Practice for test

Here are some practice problems.

1.  Consider a ball dropped from rest.  It reaches a top speed of 36 m/s.

a.  From what height was it dropped?
b.  How much time did it spend in air?

2.  forthcoming.

3.  Review these ideas:

a.  SI standards - what they are, what they were, why was there a change?  You don't need to know specific numbers, but rather things like:  the meter is now based on the speed of light, though it was once based on the distance between north pole and equator.

b.  Unit conversions

c.  Odd numbers rule (Galileo)

d.  Graph and how to interpret them - think about the lab

e.  How to use all equations of motion

f.  The related demonstrations (ballistics cart, etc.)

Worth watching.

https://www.youtube.com/watch?v=Lr_bQs4oXgU


IF you have time - it is pretty long.

Class work

Folks

Sorry to be absent.

Try these problems:

How long will it take a ball released from rest to fall a distance of 25-m?  And how fast will it me traveling before it hits ground?

Why did the 2 balls (one launched and one dropped) hit simultaneously?

If a ball is launched from a height of 10-m, at a speed (horizontally) of 5 m/s, how long will it take to hot the ground?  And how far horizontally will it travel?

What is the odd numbers rule all about?

Recall the ballistics cart on the track.  Why did the balls land in the cart?

For next class, find out some biographical info about Isaac Newton.

HW

Review the equations of motion. 

Also, revisit the demonstrations you saw today:  cart that launched and dropped the ball, 2 balls projected simultaneously.  Write a brief statement or paragraph that explains what happened and why.

stuff

http://www.xamuel.com/inverse-graphing-calculator.php?phrase=LALLY

This is just a little fun -- this website will allow you to create a complicated equation based on your name.

Also - make sure you try the additional homework problems posted yesterday.

Barometer story

Angels on a Pin

A Modern Parable
by Alexander Callandra
Saturday Review, Dec 21, 1968.

Some time ago I received a call from a colleague who asked if I would be the referee on the grading of an examination question. He was about to give a student a zero for his answer to a physics question, while the student claimed he should receive a perfect score and would if the system were not set up against the student: The instructor and the student agreed to submit this to an impartial arbiter, and I was selected.

I went to my colleague's office and read the examination question: "Show how it is possible to determine the height of a tall building with the aid of a barometer."

The student had answered: "Take a barometer to the top of the building, attach a long rope to it, lower the barometer to the street and then bring it up, measuring the length of the rope. The length of the rope is the height of the building."

I pointed out that the student really had a strong case for full credit since he had answered the question completely and correctly. On the other hand, if full credit was given, it could well contribute to a high grade for the student in his physics course. A high grade is supposed to certify competence in physics, but the answer did not confirm this. I suggested that the student have another try at answering the question I was not surprised that my colleague agreed, but I was surprised that the student did.

I gave the student six minutes to answer the question with the warning that the answer should show some knowledge of physics. At the end of five minutes, he had not written anything. I asked if he wished to give up, but he said no. He had many answers to this problem; he was just thinking of the best one. I excused myself for interrupting him and asked him to please go on. In the next minute he dashed off his answer which read:

"Take the barometer to the top of the building and lean over the edge of the roof. Drop that barometer, timing its fall with a stopwatch. Then using the formula d = ½gt², calculate the height of the building.

At this point I asked my colleague if he would give up. He conceded, and I gave the student almost full credit.

In leaving my colleague's office, I recalled that the student had said he had many other answers to the problem, so I asked him what they were. "Oh yes," said the student. "There are a great many ways of getting the height of a tall building with a barometer. For example, you could take the barometer out on a sunny day and measure the height of the barometer and the length of its shadow, and the length of the shadow of the building and by the use of a simple proportion, determine the height of the building."

"Fine," I asked. "And the others?"

"Yes," said the student. "There is a very basic measurement method that you will like. In this method you take the barometer and begin to walk up the stairs. As you climb the stairs, you mark off the length of the barometer along the wall. You then count the number of marks, and this will give you the height of the building in barometer units. A very direct method."

"Of course, if you want a more sophisticated method, you can tie the barometer to the end of a string, swing it as a pendulum, and determine the value of 'g' at the street level and at the top of the building. From the difference of the two values of `g' the height of the building can be calculated."

Finally, he concluded, there are many other ways of solving the problem. "Probably the best," he said, "is to take the barometer to the basement and knock on the superintendent's door. When the superintendent answers, you speak to him as follows: "Mr. Superintendent, here I have a fine barometer. If you tell me the height of this building, I will give you this barometer."

