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.