Thursday, December 12, 2013

Getting started on lab

P1.  Review quiz (in mailbox) - key is posted on blog.

Lab stuff -

2.  Make sure you have a neat data table (remember units).

3.  Make sure you have calculated the experimental focal lengths (f) using the lens equation:

1/f = 1/di + 1/do

In the lab, you'll need to show a sample calculation (from your data).  See sample below.

4. Start thinking about sources of error - also, why is your calculated f different from the f obtained by focusing an image of a tree outside onto your screen?

5.  Look at your data.  Is there a point when the images go from smaller to bigger?  Real to virtual?  Are those points related to the focal length?

On Monday, bring this work as well as your notebook for me to check.

Thanks, and have a great weekend!



Quiz key

Friday, December 6, 2013

Prepping for Tuesday's lab

The purpose of our new lab on lenses and mirrors is to determine how the distance between object (candle) and lens, the object distance (do), is related to the distance between image and lens (di).

We will also determine the effect on the image's:  relative size (bigger, smaller, same size), orientation (up or down), and whether or not the image is real (can be projected onto a card/screen) or virtual (can NOT be projected onto a card/screen - but CAN be seen through the lens).

In other words:

How does do affect di, and the various characteristic of the image formed?

In this lab, you will record the following:

do (candle distance to lens or mirror) - this will be a measured distance
di (image distance to lens or mirror) - this will be a measured distance
up or down (is image right-side up or upside-down)
relative size (bigger, smaller, or same size as original flame)
real or virtual (whether or not image is on a screen or only visible through the lens)

You will perform this experiment for each of the following optics:

convex lens
concave lens
concave mirror
convex mirror

See the picture below:




Monday, December 2, 2013

HW

Homework for Wednesday - note that we are having a quiz on Friday.  Start studying now.

1.  Think about today's informal lab again.  You have an initial angle of 60 degrees (coming from air) and are refracted to 40 degrees inside a block of some mystery material.

a.  Draw this scenario.
b.  What is the index of refraction of the mystery stuff?
c.  Now that you know the index of this stuff, what is the speed of the light inside of it?
d.  What happens to the wavelength of light inside the stuff?

2.  Research the following topic, bringing back a short definition (that you understand):  What is the focal length of a lens?

3.  If you have time, read about fiber optics and how they relate to total internal reflection (and/or the critical angle).

4.  Review the optional homework (if you did it).  There will likely be a trig question on the quiz - something that has to do with a right triangle.

Monday, November 25, 2013

Optional HW for practice

Happy Thanksgiving!  Here's some physics for you to play with.  I will post the answers as a "comment" below, a little later.

1.  Consider this right triangle:  36, 77, 85

Draw this and find the angles in the triangle.

2.  Repeat for this triangle:  20, 21, 29

3.  What is the index of refraction for a material that slows light to 3/4 of its vacuum speed?

4.  Lights goes from air (n = 1) to a new material, being refracted from 75 degrees to 38 degrees.  Find the index of refraction of the new material.  (Angles are measured, as always, with respect to the normal line.)


Thursday, November 21, 2013

Homework - do as many as you can.

Here's some fun stuff to do over the weekend!

Trig stuff

1.  Consider a right triangle with sides:  5, 12, 13.

a.  Draw this
b.  Find the ratios for sin, cos and tan.  (Remember SOH CAH TOA.)
c.  Use the inverse functions (sin^-1) to find the angles.  You can also use the fact that the angles of a triangle add to 180 degrees, if that is helpful.

2.  Repeat the above exercise, if helpful, for another pythagorean of your choice.  See:

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

3.  Use your calculator to find:

a.  sin 30
b.  sin 0
c.  sin 90
d.  cos 0
e.  cos 90
f.  tan 45

Index of refraction

4.  Describe WHY refraction occurs.

5.  Review the definition of index of refraction.  Review, if helpful:

http://www.physics.uoguelph.ca/applets/Intro_physics/refraction/LightRefract.html

6.  Find the index of refraction of a substance in which the speed of light goes to HALF its value in a vacuum.  (Recall that c = 3 x 10^8 m/s.)