At this point I asked the student if he really did know the conventional answer to this question. He admitted that he did, said that he was fed up with high school and college instructors trying to teach him how to think, using the "scientific method," and to explore the deep inner logic of the subject in a pedantic way, as is often done in the new mathematics, rather than teaching him the structure of the subject. With this in mind, he decided to revive scholasticism as an academic lark to challenge the Sputnik-panicked classrooms of America.

(You'll note that I tweaked the end of the story just a bit, as the last few lines don't make much sense in 2014.)

more problems

Assume no air resistance.

1.  How far can a ball fall in 5 seconds?

2.  If you want to throw a ball upwards so that it hits a window 20-m above your hand, with what speed should you throw it?  How long will it take to hit the window?  (You can solve either part first.)

3.  How does the time to fall a certain distance on the Moon compare to the time to fall the same distance on Earth?  Moon gravity is 1/6 that of Earth.

Local gravity notes

Some thoughts on the acceleration due to gravity - technically, "local gravity". It has a symbol (g), and it is approximately equal to 9.8 m/s/s, near the surface of the Earth. At higher altitudes, it becomes lower - a related phenomenon is that the air pressure becomes less (since the air molecules are less tightly constrained), and it becomes harder to breathe at higher altitudes (unless you're used to it). Also, the boiling point of water becomes lower - if you've ever read the "high altitude" directions for cooking Mac n Cheese, you might remember that you have to cook the noodles longer (since the temperature of the boiling water is lower).

On the Moon, which is a smaller body (1/4 Earth radius, 1/81 Earth mass), the acceleration at the Moon's surface is roughly 1/6 of a g (or around 1.7 m/s/s). On Jupiter, which is substantially bigger than Earth, the acceleration due to gravity is around 2.2 times that of Earth. All of these things can be calculated without ever having to visit those bodies - isn't that neat?

Consider the meaning of g = 9.8 m/s/s. After 1 second of freefall, a ball would achieve a speed of .....

9.8 m/s

After 2 seconds....

19.6 m/s

After 3 seconds....

29.4 m/s

We can calculate the speed by rearranging the acceleration equation:

vf = vi + at

In this case, vf is the speed at some time, a is 9.8 m/s/s, and t is the time in question. Note that the initial velocity is 0 m/s.  In fact, when initial velocity is 0, the expression is really simple:

vf = g t

Got it?

The distance is a bit trickier to figure. This formula is useful - it comes from combining the definitions of average speed and acceleration.

d = vi t + 0.5 at^2

Since the initial velocity is 0, this formula becomes a bit easier:

d = 0.5 at^2

Or....

d = 0.5 gt^2

Or.....

d = 4.9 t^2

(if you're near the surface of the Earth, where g = 9.8 m/s/s)

This is close enough to 10 to approximate, so:

d = 5 t^2

So, after 1 second, a freely falling body has fallen:

d = 5 m

After 2 seconds....

d = 20 m

After 3 seconds....

d = 45 m

After 4 seconds...

d = 80 m

This relationship is worth exploring. Look at the numbers for successive seconds of freefall:

0 m

5 m

20 m

45 m

80 m

125 m

180 m

If an object is accelerating down an inclined plane, the distances will follow a similar pattern - they will still be proportional to the time squared. Galileo noticed this. Being a musician, he placed bells at specific distances on an inclined plane - a ball would hit the bells. If the bells were equally spaced, he (and you) would hear successively quickly "dings" by the bells. However, if the bells were located at distances that were progressively greater (as predicted by the above equation, wherein the distance is proportional to the time squared), one would hear equally spaced 'dings."

Check this out:

Equally spaced bells:

http://www.youtube.com/watch?v=06hdPR1lfKg&feature=related

Bells spaced according to the distance formula:

http://www.youtube.com/watch?v=totpfvtbzi0

Furthermore, look at the numbers again:

0 m

5 m

20 m

45 m

80 m

125 m

180 m

Each number is divisible by 5:

0

1

4

9

16

25

36

All perfect squares, which Galileo noticed - this holds true on an inclined plane as well, and its easier to see with the naked eye (and time with a "water clock.")

Look at the differences between successive numbers:

1

3

5

7

9

All odd numbers. Neat, eh?

FYI:

http://www.mcm.edu/academic/galileo/ars/arshtml/mathofmotion1.html

gravity HW

Problems with gravity.  Assume no air resistance.

