Answers to practice problems from yesterday

1.  1.8

1.66 x 10^8 m/s

2. 3 x 10^15 Hz

3. angles are 42 and 48 degrees

4.  concave, -7.1 cm = di (making it virtual), mag = -0.7  (which makes it smaller and upside-down)

Answers to recent (turned in) homework

This will be in your mailboxes Wednesday morning.  I will post answers to the practice problems (from earlier today) later this evening.

Thanks for understanding my absence.

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1.

a.  convex (since the f is +)
b.  di = 60 cm.  Use the lens equation:  1/f = 1/di + 1/do
c.  magnification = -2.  Use the magnification equation:  mag = -di/do
d.  real (since di is +)
e.  upside-down, since the mag is -

2.  Dimmer, but still in focus.  Fewer light rays make it, but their path is not changed.

3.  Place the object AT the focal length.

4.  Either within f, or between f and 2f

Review for Thursday's test and Lab info

Folks - I am out today and tomorrow still.  Very sorry.

Finish up lab and prepare for Thursday's test.  On the blog there is a list of topics and sample questions to try.

If you want me to review your lab draft, turn it in or email it to me.  I'll pick labs up later and return them to mailboxes after school, so that you can get them tomorrow morning.  If emailing me the lab draft, attach the lab.  Do NOT share it with me.

Labs can be turned in by the end of the week.  You'll even be able to work on it, if needed, after Thursday's test.

Thanks for understanding.

- sean

Review for test:

Law of reflection – angle in = angle out

Refraction:  what is it, why it is

Index of refraction:  n = c/v

Snell’s law:  n1 sin(theta 1) = n2 sin(theta 2)

Other equations:  v = f l

Also worth remembering:  f does NOT change during refraction, but v and l will

Remember:  normal line (perpendicular to surface of optic)

Lenses and mirrors:  convex lens and concave mirror (both have +f); concave lens and convex mirror (-f)

Lens equation:  1/f = 1/di + 1/do

Magnification:  mag = -di/do

-mag = upside-down image; +mag = right-side up image

Absolute value of mag tells you whether image is larger ([mag] > 1) or smaller than object ([mag] < 1)

Real (+di) vs. virtual image (-di)

Critical angle and total internal reflection:  conceptually only
Basic trig use:  sin, cos, tan

Questions to expect:

1.       Snell’s law (like quiz)

2.       Lens (like HW)

3.       Miscellaneous:  you’ll choose 2 out of 4, more or less.  This may change.  Potential topics:

a.       Essay covering some abstract situation or problem

b.      More mathematical/theoretical question

c.       Demonstration explanation?

d.      Practical (where you have to DO something or look at something)

e.      ??? (mwa ha ha ha ha…..maniacal laugh.... just kidding!)

Sample problems which will be similar to the mathematical questions on the test; there will also be a page of short answer/essay, as before.  Answers to follow later in a separate post.

1.  Light hits a piece of plastic at an angle of 50 degrees (with respect to normal).  If the angle of refraction inside is 25 degrees, what is the index of refraction of this plastic?  Also, what is the speed of light inside the plastic.

2.  What is the frequency of light with a wavelength of 100 nm in air (where it travels pretty much at the speed of light).  (That's 100 x 10^-9 m.)

3.  Consider a right triangle:  (65, 72, 97)

Choose an angle and find the sin, cos, and tan ratios for that angle.  Then calculate the angle itself (probably using inverse sin).

4.  A lens has a focal length of -25 cm.  A candle is 10 cm in front of it.  Find the following:

a.  type of lens
b.  location of image (di)
c.  type of image
d.  magnification of image
e.  whether image is up or down
f.  whether image is real or virtual

Posting this HW early - it is due on Friday

Consider a lens with a focal length of +20 cm.  An object is 30 cm in front of it.

1.  Determine the following:

- type of lens
- location of image
- magnification of image
- type of image (real or virtual)
- whether image is right-side up or upside down

2.  What would be the effect of covering up half of the lens?

3.  Where could you place the object such that you get NO image?

4.  Where could you place the object such that you get a larger image?

Lab guidelines

Start thinking about (and writing) the formal lab.  Here is what you will need:



Basic structure of the lab report:

* Title

* Purpose of lab

* Data table - include all columns from your data, AND 1 more column:  calculated f

* Find the average f, and the percent error between your average and the expected f from the lens.

* 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.  Include the following:

 - 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).

- Discuss a technique for measuring the focal length of a concave lens (or convex mirror).

- Discuss the similarities between convex and concave mirrors and lenses.

 - Give a general conclusion.

Homework for Monday

Questions to think about, based on the recent lab stuff:

Under what circumstances do images form at the focal point of a lens, if ever?

What is a virtual image?

Can a focal point/length be negative?  What would that mean?

In our next class, we'll look at curved mirrors and see how they compare. 

Expect a quiz next Friday, and the formal lab will be due a class after that (more or less).

Happy weekend!

HW for Thursday

Hi there!

Have a look through your lab data to see if there is anything interesting.

Also, look up the term "focal point" (or "focal length").  How is this related?

Thanks!