7.  The speed of light in a piece of plastic is 2.5 x 10^8 m/s.  What is its index of refraction?

Snell's Law

8.  Light passes from air (n = 1) to a block of new stuff, with the angle going from 60 degrees outside the block to 35 degrees inside the block.  The angles are measured with respect to a normal (perpendicular) line.

a.  Draw this.
b.  Calculate the index of refraction of the stuff.

Tuesday, November 19, 2013

Refraction homework

1.  Find a definition for "index of refraction." 

2.  Look up "Snell's Law".  It will likely have a "sin" of an angle in the equation.  See how much of this makes sense to you.  If you find an equation for it, make sure you have a picture that goes with it.

3.  Play around with this applet (from class):


4.  You have a sense of what happens when light goes from air (or a vacuum) into water (or plastic, glass, etc.).  What do you suppose happens when light goes from water into air?  Draw this out and explain.


Friday, November 15, 2013

HW for Tuesday

1.  Come up with a definition of (wave) reflection.

2.  Come up with a definition of (wave) refraction.

You can look online, but make sure that the topics are clear to you (and written in your own voice).  USE PICTURES, too.

Monday, November 11, 2013

hw

http://www.darvill.clara.net/emag/index.htm

Visit this page and read about the various types of electromagnetic radiation.

And if you have more time:

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



Also, bring test-related questions on Wednesday.

Friday, November 1, 2013

For next class

Read up on the Doppler effect.  Play with the applets, if you wish:

http://falstad.com/ripple/




And - some practice problems to try before the test (which will be the week after the upcoming short week.)

1.  B is 466.1 Hz, approximately.  Find the following frequencies:

a.  the next 2 octaves
b.  the 2 octaves below
c.  the C above (one semi-tone)
d.  the D above (3 semi-tones)

2.  Be able to conceptually/physically explain the Doppler effect.

3.  Consider a tube open on both ends, 0.4-m in length.  Find the relevant stuff for the first 3 harmonics:  wavelengths, frequencies, shapes.  How would answers change for the same tube capped on one end?

4.  Review the recent quiz and make sure you understand the string/harmonics stuff.

5.  Review the speed of sound lab in your notes.

Remember:  Proper prior preparation prevents poor performance.



Wednesday, October 30, 2013

HW to turn in Friday

1.  Middle C is 261.6 Hz.  Find the following:

a.  the next two C's above this note (one and two octaves above)
b.  the C one octave below middle C
c.  C#, which is one semi-tone (or piano key or guitar fret) above C
d.  D, which is two semi-tones above C
e.  The wavelength of middle C, if the speed of sound is 340 m/s

2.  Consider an organ pipe 0.7-m long.  Find the following:

a.  the wavelengths of the first 3 harmonics
b.  the frequencies of the first 3 harmonics (speed of sound is 340 m/s)
c.  the wave shapes associated with the first 3 harmonics - draw them
d.  What would happen if you cap this pipe on one end?



Monday, October 28, 2013

HW for Wednesday

We did not chat about the well (or equal) tempered scale on Monday.  Be sure to bring information about it.

HW related to today's discussion of organ pipes:

Find out the difference between longitudinal and transverse waves.  Which type of sound (in air)?

Play around with the applet from class:

http://www.physics.smu.edu/~olness/www/05fall1320/applet/pipe-waves.html

Note that the mathematics of organ pipes is exactly the same as that of guitar strings.  The only difference is that there are anti-nodes on each end of an organ pipe.  (On a guitar string, there are nodes on each end, since the string is fixed to the guitar neck at top and bottom).

With that in mind.  Find the wavelength and frequencies of the first 3 harmonics in an organ pipe that is 50 cm long.  Assume that the speed of sound is 340 m/s.

Friday, October 25, 2013

HW

Find out what you can about the equal-tempered scale (in music).  It is sometimes called well-tempered.

Tuesday, October 22, 2013

just cool.

http://www.slate.com/blogs/bad_astronomy/2013/10/21/three_illusions_that_will_destroy_your_brain.html


Monday, October 21, 2013

practice pre-quiz

To prep for Thursday's quiz:

1.  Consider a string, 0.2-m long.  The fundamental frequency of this string is 40 Hz.
a.  Draw the first 3 harmonics.
b.  Calculate the wavelengths, frequencies and speeds of the first 3 harmonics.

