Practice for test

Consider 700 nm light ray, hitting a block of plastic at 40 degrees.  It refracts to an angle of 25 degrees.  Find:

- Index of refraction of plastic

- speed of light inside plastic

- wavelength inside plastic

- frequency of light

Consider a lens, focal length of -20 cm. An object is 30 cm in front of it.  Find:

- Image location

- type of image

- magnification of image

Attention A block

Your lab draft will be due next Monday (NOT this Thursday).  Lab will then be due next Wednesday.  The test will still be on Friday of next week.

I'll gladly look at labs on Thursday if you like, but we spent too much time on other problems today.  You would benefit from more class time on the lab - which you'll have on Thursday.

So in conclusion:

Monday - lab draft due
Wednesday - final lab due
Friday - test

We'll review for the test Monday and/or Wednesday.

Reminder about formulas for the lab

Thin Len (or Mirror) Equation:

1/f  =  1/di  +  1/do

Remember:  it's quickest to use the x^-1 key on your calculator (4th key down on the left on most calculators).


Percent error = [(experimental - theoretical)/theoretical] * 100

Or if you prefer:

[ (exp - theo) / theo ] * 100

In our experiment, the calculated f (using the thin lens equation) is "experimental".  The "theoretical" value is the one that was printed on the lens or mirror.  (The mirror focal lengths were all 20 cm, except for the one that was 22 cm.)

You'll need to calculate the experimental focal lengths for EACH trial where you have numerical do and di data.  Do these calculations before the next class and add them to your data table (along with the percent error).

Lab guidelines

Basic structure of the lab report:

* Title

* Purpose of lab

* Your hypothesis (from the earlier homework)

* Data table - include all columns from your data, AND 2 more columns:  calculated f, and percent error
   - be sure to calculate these for all trials where you have numerical data for real images

* Sample calculation for focal length (your data table will have ALL of the calculated values, but there is only need for one calculation to be shown)

* Graph(s) if you made any (they are not required, but a graph of di versus do might be instructive)

* Conclusion - probably the biggest, most detailed part of the lab:
 - Address your hypothesis.
 - Give sources of error.
 - Discuss how the image formation depends on object distance.  Note if there "transition points", etc.  This is the tough part of the conclusion (and the most mathematical).
 - Give a general conclusion.

HW for Tuesday A and Wednesday F

Use the lens equation to calculate experimental focal lengths (f) for all of your trials with di and do values - for both convex lenses and concave mirrors.

Play around with this, if your computer will run it:

http://phet.colorado.edu/sims/geometric-optics/geometric-optics_en.html

HW for A (Friday) and Monday (F)

Look at your lab data - note trends, if you can see any.  Things like:  does the images become virtual at a certain point, does the image go from smaller to bigger at a certain point, etc.  

Look up the "thin lens equation."  It will have the variables:  di, do, and f.

Thanks!

Lab image

See also the previous blog notes about the hypothesis.

Note above - the light source ("object") is on the left, the lens is in the middle, and the screen (where any "real image" forms) is on the right.

Distance between object and lens is do.

Distance between image and lens is di.

In your lab, you will record this data (at least):

do

di

Real or virtual image

Up or down image

Relative size of image (bigger, smaller, same)

Other info

You will perform the experiment for a convex lens, concave lens, concave mirror, and convex mirror.

HW for A (for Wednesday's class). F can do this if you didn't finish it earlier.

Find a formal definition for "focal point" and/or "focal length" of a lens.  Does this apply to all lenses (convex and concave)?

Also, please revisit the hypothesis and tweak it, if you like.

Finally, if you are still unclear about real and virtual images, do a little research about this, too.

As always, this site is helpful:

http://www.physicsclassroom.com/

Also, my apologies for not discussing the hypothesis - when I wrote it on the blog, I forgot to mention that it was optional.  I really didn't want to give you work over Thanksgiving!  Sorry to stress anyone out.