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AP: Magnetic Induction Quiz Problem KEY

B-induction problem KEY


Optics quiz KEY–2nd attempt



PreAP Optics review worksheet answers

1. Probably not.  If any image is visible it will be diffuse.  A surface that feels rough is too irregular to create a coherent reflection of light waves–the lumps and bumps are simply too large.

2.  sketch

3. 63 degrees

4. This can be done with a diagram and the premise of alternate interior angles since the opposing sides of the square block are themselves parallel.

5. You have two interfaces to consider.  Air –>Glass, then Glass–>Water.  The angle of refraction for the first interface becomes the angle of incidence for the second interface.
Theta-refracted-1 = 14.3 degrees = Theta-incident-2
Glass–>Water, theta refracted 2 = 16.4, which makes sense that the final bend is AWAY from normal since water has a slightly smaller index of refraction compared to glass.

6. Your eyes cannot comprehend the bend (refraction) of light cause by the change in medium from water to air, so to your eyes, sunlight reflecting off the fish comes straight out of the water towards you.  However, you know that since water is more optically dense than air, the light reflecting off the fish actually bends AWAY from normal as it leaves the water.  This fact means that the only possible path the light takes is the one closest to the dock.  This also means that you see the fish slightly higher and slightly farther away than it actually is (trace that ray back into the water).

7. Total internal reflection.  Theta-critical for water–>glass is 61 degrees

8.  Do Snell’s law, solve for n2.  n2 = 2.09 (this may not be accurate to real life, I think I made this problem up)

9. Assuming REAL images, image distance is positive, focal length is 12cm, assuming VIRTUAL image (negative i), f=60cm

10. Let’s go with a real image since the question does not specify.  M = hi/ho = -i/o.  If the image is real the image distance is positive so M = -30/20 = -1.5.  Remember the negative means inverted, which makes sense since real images are always upside down.
b.  3.3 m
c. Since the problem in #9 does not specify what type of lens it is, nor whether the image is real or virtual, no, there is no way to tell.  However, as indicated in part A, the negative magnification is correct on the assumption of a real image.  If we went with the virtual image case then the magnification would turn out postive, which makes sense since virtual images are always upright/erect.

***Disclaimer: I did these quickly on my iphone calc while riding in a car. I hope all the math is correct, but if you just cannot figure out why I’m getting my answers it is possible that my fat fingers hit cos instead of sin once or twice.



PreAP Optics HW #2, problem 27

A dentist uses a small mirror of radius X mm to locate a cavity in a patient’s tooth. If the mirror is concave and is held Y mm from the tooth, what is the magnification of the image?

Step 1: Find the focal distance.  When they say “radius” in this context they mean “radius of curvature” for the mirror, not the circular size of the mirror.  Radius of curvature is 2 times the focal distance, so f = 0.5*X

Step 2: We need to determine if this will be real or virtual before we do any math!  Draw a ray diagram.  Use your focal length from step 1 vs. the value of Y in your problem to determine where the object (the tooth) is relative to the focal point.  Since this is a concave mirror, if the object distance is greater than the focal distance then it is a real image.  If the object distance is smaller than the focal distance then this will be a virtual image.  Draw a ray diagram to prove it.  It will be good practice for your test.

Real images: all values positive

Virtual images: images distance (only) is negative

Step 3:  Apply the thin lens equation to find the image distance

Step 4: Apply the magnification equation M = -i/o to find the magnification.  This should be your answer.


PreAP HW Optics 1

There are two particularly difficult problems in the first optics assignment: #26 & 27

I will NOT assist you with number 27 except to give you a few pieces of information:

  • The ray that leaves the window is parallel to the incident ray
  • You are looking for the separation distance between the two rays, not the distance apart at the points of exit on the window itself.  If you draw a diagram with the window thickness oriented vertically, the distance you are looking for is sort of a diagonal line, it is NOT a vertical distace along the window.
  • You will need to create two right traingles INSIDE the window to find the distance you are looking for.
  • Note your units. Conversions may or may not be required.


Click on the diagram for a larger picture

You are looking for theta-exit as indicated.  You know the angle of incidence with the 1st face of the prism.  You can easily find angle A using Snell’s Law.

  1. Angle A and B share a special relationship.  Figure it out and you can find angle B.  Hint: It has something to do with the dotted line.  What do we call that again?
  2. Once you know angle B, notice that you have a triangle at the top of the prism.  It has interior angles B, 60 and C.  Since you know B and 60, you can find C.
  3. Once you know angle C you can easily compute angle D.  They share the same relationship that A has with B.
  4. Angle D is the incident angle for the second refraction.  Make sure you use the right indexes of refraction for the prism and air on the outside and Snell’s Law takes car of the rest.


PreAP: Post Spring Break Class Schedule

Monday (3/17): Introduce Projects.  Projects due on March 28.

Tuesday- Thursday: Project work in Library.  Go directly to Library.

Friday: The EM Spectrum.  Read Young’s Experiment notes over weekend. notes–youngs double slit

Monday (3/24): Quiz(es): Over Color & EM Spectrum.  Class topic: Young’s Experiment

Tuesday: Wrap up Young’s Experiment

Wed:Quiz: Young’s Experiment and diffraction

Thursday: Exam: The Nature of Light

Friday: Projects Due


Update to Calendar, week before spring break

PreAP Physics:

Video which continues our discussion of the “Dual Nature of Light” from Friday

Quiz: Nature of  & Speed of Light (read notes on website),
Illumination, polarization,

Polarization (cont’d), Color (demos)
The E/M Spectrum
Read Young’s Experiment notes for Friday
HW moved to end of school day Friday.

Quiz: Color
Young’s Experiment demo & details
Online HW due by end of school day.

Lab, Ohm’s Law

Quiz: “Simple Circuits”
Kirchoff’s Rules
Multi-loop, multi-battery circuits

Quiz: Multi-loop circuits
RC circuits introduced
HW moved to end of school day Friday.

Continue RC circuits, quiz moved to Tuesday after spring break.
HW Due at end of school day.

Happy spring break!


PreAP Waves & Sound HW #2 problem 15

A rock band plays at a(n) 85 dB sound level. (a) How many times greater is the sound pressure from another rock band playing at 100 dB? etc.

So, this is your “challenge problem” on this assignment.  As such I will not help you complete it, but I will help you get started.

If you don’t know what to do with the logarithmic equation, this may help a bit:

Let x = your variable and y = a number.

Log 10 x = y

Raise both sides as exponents of ten (the log base)

10 Log 10 x = 10 y

Raising the log expression to be the exponent of the log base effectively undoes the logarithmic operator, leaving behind the result

x = 10 y

You can solve the equation with any log base in exactly the same way.

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