2.  What is the frequency of an 89.7 MHz radio wave?

3.  Pendulum problem.  Find the period of a 10-m long pendulum.

4.  What is the difference between mechanical and electromagnetic waves.  Give examples.

5.  Consider concert A, vibrating at 440 Hz.  What are the frequencies of:  the next 2 octaves above this note, and the octave below it.


Wednesday, October 9, 2013

Lab Guidelines

Your first formal lab will be due in 3 classes.  If you want me to have a quick look at it, show it to me within 2 classes.

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.

It's not that different from the first lab - just a couple of extra things.  Make sure it is neat.

Tuesday, October 8, 2013

Waves - part 1

There are 2 primary categories of waves:

Mechanical – these require a medium (e.g., sound, guitar strings, water, etc.)

Electromagnetic – these do NOT require a medium and, in fact, travel fastest where is there is nothing in the way (a vacuum). All e/m waves travel at the same speed in a vacuum (c, the speed of light)

General breakdown of e/m waves from low frequency (and long wavelength) to high frequency (and short wavelength):

Radio
Microwave
IR (infrared)
Visible (ROYGBV)
UV (ultraviolet)
X-rays
Gamma rays

In detail, particularly the last image:






Waves have several characteristics associated with them, most notably: wavelength, frequency, speed. These variables are related by the expression:


v = f l


speed = frequency x wavelength

(Note that the 'l' above should be the Greek letter 'lambda'.)


For e/m waves, the speed is the speed of light, so the expression becomes:

c = f l

Again, the 'l' should be Greek letter 'lambda'.

Note that for a given medium (constant speed), as the frequency increases, the wavelength decreases.


Note the units:


Frequency is in hertz (Hz), also known as a cycle per second.

Wavelength is in meters or some unit of length.

Speed is typically in meters/second (m/s) or cm/s.


Sound waves


In music, the concept of “octave” is defined as doubling the frequency. For example, a concert A is defined as 440 Hz. The next A on the piano would have a frequency of 880 Hz. The A after that? 1760 Hz. The A below concert A? 220 Hz. Finding the other notes that exist is trickier and we’ll get to that later.

Interference

Waves can “interfere” with each other – run into each other. This is true for both mechanical and e/m waves, but it is easiest to visualize with mechanical waves. When this happens, they instantaneously “add”, producing a new wave. This new wave may be bigger, smaller or simply the mathematical sum of the 2 (or more) waves. For example, 2 identical sine waves add to produce a new sine wave that is twice as tall as one alone (as in, 1 sin x + 2 sin x = 3 sin x). Most cases are more complicated (1 sin x + 3 cos x = .....).

In music, waves can add nicely to produce chords, as long as the frequencies are in particular ratios. For example, a major chord is produced when a note is played simultaneously with 2 other notes of ratios 5/4 and 3/2. (In a C chord, that requires the C, E and G to be played simultaneously.) Of course, there are many types of chords (major, minor, 7ths, 6ths,…..) but all have similar rules. In general, musicians don’t remember the ratios, but remember that a major chord is made from the 1 (DO), the 3 (MI) and the 5 (SO). It gets complicated pretty quickly.

We looked at specific cases of waves interfering with each other – the case of “standing waves” or “harmonics.” Here we see that certain frequencies produce larger amplitudes than other frequencies. There is a lowest possible frequency (the resonant frequency) that gives a “half wave” or “single hump”. Every other harmonic has a frequency that is an integer multiple of the resonant frequency. So, if the lowest frequency is 25 Hz, the next harmonic will be found at 50 Hz – note that that is 1 octave higher than 25 Hz. Guitar players find this by hitting the 12th fret on the neck of the guitar. The next harmonics in this series are at 75 Hz, 100 Hz and so on.  Or if you prefer, fn = n f1.


Friday, September 27, 2013

Homework to be turned in on Tuesday

Please write your work and answers on a sheet of paper that you can turn in.  Thanks!

