HW for Friday

Revisit the Ripple applet and play with the "two sources" set-up.  It may be helpful to look at the different color schemes.

http://falstad.com/ripple/

You are witnessing waves "interfering" with each other.

This can happen quite easily with light - passing light through small openings creates "diffraction" patterns.

Look into this phenomenon, checking out images online.

Also, see if you find out some information about this equation:





Detailed info here - you don't have to read the entire thing, but have a look if there is time:

http://en.wikipedia.org/wiki/Diffraction


Finally, if you're bored over break and want to read ahead, look into HOLOGRAPHY.

Today / Thursday

This is the last day I expect to be out.  I hope to return tomorrow (Friday) and will keep G and X block open for lab questions, etc.

TURN IN A DRAFT TODAY IF YOU WANT IT BACK FOR COMMENTS TOMORROW.  There will be a box on the front desk in the classroom - turn them in there.

Formal lab is due next Wednesday.


Problems to try in class TODAY.  Work on these and discuss.  These are NOT to turn in, but you should be discussing them in your lab group. 

The following equations will be needed:

Lens/mirror equation:

1/f = 1/do + 1/di

Magnification equation:

M = -di/do


1.  You have a convex lens with focal length (f = 20 cm).  Where do you find the image (di) if the object is located (do):

a.  100 cm away
b.  40 cm away
c.  30 cm away
d.  20 cm away
e.  10 cm away

Solve first, then check your answers below.

Look at cases b, d and e in particular.  Is there anything strange about the answers these cases?  Do you notice any similarities with your lab data?

Looking at e -- if you've done it correctly, you calculated a negative answer.  What do you think a negative di means about the image?

Looking at d -- this one is hard to think about, but the answer is "undefined."  What does this mean?

Answers:

a.  25 cm
b.  40 cm
c.  60 cm
d.  undefined (or di = infinity); no image is seen, or the light rays emerge parallel (never converging, or rather, converging at infinity)
e.  -20 cm


2.  Now go back to problem 1 and calculate the magnifications in each of the cases. 

Answers:

a.  -0.25
b.   -1
c.  -2
d.  no image
e.  2

Worth noting:

Magnification means how much the image is magnified (compared to the object). 
Magnification has no units - it is a ratio that represents how many times the image is larger than the object.
A negative magnification means that the image is upside-down. 
When the absolute value of the magnification is greater than 1, the image is larger than the object.
When the absolute value of the magnification is less than 1, the images is smaller than the object. 
If the absolute value of the magnification equals 1, the image is the same size as the object.

If there is time in class, you may continue to work on the lab draft, but it must be turned in today (in the box on my front lab desk) for me to look at it tonight and return it tomorrow (to your mailbox).

Revisit the applet and play around.  Note that you can change the optic from convex to concave, and from lens to mirror.  Slide the arrow (object) from left to right and note how the image changes.

http://www.physics.metu.edu.tr/~bucurgat/ntnujava/Lens/lens_e.html

 

For homework, continue to work on lab report - I will return your labs to your mailboxes on Friday.  You can also look at these problems and the applet again.

About the lab

There is apparently panic brewing.  The DRAFT of the lab is due on Thursday.  You will have time tomorrow to work on it, though you should have done a good amount of it by now. 

The FINAL copy of the lab will be due NEXT Wednesday (before winter break begins).

Got it? 

In your lab, be sure to have all the important lab stuff:

purpose
hypothesis (this was hw from before the beginning of the lab)
data tables
sample calculations
questions that were posted last time
conclusion, etc.

I will be out tomorrow (Tuesday).

Lab questions

Questions for the lens/mirror lab:

1.       1,  You’ve no doubt seen that the size of the image changes.  Comment on how/where it changes.   Knowing the theoretical focal length, can you determine a rule (or an approximate rule) – something like, the image goes from smaller to larger when do is equal to the focal length (or something like that).  Be specific.

2.      2.  How does the aperture affect the image?  That is, if you were to block the top part of the lens or mirror, how would the image change?  How about if you cover the bottom part?

3.      3.  You should calculate theoretical focal lengths for every point where you have data (do and di).  How do these experimental focal lengths compare to the theoretical focal length?  Do a percent difference calculation for each.

4.       4,  In general, have you found your results for lenses and mirrors to be similar?  Discuss.

5.       5.  In this lab, where did you see virtual images?  How did you know they were virtual?

6.      6.   How could one determine (experimentally) the focal length of a convex mirror or concave lens?

7.      7.   Mathematically, there are worthwhile cases to consider – key points – where do = f, do = 2f, do is much, much greater than f.  What is going on mathematically with each of these cases?

8.      8.  Don’t forget to give errors and a general conclusion.  In your conclusion, discuss the extent to which your initial suspicions (hypothesis) were correct or incorrect.

Play around with this applet and note what happens.  This may make things easier to visualize, even though only 2 or 3 rays are depicted as coming from the object.

http://www.physics.metu.edu.tr/~bucurgat/ntnujava/Lens/lens_e.html

There is a useful expression, the magnification equation:

Magnification = -di/do

In this equation, a negative magnification indicates an upside down image.  If the absolute value of the magnification is greater than 1, the image is larger.  You may want to calculate magnifications to see how well they match with what you find to be in the lab data.

HW

Don't forget - brief quiz (Snell's law) on Friday.  Invest a few minutes in reviewing/studying.

To prepare for last day of lab stuff:

Investigate the difference between real images and virtual images.  Define each.

If you have time, calculate experimental focal lengths, using the thin lens equation:

1/f = 1/do + 1/di

(It's easiest if you use the x^-1 key on the calculator.)

Continue to examine lab data and look for trend(s).

HW

1.  Examine lab data; look for trends.

2.  Look up:  thin lens equation.  Find out how what it means, how to use it, etc.

3.  Look up concave and convex mirrors - know the difference.

Also - suggested viewing from Alexis (thanks!).  You may find some useful videos here:

http://www.khanacademy.org/science/physics