Basic practice problems

1.  Light hits a piece of plastic at an angle of 50 degrees (with respect to normal).  If the angle of refraction inside is 25 degrees, what is the index of refraction of this plastic?  Also, what is the speed of light inside the plastic.

2.  What is the frequency of light with a wavelength of 100 nm in air (where it travels pretty much at the speed of light).  (That's 100 x 10^-9 m.)

3.  A lens has a focal length of -25 cm.  A candle is 10 cm in front of it.  Find the following:

a.  type of lens
b.  location of image (di)
c.  type of image
d.  magnification of image
e.  whether image is up or down
f.  whether image is real or virtual

Answers

1. 

1.8

1.66 x 10^8 m/s

2. 3 x 10^15 Hz

  

3.  concave, -7.1 cm = di (making it virtual), mag = -0.7  (which makes it smaller and upside-down)

Answers from recently turned in homework

1.

a.  convex (since the f is +)
b.  di = 60 cm.  Use the lens equation:  1/f = 1/di + 1/do
c.  magnification = -2.  Use the magnification equation:  mag = -di/do
d.  real (since di is +)
e.  upside-down, since the mag is -

2.  Dimmer, but still in focus.  Fewer light rays make it, but their path is not changed.

3.  Place the object AT the focal length.

4.  Either within f, or between f and 2f

Test review stuff.

Review for test:

Law of reflection – angle in = angle out

Refraction:  what is it, why it is

Index of refraction:  n = c/v

Snell’s law:  n1 sin(theta 1) = n2 sin(theta 2)

Other equations:  v = f l

Also worth remembering:  f does NOT change during refraction, but v and l will

Remember:  normal line (perpendicular to surface of optic)

Lenses and mirrors:  convex lens and concave mirror (both have +f); concave lens and convex mirror (-f)

Lens equation:  1/f = 1/di + 1/do

Magnification:  mag = -di/do.  Also, hi/ho (height of image divided by height of object)

-mag = upside-down image; +mag = right-side up image

Absolute value of mag tells you whether image is larger ([mag] > 1) or smaller than object ([mag] < 1)

Real (+di) vs. virtual image (-di)

Critical angle and total internal reflection:  sin(critical angle) = 1/n

Questions to expect:

1.       Snell’s law (like quiz)

2.       Lens (like HW)

3.       Miscellaneous:  you’ll choose 1 or 3 (or so) 

a.       Essay covering some abstract situation or problem

b.      More mathematical/theoretical question

c.       Demonstration?

d.      Practical (where you have to DO something or look at something)

e.      ??? (mwa ha ha ha ha…..maniacal laugh)

HW to TURN IN next class

Consider a lens with a focal length of +20 cm.  An object is 30 cm in front of it.

1.  Determine the following:

- type of lens
- location of image
- magnification of image
- type of image (real or virtual)
- whether image is right-side up or upside down

2.  What would be the effect of covering up half of the lens?

3.  Where could you place the object such that you get NO image?

4.  Where could you place the object such that you get a larger image?

HW - start thinking about the lab report

In the next 2 weeks, we'll have a quiz (or graded homework), test and formal lab.  The goal is to finish optics before winter break - there is enough time for this, without rushing.

Start thinking about (and writing) the formal lab.  Here is what you will need:



Basic structure of the lab report:

* Title

* Purpose of lab

* Data table - include all columns from your data, AND 1 more column:  calculated f

* Find the average f, and the percent error between your average and the expected f from the lens.

* Sample calculation for focal length (your data table will have ALL of the calculated values, but there is only need for one calculation to be shown)

* Graph(s) if you made any (they are not required, but a graph of di versus do might be instructive)

* Conclusion - probably the biggest, most detailed part of the lab.  Include the following:

 - Give sources of error.

 - Discuss how the image formation depends on object distance.  Note if there "transition points", etc.  This is the tough part of the conclusion (and the most mathematical).

- Discuss a technique for measuring the focal length of a concave lens (or convex mirror).

- Discuss the similarities between convex and concave mirrors and lenses.

 - Give a general conclusion.

HW

Think about the things from today.  Also:

Under what circumstances do images form at the focal point of a lens, if ever?

What is a virtual image?

Can a focal point/length be negative?  What would that mean?

HW

Hi there!

Have a look through your lab data to see if there is anything interesting.

Also, look up the term "focal point" (or "focal length").  How is this related?

Thanks!

>

Sorry for delay - this was posted to the wrong blog.