homework for Monday

A few things to do for Monday:

1.  Review the concept of critical angle and the related formula.

2.  Calculate the critical angle of diamond (n = 2.417).

3.  If a substance has a critical angle of 60 degrees, what is its index of refraction?

4.  What is the critical angle of air (n = 1.0028).  One of the classes did this already.

5.  (Review problem.)  A 475 nm light ray hits a tank of water (n = 1.33) at an angle of 30 degrees with respect to the normal line.  Find the angle of refraction, speed and wavelength inside the water.

6.  Prep for the next formal lab.  To do this, research the following:

a.  focal length.  What is the definition?

b.  Our next lab will investigate the question:  what is the relationship between the location of an object (how close it is to a lens) and whether or not an image forms, and the nature of the image that forms (bigger or smaller image, where it forms, etc.).  Decide on a type of protocol to investigate these questions.

More Snell's Law fun!

(Trig practice)

1.  Consider a triangle with sides 20,21, and 29.  Find all the angles in the triangle.

2.  Choose any other Pythagorean triple and find the angles in the triangle:

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

(Snell's Law)

3.  A 450 nm blue light ray hits a piece of Plexiglas (n = 1.6) at an angle of 55 degrees with respect to a normal line.  Find the following:

a.  picture that represents what happens

b.  angle inside Plexiglas

c.  wavelength inside Plexiglas

d.  speed of light inside Plexiglas

4.  Light hits a piece of unknown material, such that the light refracts from 60 degrees to 35 degrees.  If the light enters from air, what is the index of refraction of the material?

5. Review all of the relevant formulas discussed so far to make sure you understand each one.  Include the new ones from class today.

Homework for after test

Try these, even if your trig is poor or nonexistent.

Snell's Law images from today's class

Sample Doppler problem

Consider a siren that has a 1200 Hz tone attached to an ambulance traveling at 35 m/s.  Find the following frequencies:

a.  what you hear when the ambulance approaches you
b.  what you hear when the ambulance travels away from
c.  what you hear if you were in a car traveling toward the ambulance, you moving at 15 m/s

Interesting and cool - not physics related, but math and politics related

http://www-personal.umich.edu/~mejn/election/2012/

Test Friday - practice problems for recent stuff

1.  Doppler effect.  A police car approaches you.  According to the siren's manufacturer, the frequency is 1000 Hz.  How would the frequency change if you were located:
a.  behind the car as it passed you
b.  in front of the car as it approached you
c.  in the car
d.  running toward the car, if it were at rest
e.  running away from the car, if it were at rest

2.  What does the red shift of distant galaxies suggest?

3.  What do red shift and blue shift mean?

4.  Explain the Doppler effect.

5.  Consider a 320 Hz note (E, approximately).  What is the frequency of:
a.  the next E (one octave above)
b.  the E one octave below
c.  an E that is 3 octaves above
d.  an F, one semi-tone above
e.  a G, three semi-tones above

Don't forget to review strings, organ pipes and all other wave phenomena.

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

Reflection - light "bouncing" off a reflective surface. This obeys a simple law, the law of reflection!

The incident (incoming) angle equals the reflected angle. Angles are generally measured with respect to a "normal" line (line perpendicular to the surface).

Note that this works for curved mirrors as well, though we must think of a the surface as a series of flat surfaces - in this way, we can see that the light can reflect in a different direction, depending on where it hits the surface of the curved mirror. More to come here.

Refraction is much different. In refraction, light enters a NEW medium. In the new medium, the speed changes. We define the extent to which this new medium changes the speed by a simple ratio, the index of refraction:

n = c/v

In this equation, n is the index of refraction (a number always 1 or greater), c is the speed of light (in a vacuum) and v is the speed of light in the new medium.

The index of refraction for some familiar substances:

vacuum, defined as 1

air, approximately 1

water, 1.33

glass, 1.5

polycarbonate ("high index" lenses), 1.67

diamond, 2.2

The index of refraction is a way of expressing how optically dense a medium is. The actual index of refraction (other than in a vacuum) depends on the incoming wavelength. Different wavelengths have slightly different speeds in (non-vacuum) mediums. For example, red slows down by a certain amount, but violet slows down by a slightly lower amount - meaning that red light goes through a material (glass, for example) a bit faster than violet light. Red light exits first.

