Remember - labs due this week

Gang,

I am out today (Monday, 12th). Your labs are due this week. If you want feedback on a draft, get it to me by Wednesday.

For C block, even though we don't meet on Friday, you may turn in your lab by then.

Okily dokily?

sean

practice problem

This is due on Friday for C block, and next Monday for E block.

Also, the formal lab is due NEXT Thursday for C block and NEXT Friday for E block. Drafts (for those who wish to submit them) should be turned in by no later than one class earlier.

In addition to finishing the lab questions, try this problem:

A convex lens has a focal length of 20 cm. Calculate di under these circumstances:

do = 60 cm

do = 40 cm

do = 30 cm

do = 20 cm

do = 10 cm

This may seem tedious, but the idea is to help you see what happens to di as the object approaches the focal length.

Lab questions

First, play around with this applet:

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

Here are questions to ponder for this formal lab:

1. How are convex lenses and concave mirrors similar?

2. What would happen to a real image if a mirror or lens were half-covered? Does it matter HOW you cover it?

3. What is going on with convex mirrors and concave lenses?

4. Pick a data point from your convex lens trials. Use the measured do and di to calculate an experimental focal length (f), using the equation:

1/f = 1/do + 1/di

5. Repeat 4 for a data point from your concave mirror trials.

6. How close are your calculated focal lengths to your "theoretical" (taken outside) focal lengths? Calculate percent errors:

% Error = (experimental - theoretical) / theoretical x 100

7. What does the lens equation have to say about the following cases? If helpful, make up a focal length and plug it into the lens equation.

do is much, much greater than f

do = 2f

do = f

do < f

8. Do these answers (to 7) make sense in light of your data?

9. How can you tell mathematically if an image is virtual or real?

10. How could one determine the focal length of a convex mirror or concave lens?

11. So, what can you say about how images form (in a convex lens or concave mirror) as do progresses from greater than 2f to within f?

12. Don't forget to address sources of error in your report and write a general conclusion about the lab.

Take-home quiz

Due Monday (for C block) and Tuesday (for E block):

Write out your work on a separate sheet of paper - you will be turning this in. It is due IN class at the beginning of class.

You may NOT use your notes for this, NOR may you use the blog or other online resources. Use the equations given. This is a low-stress way to give me a sense of what you know today.

n = c/v

c = 3 x 10^8 m/s

n1 sin (theta 1) = n2 sin (theta 2)

sin (critical angle) = 1/n

1. Consider a rectangular (but 3-d) piece of amber (n = 1.55). Light hits it an angle of 70 degrees (with respect to normal). The block is large enough so that the light can enter and exit without interference. Find the following:

a. a drawing that represents this problem

b. the angle of refraction inside the amber

c. the angle as the light ray leaves the amber

d. the critical angle of amber

e. speed of light inside amber

2. Consider a set of parallel lines hitting a convex lens. Draw this, showing exactly what the light rays do inside the lens and outside the lens (after they leave). Be clear and specific in your drawing.

Part 2 of homework. Play around with this applet (if time allows):

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

Unrelated, but fairly awesome.

http://gizmodo.com/5850643/what-the-hell-magnets-why-are-you-so-amazing?tag=science

http://andrewsullivan.thedailybeast.com/2011/11/ment-5.html

homework: C and E

Apologies to C-block for not posting this sooner. E-block, this is for Friday's class.

Before next class, find the definitions of:

focal point (or focal length)

thin lens (or mirror) equation - show the equation and tell what the variables represent.

Thanks very much. If you do not get to it, because you expected it last night, I understand.

In the lab, we will take this approach:

1. Set up optical bench, using a convex lens first. Your "object" will be a candle. Find a theoretical focal length, using the method discussed in class.

2. Vary the object distance (do) and note how the resulting images (if there are images) change.

3. Watch for these things, and make notes (in a table). Is the image:

a. bigger, smaller or same size as object

b. right-side up or upside down

c. real or virtual

d. where is the image located (di)?

4. You will repeat this for a concave lens, concave mirror and convex mirror.

5. In general, you will be seeking to determine how lenses and mirrors form images, when they form images and how they form images.

6. The lens/mirror equation is very good for making predictions as to how images are formed (and what types of images are formed). You will use the lens equation to calculate an experimental focal length and compare it to your theoretical focal length (with percent error).

7. Other lab questions will be posted.

Prep for Next Formal Lab homework

In our next formal lab, we will investigate how images form. Find out the meaning of these terms:

real image
virtual image
convex lens
concave lens
convex mirror
concave mirror
plane mirror

This is homework mainly for C block class, but E block class should do it if they have time (after the 5 critical angle problems).