1.  An object is dropped from rest.  How far could it fall in 3 seconds?

2.  How fast would it be moving after 3 seconds?

3.  If you drop a rock from a 30-m high bridge into the water below, how long will it take to hit water?

4.  Imagine throwing a ball straight up into the air, with an initial speed of 25 m/s.  Hint:  it may be wisest to call UP positive, which makes gravity negative.

a.  How long will it take to reach apogee?
b.  How high will it rise?
c.  How long will it take for the ball to return to your hands?

Think about this

HW stuff

Remember - the lab draft is due on Wednesday.  Final lab report due Friday.

A problem to play with (based on today's new material):

Consider an object that is accelerating at 2 m/s/s*.  It starts from rest and accelerates until it reaches a speed of 24 m/s.

- how long will this take?
- how far will it go during this time?  (Think about the equation for average velocity before doing this part.)

* Note that this unit could also be:  m/s^2.

See you Wednesday!

quiz reminder and hw

Quiz this Thursday:

- conversions (like m/s to furlongs/fortnight, etc.)
- conversion (simpler one - like mico-fortnight, etc.)
- the meaning of SI units

Lab:

If possible, finish your graphs.

Lab draft will be due next Wednesday.  Final lab due next Friday.

Hooray!

Also...

Quiz next Thursday.

Formal lab on Tuesday. Read earlier blog post.

Prep for new lab

Velocity Lab

Formal Lab – The Measurement of Velocity

In this experiment, we will determine the velocity of a cart by 2 methods:

·              Photogate timer

·              Ticker tape timer

Each method can be quite accurate, though what is actually being measured by each is worth some discussion. 

Recall that velocity is calculated by knowing the displacement and the amount of time required to traverse it:

v = d / t

Strictly speaking, this is average velocity.  In theory, the average velocity is a mathematical average of all (if that were possible) instantaneous velocity points throughout the trip.

Instantaneous velocity is the type of velocity you receive from a speedometer – it is the velocity at that instant.  In the case where the object moves at a constant rate, the instantaneous velocity (at all points) is equal to the average velocity.  That should be the case (approximately) for this lab.  We will determine the extent to which this idea is true in this lab.  In this lab, you may work in cm OR m – be consistent.

Procedure

1.      Set up a path for your car to travel – 1 meter should be long enough.  Place your motorized car on it.

2.      Attach a piece of timer tape to the card and ready the cart for motion.

3.      Place a photogate timer at some point along the cart’s path.  Place a flag on the cart – it must break the light gate fully.  Ready the photogate for timing.  Measure the width of the flag for future reference.

4.      Set the tape timer and note the frequency of operation.  Turn on the tape timer.

5.      Turn on the car and allow it to run the length of the path.

6.      Remove the tape and write the time value from the photogate on the tape for future reference.

7.      Repeat for 2 different cart trials, using new tapes each time.

Analysis I – the Ticker Tape Timer

·              Examine the ticker tape.  If the car is traveling at a uniform velocity, how should the dots appear? Verify that this does occur. 

·              Starting with the first clear dot, measure the distance that each consecutive dot is from the firstdot.  Recall the frequency of the timer – this determines the time intervals.  For example, if it is set at 10 Hz, the time between each dot is 1/10 of a second.  With this in mind, write down the first 30 or so total displacements from the first point.  The corresponding times (for 10 Hz) are 1/10, 2/10, 3/10, and so on.

·              Plot total displacement versus time on a graph. What type of relationship is it?  Does this seem correct?

·              Find the slope of the graph.  What does this represent?

·              What would a (displacement vs. time) graph of an accelerating car look like?  How about a decelerating car?  How about a car moving backwards with constant velocity?  Draw these in your lab.

Analysis II – the Photogate Timer

·              Calculate the instantaneous velocity of the car using the time and width of flag.

·              Compare, by means of % difference, the velocities from both methods.  Percent difference is found by taking one value minus the other value, divided by the average of the two values, and multiplied by 100.

In your conclusion, discuss the relative accuracy of the two methods and give methods for improving the lab.

Hw

1.  Convert your speed values from m/s to miles per hour.  The technique is shown below.

2.  See if you can create your own conversion factor for converting from m/s to furlongs per fortnight.  You may need to look up the definition of furlong and fortnight.

3.  How many seconds is a microfortnight?  Recall that micro means one millionth.  Show the work for this.

4.  How great a distance is a nano-light-year?  Work it out.  Recall that nano means one billionth.  You may have to look up light-year (in m).  Also, if the radius of the Earth is 6.4 x 10^6 m, how close is a nano-light-year?