In addition, different wavelengths of light are "bent" by slightly different amounts. This is trickier to see. We will explore it soon.

FYI:

http://www.physicsclassroom.com/class/refln/

>

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

 

Refraction, in gross gory detail

Consider a wave hitting a new medium - one in which is travels more slowly. This would be like light going from air into water. The light has a certain frequency (which is unchangeable, since its set by whatever atomic process causes it to be emitted). The wavelength has a certain amount set by the equation, c = f l, where l is the wavelength (Greek symbol, lambda).

When the wave enters the new medium it is slowed - the speed becomes lower, but the frequency is fixed. Therefore, the wavelength becomes smaller (in a more dense medium).

Note also that the wave becomes "bent." Look at the image above: in order for the wave front to stay together, part of the wave front is slowed before the remaining part of it hits the surface. This necessarily results in a bend.

The general rule - if a wave is going from a lower density medium to one of higher density, the wave is refracted TOWARD the normal (perpendicular to surface) line. See picture above.

HW and test

Test in 3 classes - next Friday.

HW - each student should attempt to summarize what was seen in the lab.  Use the falstad website to help "see" the phenomena, especially the difficult stuff.  Write down some bullet-points or comments about each of the parts of the lab.

This semi-formal group lab will be submitted in 2 classes - you'll have some time next class to pull together your data/conclusions/thoughts.

Hooray!

for Thursday

pre-lab HW

You have a temporary definition of the relevant wave terms from today's class.  Now look up formal definitions.

physicsclassroom.com may be useful.

Be prepared for some wave fun on Thursday.  Check out the falstad website to get a sense of what you should be seeing.

TEXT OF LAB IS BELOW.

text of the lab

Waves in a Ripple Tank - a semi-formal group lab

This is a bit of a strange lab.  First, it is a group lab - to be submitted by the group as one document.  You will witness waves in a different fashion than we have in the last several classes:  waves in water.  This is useful, as light (our next topic) behaves in a wave-like fashion under several circumstances.

For this lab, I would like you to begin by writing your own personal definitions of these words: 

Reflection, refraction, diffraction, interference

This will serve as a hypothesis.  How do you expect to see these phenomena represented in water?  This is a largely visual, non-quantitative lab.  Enjoy!

For your "data", draw everything you see and make relevant comments.  Each lab partner should do this in his or her lab notebook.

1.  Propagation of waves

Dip your finger in the water repeatedly, with constant frequency.  Comment.  We will call these "circular waves" in this lab.

Do the same with a ruler, commenting on what is seen.  We will call these "straight waves" in this lab.

2.  Reflection of waves

Send a wave (or waves) into a barrier.  Try this directly (no angle) at first, and then at some angle.  Discuss.

3.  Refraction of waves

This can be tricky to see, but there is something to be observed.  Set up an area with deep water and shallow water (using a piece of Plexiglas) to see what happens when water passes from one medium (deep) to another (shallow), or vice versa.  Try this directly and also at an angle (as above).

4.  Diffraction of waves

Send straight waves to a barrier as shown.

Now send them through a small opening, changing the size of the opening and noting the effects.  Comment on all scenarios.

5.  Interference of waves

This can also be tough to see.  Instead of creating one set of circular waves, create 2 sets (with 2 fingers) at the same frequency.  Watch what happens when they "interact" with each other.  Comment and draw if at all possible

Some of these things can be tough to see.  You may want to play with the applet below to see what would be seen under ideal circumstances for some of these cases.

http://falstad.com/ripple/

In your group's conclusion, comment on what you saw, especially as it relates to what you thought you'd see.  Give updated definitions of the words initially mentioned in the lab introduction.

pre-lab HW

You have a temporary definition of the relevant wave terms from today's class.  Now look up formal definitions.

physicsclassroom.com may be useful.

Be prepared for some wave fun on Thursday.  Check out the falstad website to get a sense of what you should be seeing.

FYI - just for fun

http://www.youtube.com/watch?v=AcS3NOQnsQM&feature=related
The classic "Elements song".

http://www.youtube.com/watch?v=U0kXkWXSXRA&feature=related
A new "classic"?  Or not.