1.  What factor(s) does the period of a pendulum depend on?

2.  Draw the approximate shape of a graph of period vs. length for a pendulum.

3.  The equation we've seen in class is the period of a "simple" pendulum.  What do you suppose makes our pendulums "simple"?

4.  Calculate the period of a 5-m long pendulum.

5.  If the pendulum in #4 above were taken to Mars, where the gravity is roughly 40% Earth's gravity, how would the period be different (if at all)? 

6.  Consider a 261.6 Hz sound wave (this is approximately the note 'middle C').  Assume that the speed of sound at sea level is 340 m/s.
a.  What is the wavelength of this sound wave? 
b.  Do you think the wavelength of this sound wave would be different on the Moon?

Wednesday, September 25, 2013

Homework

Hi - some homework for you.

1.  Be able to define these terms:  amplitude, crest, trough, wavelength, frequency, wave speed.

2.  The equation for speed of a wave is:

speed = frequency times wavelength

Write this symbolically.

3.  Solve.  What is the wavelength of a 440 Hz sound wave (speed is 340 m/s)?

Play around with this applet:

http://phet.colorado.edu/sims/wave-on-a-string/wave-on-a-string_en.html


Thursday, September 19, 2013

Lab homework

You will be submitting an informal lab report for your pendulum work - this will be submitted individually, 2 classes from now.

In it, you should have:

Graph of Time vs. Length, with a curve fit
Graph of Time vs. Square root of length, with a linear regression line (and slope)
Graph of Time vs. Angle (if you took that data)

On the second graph, the regression line equation IS the equation for your experiment.  Write down that equation and talk (in a conclusion) about how well it compares to the ACTUAL equation for the period/time of a pendulum:



Note that g is a local gravitation constant, equal to around 9.8 m/s/s (that's how quickly gravity makes object accelerate, in m/s per second).  If you work out the constants (2 pi divided by the square root of 9.8), you get a coefficient of around 2.0.  So a local version of the equation becomes:

T = 2 sqrt (L)

In your conclusion (which will be 2-3 paragraphs at minimum), discuss:

- the extent to which your equation resembles the one above
- sources of error in your experiment
- ways to improve your experiment
- anything else you wish to talk about

Finally, answer these 2 questions in your short report:

1.  Knowing the formula above, what is the period of a 3-m long pendulum?  Calculate this.
2.  (This requires some algebra.)  How long should a pendulum be for it to have a 1 second period?

Again, here's what is due in 2 classes:

Graphs
Conclusion
Question to answer

And it should be neat - typed is nice, but handwritten is ok, too.

Tuesday, September 17, 2013

Also

After you download Logger Pro, play around with it. Bring a graph of your data next class.

Logger Pro download

Logger Pro 3.8.6.1 with sample movies (Windows)
Password: extrapolate

Logger Pro 3.8.6.1 with sample movies (Mac OS X)
Password: extrapolate

Friday, September 13, 2013

The relationship?

You now have some pendulum data.  Examine it and decide which (if any) variables are relevant.  You may find that some are more relevant than others - perhaps some have differences that can be explained by timing issues.

How will you get a relationship out of this data?  What is the best way to formulate a "rule" for your data?  How can you turn your data into a predictive tool?  Would a graph be helpful?

Think about these questions and make a first attempt at formulating a rule.  Bring this to class.  If you are feeling more ambitious, consider how you might make a mathematical tool (equation?) out of your data.

Tuesday, September 10, 2013

Lab

Our goal will be to determine a relationship that describes the motion of a pendulum - specifically, how the time (for a complete swing - the period) depends on particular variables.  Ideally, we will have a relationship that has predictive ability - something that we can use to predict the period of any theoretical pendulum.

Think about what variables you would test to find out whether or not a pendulum's period is altered (and how)?

Think also about how many trials you may need to take, the range of values (for example, if we're talking about length - how short to how long?) and how you will represent your data (chart, graph, both?).

Monday, September 9, 2013

HW for after Monday's class


http://www.youtube.com/watch?v=NwyeK36Gh-s

Watch as much as you can stand - comment on what makes it believable or NOT believable.