HW

Find out something about the current SI standard of mass (the kilogram), and the plan to change that standard over the next few years.

SI Units info.

Some comments on standards.

Mass is measured based on a kilogram (kg) standard.
Length (or displacement or position) is based on a meter (m) standard.
Time is based on a second (s) standard.

How do we get these standards?

Length - meter (m)

- originally 1 ten-millionth the distance from north pole (of Earth) to equator
- then a distance between two fine lines engraved on a platinum-iridium bar
- (1960): 1,650,763.73 wavelengths of a particular orange-red light emitted by atoms of Kr-86 in a gas discharge tube
- (1983, current standard): the length of path traveled by light during a time interval of 1/299,792,458 seconds

That is, the speed of light is 299,792,458 m/s. This is the fastest speed that exists. Why this is is quite a subtle thing. Short answer: the only things that can travel that fast aren't "things" at all, but rather massless electromagnetic radiation. Low-mass things (particles) can travel in excess of 99% the speed of light.

Long answer: See relativity.

Time - second (s)

- Originally, the time for a pendulum (1-m long) to swing from one side of path to other
- Later, a fraction of mean solar day
- (1967): the time taken by 9,192,631,770 vibrations of a specific wavelength of light emitted by a cesium-133 atom

Mass - kilogram (kg)

- originally based on the mass of a cubic decimeter of water
- standard of mass is now the platinum-iridium cylinder kept at the International Bureau of Weights and Measures near Paris
- secondary standards are based on this
- 1 u (atomic mass unit, or AMU) = 1.6605402 x 10^-27 kg
- so, the Carbon-12 atom is 12 u in mass

Volume - liter (l)

- volume occupied by a mass of 1 kg of pure water at certain conditions
- 1.000028 decimeters cubed
- ml is approximately 1 cc

Temperature - kelvin (K)

- 1/273.16 of the thermodynamic temperature of the triple point of water (1 K = 1 degree C)
- degrees C + 273.15
- 0 K = absolute zero

For further reading:

http://en.wikipedia.org/wiki/SI_units

http://en.wikipedia.org/wiki/Metric_system#History

>

In addition, we spoke about the spherocity of the Earth and how we know its size. I've written about this previously. Please see the blog entries below:

http://howdoweknowthat.blogspot.com/2009/07/how-do-we-know-that-earth-is-spherical.html

http://howdoweknowthat.blogspot.com/2009/07/so-how-big-is-earth.html

Papers due Friday

You can still submit them tomorrow if you wish, but I'd like to chat about e/m induction a bit - and your devices might use this idea, so your paper may benefit.

Pick a device (paper)

Pick one of the following devices to research and write about in a 1-2 page paper.  Make sure to include a helpful diagram or two.  It is ok to talk about the history, but be sure to go into the physics - how does it work?

speaker or headphones/earbuds

telephone (regular, not cell)

guitar pickup (standard electromagnetic, not piezo)

microphone

transformer

MRI

magnetic tape recording
ruining electronics with magnets
hard drive
generator
turbine - wind, steam, water
metal detection
telegraph
phonograph (record player)

maglev trains

If you think of other ideas, let me know so I can tell you if you're in 'over your head'.

This will be due in 2 classes..

HW / motor

1.  Explain how/why the motor works.  It will be important to think about magnetic fields in coils.  In other words, why was it important to wrap the red wire into a coil?  What effect does that have on the magnetic field in the coil?  Is it similar to the electromagnet seen in class?

2.  Real motors are a bit more complicated.  Find images and/or descriptions of real motors, and comment on how they compare to yours.

HW

Sorry for the delay in posting this.

Find out about electromagnetism and how it works.

MAGNETISM HW

Some questions for research:

1.  What exactly is a compass?

2.  Where is "magnetic north" on Earth?  Magnetic south?

Yet more things to think about:

3.  Must there always be at least one north pole with every south pole (and vice versa)?  Can magnetic monopoles exist?  This is not something you'd necessarily know, so feel free to research.

4.  How are magnetic fields different from electric fields?

Test practice

1.  Consider three resistors:  2, 6 and 4 ohms, connected in series to a 36 volt battery.  Find the total resistance, current and all voltages.

