Calculate a standard deviation. The higher the standard deviation, the more inaccurate one or more the measurements or calculations are.
14) On any that have a high standard deviation, calculate the variance of each individual measurement to determine which measurements were troublesome.

Craig is correct that this is easier- if you understand it- since there you measure the angle directly- however- if you don't understand it and are trying to copare with the prism instructions then the confusion will get worse - I personally think that we've spent a lot of time on the first experiment and we should stick to that- but the choice is yours


A picture is worth a thousand words- so look at these websites and see if they make what Craig and I have said clearer- These use java so you can 'do' the experiment on the computer and understand first! Then do it for real


http://freespace.virgin.net/gareth.jame ... ction.html

This is a site that lets you do a refractive index (RI) change- increasing the salt in the 'real' experiment changes the RI so you can test the effect:
http://micro.magnet.fsu.edu/primer/java ... index.html

If you use these virtual labs I think you will understand the project better- then read this thread again and I think everything will be clearer

Louise

Honestly I don't know how to thank both of you, Thank you so much for your help and effort that you put to help me. All right I will see those website that you gave me.

once again THANKS !!!!!!!

Alaa1991 wrote:Honestly I don't know how to thank both of you, Thank you so much for your help and effort that you put to help me. All right I will see those website that you gave me.

once again THANKS !!!!!!!

No problem. We are glad to help! And we really want you to understand your project and do well! So, if you have questions about refractive index or snell's law after you look at those websites, ask!

I understand things best by doing something. For example, I could read the instructions for this project at science buddies, but I would not _really_ understand them until I do them. Luckily, I have used lasers and prisms and found the angle of minimum deviation, so I understand that well. When you posted the picture of the refraction box, the first thing I did was look around to see if I could find something like that in my house or at school so I could try it myself. I could not find anything, so I looked for a "virtual demo". You may learn differently than I do, but I get the feeling that you also learn better from doing instead of reading a long text.

I really hope these websites help, because I am afraid I've only confused you more or not un-confused you!

Louise