Also, our first lab begins next class.  Think about the following question:

What is a relationship that describes the motion (time) of a pendulum swing?  How can we determine this?  How can we know if our model/relationship/equation is "correct"?

We will spend the next 2-3 classes treating this problem.


https://www.youtube.com/watch?v=2MFAvH8m8aI&feature=player_embedded
If you ever have an hour to kill - the best documentary on pseudoscience ever.

Wednesday, September 4, 2013

Pseudoscience!



What Is Pseudoscience?
Distinguishing between science and pseudoscience is problematic

By Michael Shermer

Climate deniers are accused of practicing pseudoscience, as are intelligent design creationists, astrologers, UFOlogists, parapsychologists, practitioners of alternative medicine, and often anyone who strays far from the scientific mainstream. The boundary problem between science and pseudoscience, in fact, is notoriously fraught with definitional disagreements because the categories are too broad and fuzzy on the edges, and the term “pseudoscience” is subject to adjectival abuse against any claim one happens to dislike for any reason. In his 2010 book Nonsense on Stilts (University of Chicago Press), philosopher of science Massimo Pigliucci concedes that there is “no litmus test,” because “the boundaries separating science, nonscience, and pseudoscience are much fuzzier and more permeable than Popper (or, for that matter, most scientists) would have us believe.”

It was Karl Popper who first identified what he called “the demarcation problem” of finding a criterion to distinguish between empirical science, such as the successful 1919 test of Einstein’s general theory of relativity, and pseudoscience, such as Freud’s theories, whose adherents sought only confirming evidence while ignoring disconfirming cases. Einstein’s theory might have been falsified had solar-eclipse data not shown the requisite deflection of starlight bent by the sun’s gravitational field. Freud’s theories, however, could never be disproved, because there was no testable hypothesis open to refutability. Thus, Popper famously declared “falsifiability” as the ultimate criterion of demarcation.

The problem is that many sciences are nonfalsifiable, such as string theory, the neuroscience surrounding consciousness, grand economic models and the extraterrestrial hypothesis. On the last, short of searching every planet around every star in every galaxy in the cosmos, can we ever say with certainty that E.T.s do not exist?

Princeton University historian of science Michael D. Gordin adds in his forthcoming book The Pseudoscience Wars (University of Chicago Press, 2012), “No one in the history of the world has ever self-identified as a pseudoscientist. There is no person who wakes up in the morning and thinks to himself, ‘I’ll just head into my pseudolaboratory and perform some pseudoexperiments to try to confirm my pseudotheories with pseudofacts.’” As Gordin documents with detailed examples, “individual scientists (as distinct from the monolithic ‘scientific community’) designate a doctrine a ‘pseudoscience’ only when they perceive themselves to be threatened—not necessarily by the new ideas themselves, but by what those ideas represent about the authority of science, science’s access to resources, or some other broader social trend. If one is not threatened, there is no need to lash out at the perceived pseudoscience; instead, one continues with one’s work and happily ignores the cranks.”

I call creationism “pseudoscience” not because its proponents are doing bad science—they are not doing science at all—but because they threaten science education in America, they breach the wall separating church and state, and they confuse the public about the nature of evolutionary theory and how science is conducted.

Here, perhaps, is a practical criterion for resolving the demarcation problem: the conduct of scientists as reflected in the pragmatic usefulness of an idea. That is, does the revolutionary new idea generate any interest on the part of working scientists for adoption in their research programs, produce any new lines of research, lead to any new discoveries, or influence any existing hypotheses, models, paradigms or world­views? If not, chances are it is pseudoscience.

We can demarcate science from pseudoscience less by what science is and more by what scientists do. Science is a set of methods aimed at testing hypotheses and building theories. If a community of scientists actively adopts a new idea and if that idea then spreads through the field and is incorporated into research that produces useful knowledge reflected in presentations, publications, and especially new lines of inquiry and research, chances are it is science.

>


http://www.randi.org/site/index.php/encyclopedia.html

http://www.quackwatch.com/01QuackeryRelatedTopics/pseudo.html

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

http://www.skepdic.com/pseudosc.html