2.  Repeat the above problem, if the resistors are in parallel with the 36 volt battery.

3.  Charge review.  Consider a chunk of charge:  -15 C.  

A.  What kind of particles are these?

B.  How many of these particles are there

C.  If a 10 C charge is brought nearby, 0.1 m away, what is the force between the two chunks of charge?

D.  Draw the electric field between the charges.

Test heads-up

Test on circuits (and earlier charge stuff) is next Friday, right before spring break.  Sorry for the final push - cant be helped.

The birthday problem revisited

So, I misspoke in class about the birthday problem.  The probability that gets so large so quickly is that there will be ANY two people in the group with the same birthday.  NOT that Atira (for example) will find someone in a group that shares her birthday.

That's a pretty big difference and I didn't mean to misrepresent it.  Giant OOPS!  Sorry!  Here's the gist of it, followed by the reference:

In probability theory, the birthday problem or birthday paradox^[1] concerns the probability that, in a set of n randomlychosen people, some pair of them will have the same birthday. By the pigeonhole principle, the probability reaches 100% when the number of people reaches 367 (since there are 366 possible birthdays, including February 29). However, 99.9% probability is reached with just 70 people, and 50% probability with 23 people



http://en.wikipedia.org/wiki/Birthday_problem

Ohm's law lab guidelines

Plan on submitting a draft by Tuesday's class.

Final lab will be due the class after the draft is due.

Lab Questions for "Ohm's Law" lab - please change the title.  If you use "shocking" as part of your title, I'll deduct a point for lazy punning.

So far, you've done 2 graphs: I vs. R, V vs. R. They will be included in the lab. Don't forget titles, units, etc.

Lab questions

1. Examine the 2 graphs. Do they make sense? Why? What's going on in them? Do they appear to obey any mathematical relationship/equation?   Is one graph stranger (or more unexpected) than the other?  Discuss.

2.  Calculate experimental resistances for each pair of V and I - use the equation R = V/I

3. You've just determined experimental resistances for each trial. Are they within 1% (the supposed tolerance) of the expected/theoretical values (the ones on the box)? Should they be? If not, why are they not so good?  Don't forget sources of error, in general.

4. What does it mean exactly if something follows Ohm's Law? Do all electrical devices follow this law? Are there substances that definitely are not "ohm-ic"?

5. Other than the batteries "dying," what might happen as the batteries are connected to the resistors? Would the V and I values change?  Discuss.

6. What is meant by "internal resistance" of the battery, and how does it affect this experiment (your results)?

7. Anything else you want to conclude or talk about.

8.  Hooray!

Lab guidelines (in general) - reposted

Lab Guidelines

The lab writeup should have each of the following items:

Title of Experiment - this is up to you
Your name
Lab partner(s)
Date(s) performed

Purpose - the purpose of the experiment, as it appears to you

Data - in table form, with units.  Give table a title as well.

Graph(s), where relevant - for this harmonic lab, graphs are optional.  They may make your point(s) stronger.

Answers to lab questions - see lab handout

Sources of error and ways to eliminate/reduce error

General conclusion - Talk about what you learned in the experiment.  Analyze data.  Give thoughts and reasoning, where appropriate.  Talk about applications or places where this new knowledge applies.

Make sure it is neat.

LoggerPro download

https://parkscience.pbworks.com/w/page/351271/LoggerPro

password is extrapolate

lab HW

Create 2 graphs (using Logger Pro, or whatever program you like):

Current (I) vs. Resistance (R)
Voltage (V) vs. Resistance (R)

Look at the curves.  Do they make sense?  Start thinking about them.

Also, if you have time:  calculate the experimental resistance values for each trial.  Do this by:

R = V/I

The answers will be your 4th column in your data table.

Hooray!

HW

Please investigate the concept of resistance. Prepare to run a lab comparing voltage, current and resistance (next Monday).

HW

Read about the physics and chemistry of the simple battery.

hw

For next class:

Research L. Galvani's frog experiment.
Find out how this relates to the battery - it may be useful to read about A. Volta.

Enjoy the long weekend!

hw for Monday

1.  Recall the electric field concept.  Draw the field that you would see under these circumstances.  The first two were shown in class:

a.  a positive hunk of charge by itself

b.  a negative hunk of charge by itself

c.  2 hunks of charge close to each other; both are negative

d.  2 hunks of charge close to each other; both are positive

e.  a hunk of positive charge close to a hunk of negative charge

If you want to know quickly if your answers a close to correct, do a google image search for "electric field".  The first few images that show up will have these among them.

