Name: ______________________________
March 23, 2012
College Preparatory Physics
Mr. Richter
Chapter 14 Study Guide: Light
There will be very little calculation problems on Tuesday’s test. The vast majority of
what you will be assessed upon will be conceptual questions. Practice the problem
solving below, then address the concepts in the boxes provided. If you can verbally
explain the concepts in one sentence and use an example, you should be well
prepared for Tuesday’s test.
Formula: c  f; c  1.0  10 8 m /s
Problem Solving:
 1. What is the wavelength for an FM radio signal if the number on the dial reads
99.5 MHz?
300,000,000 m/s = (99,500,000 Hz)*λ
λ=3.02m
2. A concave shaving mirror has a focal point 25 cm away from its center.
Describe the image of a pencil placed at the following distances from the
mirror:
a. 15cm
Magnified, virtual, non-inverted
b. 25 cm
Equal in size to the pencil, real, projected onto pencil.
c. 40 cm
Shrunken, real, inverted.
3. If you are standing 2 meters from flat mirror, how far away does your image
appear from you?
4 meters. Two to the mirror, and then two more to the image.
Concepts: For each question or concept, write 1-2 complete sentences with an
example. If you can do this, you will be fine for the test.
1. A blue light, a white light, and a red light are shined one-at-a-time on a blue
button. What will the button look like under each light and why?
The button will look blue under blue light and white light, because the button will
reflect blue light and absorb all other colors. Under red light, the button will appear to
be black, because the red light contains no blue light, so the button has no light to
reflect.
2. How is mixing red light and green light different from mixing red paint and
green paint?
Colors of light are additive, so when I combine red and green light, I am actually
adding the frequencies of red and green light together, creating a new frequency of
yellow. Red and green paint absorb all colors of light except red and green
respectively, so when they combine, they want to absorb all colors of light including
red and green, making a brown or black.
3. What is the difference between a virtual image in a mirror and a real image?
A real image can be projected onto another surfaces, whereas a virtual image only
exists in a space “behind” the mirror.
4. Which types of mirrors can form real images and why?
Concave spherical and parabolic mirrors can form real images, because the rays of
reflected light from all angles intersect at points in front of the mirrors themselves.
These images can be projected onto other surfaces.
5. Describe the four types of mirrors we have discussed in class, describe the
images formed by them, and give examples of where they are used.




Flat. Image is left-right reversed, same size. Ballet studio.
Concave spherical. See image descriptions below. Shaving mirror.
Convex spherical. Image is shrunken, distorted, and left-right reversed.
Passenger side mirror.
Parabolic. Images are the same as convex spherical, but more sharply
concentrated; less blurry. Satellite dish.
6. What are the three types of images that can be formed in a concave spherical
mirror? Under what conditions are those images formed?
If the object is closer to the mirror than the focal point, the image is magnified, virtual,
and non-inverted. If the object is further from the mirror than the focal point, the
image is shrunken, real, and inverted. If the object is at exactly the focal length of the
mirror, the image is real, projected onto the object, and is the same size as the object.
7. Explain how polarized sunglasses work to reduce glare.
Most of the light reflecting off a surface is polarized in one direction (say, horizontal).
If sunglasses are polarized vertically, they will block out all of the glaring, horizontally
polarized light and allow the viewer to absorb/see all other light with different
polarizations.
8. What is the difference between specular and diffuse light?
Specular reflection is light reflecting off a smooth surface, in which all of the reflected
rays of light travel in the same direction. Diffuse reflection is light reflecting off a
rough or uneven surface, and the rays of light do not travel in predictable directions.
This is the difference between light reflecting off of a calm, still lake, and light
reflecting off a lake with many ripples and disturbances.
9. What is the relationship between how bright an object appears to be and the
distance from the object?
The brightness decreases by a factor of the square of the distance from the source. For
example, if the distance is tripled, the brightness is decreased by a factor of 9 (32).
10. What is the major difference in an image from a parabolic mirror as opposed
to a concave spherical mirror?
Parabolic mirrors concentrate all the rays of light into one focal point exactly. A
spherical mirror reflects rays of light to points near the focal point, but the reflection is
more distorted the farther away from the center of the mirror. This is called spherical
aberration.