Test practice

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

To play with.....

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

Move the object to see how the image formation changes.  Choosing "principal rays" makes it easiest to see things.

Feel free to play around with the other options.

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 Monday

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.

In your conclusion, one thing you will address will be:  how well do your results match your prediction/hypothesis?

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

Thanks!

HW for Thursday

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

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

As always, this site is helpful:

http://www.physicsclassroom.com/

And now, the lab:

On Thursday we will begin a new formal lab.  This lab is all about convex and concave lenses and mirrors.  You have played with lenses and mirrors a bit in class.  For our next class, please write a formal hypothesis for this experiment.  It will likely be informed by what you saw today in class.  The hypothesis should address the following:

How will the distance between object and lens affect whether or not an image is formed, how (relatively) large the image is (bigger/smaller) and whether or not the image is right-side up or upside-down?  Also, address what types of optics (concave or convex lenses and mirrors) will produce images (real or virtual).

FYI - a "real" image can be projected onto a screen (think of setting fire with a magnifying glass), whereas "virtual" images are only seen "inside" lenses or mirrors (think of looking at yourself in a bathroom mirror).

Answers may be something like:

- I believe that all images will form at the focal point, and I think that all images will be real and smaller than the object.
- I believe that images will only form if the object is very close, but that these images will be virtual.
- I believe that images will be formed only when the object is between the theoretical focal point and twice the theoretical focal point, and that these real images will always be smaller than the object.
- etc.

Be sure to type out your hypothesis and keep a copy for your formal lab report - I will be checking these at the beginning of class.

For next class (if you see this in time)

Please look up the definition of "focal point" and/or "focal length."

Also look up the definitions of "real image" and "virtual image."

Thanks!  Sorry for the late notice.  If you don't have time to do this, it'll be ok.

sean