2.  Review for quiz:

a.  Calculate the force between 2 charges (5E-6 C, -15E-6 C), when separated by a distance of 0.004 m.

b.  If the distance between these 2 charges were changed to 5 time the original amount, how would the force be different?  Do this without a formal calculation.

c.  Consider the 5E-6 coulomb charge.  How many protons is this?  Recall that 1 proton has a charge of 1.6E-19 C.

d.  What does it mean to be a fundamental particle?  

hw for Thursday

1.  Revisit Coulomb's law - make sure you understand it, and what "inverse square" law means.

2.  Work this problem: Two charges (4 x 10^-9 C, and 9 x 10^-9 C) are separated by a distance of 0.01 m.

a.  What is the force between these charges?
b.  Is this force attractive or repulsive?  Why?
c.  Without exactly calculating it, what would happen to the force if the distance between the charges was tripled?
d.  What would happen to the force if the distance was cut to half the original value?  Try to determine this without calculating.

3.  How many electrons are in -1 C of charge?  How about in -6 C?

4.  Look up electric fields and try to understand:  what they are, and how to draw them.

Let's have a quiz next Wednesday on everything up to and including this Thursday's class.

HW for Wednesday

1.  Think about this question.  If the charge of a proton is 1.6 x 10^-19 coulombs, how many protons would be required to make 1 coulomb of charge?  It's a big number, by the way.  (The answer is the same for -1 C and the number of electrons required.)

2.  Look up Coulomb's law and write a related equation.  Identify what the variables mean.

3.  What are the main differences between an electron and a proton?

4.  Look at the Standard Model chart, posted earlier - see if anything jumps out at you.

FYI

HW on charge

Questions to ponder.  Please write out your answers in your notebook.

1.  Write some type of definition of "charge".  It's ok to look something up, but try to formulate your own definition of charge first.

2.  Find the actual distance between an electron and a proton in a typical atom - it's ok to use hydrogen as your example.

3.  Are protons, electrons and neutrons are "fundamental"?  That is, can any of them be broken into something smaller?  If so, talk about it.

4.  Review the rotating 2x4 demonstration from today - why does the board rotate?

5.  What is the official standard unit for charge?  How is it defined?  This may be tough to put into words.  Try your best.

6.  How do the mass of proton, neutron and electron compare?  Look up their values, if that's helpful.

*7.  (If time allows.)  Look up the Heisenberg Uncertainty Principle.  How does it relate to the measuring of particles?  If you don't follow it, keep in mind that it was new and challenging enough to win Heisenberg a Nobel Prize (so don't feel bad).

for fun.

http://sploid.gizmodo.com/the-sum-of-1-2-3-4-5-until-infinity-is-so-1503066071/@caseychan

(Thanks, Kyle!)

http://geekdad.com/2014/01/1000000000000000000000-years/

pre-quiz homework

quiz practice

1.  Make sure you finish the diffraction (informal) lab for wavelength of laser light.  There may be a related question on the quiz.

2.  Speaking of which..... A 400 nm laser hits a diffraction grating (750 slits/mm).  The wall/screen is 2-m away from the grating.  Find:

a.  diffraction angle for a first order image
b.  distance between the central/primary (n = 0) image and the first order image

3.  Consider a 40-cm focal length mirror.  An object (5 cm in height) is 100 cm in front of it.  Find:

a.  image location
b.  magnification of image
c.  image characteristics
d.  Give the do(s) that would yield only virtual images.
e.  Give the do(s) that would yield NO images.

During the quiz day, I'll also ask you to write a short self-reflection/"what I've found interesting" paragraph.  This will be included in your semester grade report.  Feel free to start writing this now.  It needs to be emailed to me before the end of the week.  Thanks!

HW

Have a look at these pages:

Two things for homework.

Thing 1 - try these problems:

You have a lens with focal length 24 cm.  A plastic toy is located 50 cm from it.  Find the following:

a.  type of lens.  Recall that the sign of the focal length means something.

b.  location of image (di).  Use the lens equation.

c.  type of image (real or virtual).  Hint - a positive di means real; negative di means virtual.

d.  magnification of image (mag = - di/do)

e.  whether image is right-side up (positive magnification) or upside-down (negative magnification)

f.  whether image is larger (absolute value of magnification is bigger than 1) or smaller than object

g.  location(s) where you could move object and get NO image at all

h.  location(s) where you could move object and get only virtual image(s)

Thing 2 - Have a look at these pages.  Come to class with a definition of "diffraction" of light.

http://www.physicsclassroom.com/Class/light/u12l1a.cfm

http://www.physicsclassroom.com/Class/light/u12l1b.cfm