Chapters 27 & 28
LIGHT & COLOR
Mr. Kuffer
Concepts of Physics
Opaque Materials & Shadows
When you walk under a street lamp, you see a shadow of yourself. How do shadows
form? What would you see if there were two lamps close to one another? This
activity will give you a chance to explore some interesting properties of shadows.
Materials: Two mini maglites, maglite holder, large white board screen, ruler, index card, about 5 cm
square (with the equipment to mount it), a few white board markers, colored pencils
1. Unscrew the top of a maglite, and mount the maglite in the middle hole of the maglite holder. Mount
the screen about 30 cm away from the tip of the maglite. With the room lights dimmed you should
note that the screen is fully illuminated by light coming from the tip of the maglite. (You can ignore
small variations in the screen illumination, perhaps due to small imperfections in the maglite tip.)
Why do you think the screen is fully illuminated rather
than just showing a single spot of light?
2. To the right is a side view diagram of the tip of the maglite and the screen. Draw how you think light
goes from the tip to the screen, to fully illuminate it.
3. Mount the small 5-cm square card so its center is at the same height as the maglite, and closer to the
screen than the maglite.
With the room lights turned off you should
observe a sharp shadow of the card on the screen.
Explain how the shadow is formed.
20 cm
15 cm
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4. Below, to the left, is a side view diagram of the maglite tip, card and screen. To the right is a picture of
what the shadow looks like on the screen. (It's what you would see if you were to look directly at the
screen from the front.) Draw how you think light leaves the source and goes past the card to the (side
view of the) screen, causing a shadow to appear there. By looking at your diagram in the side view, a
person should be able to infer which part(s) of the screen would be illuminated with light, and which
part(s) would be in shadow.
How does your diagram show that the size of the shadow on the screen is larger than the
size of the card? If it does not show that, try to modify your diagram so that it does.
5. Outline with the whiteboard markers on the screen at the top and bottom borders of the shadow.
Imagine moving the maglite downward, from hole #3 to hole #2. Predict how the shadow would
change, if at all.
6. Test your prediction. Remove the maglite and reinsert it through hole #2 (but do not move the maglite
holder). What happened to the shadow? (Outline the additional shadow on the screen, with a different
color marker, at the top and bottom borders of this new shadow.
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7. Imagine using two maglites, one in hole #2 and one in hole #3.
To the right is a picture of the front of the screen. The
dashed lines indicate the boundaries of the two shadows
that were formed when the maglite was first in hole #3, and
then in hole #2.
Predict what the screen will look like if both maglites
are turned on at the same time. Shade and label the
different regions according to what you predict will
happen.
What idea(s) about light are you using to guide your prediction?
8. Test your prediction. Shade and label the different regions according to what actually appears on the
screen.
Below is a side view diagram representing the two maglite tips, the card and the screen. Again shade in
the screen so it appears the way you observed it to be. Then draw lines showing how light travels from
each maglite tip to the screen. You may wish to color the lines from each source differently. A person
looking at your diagram should be able to infer the different shaded regions on the screen.
9. Imagine using four maglites in holes #1, 2, 3 and 4, as shown below to the left. Predict what you
would expect to see on the screen by shading in the picture of the screen below to the right. Explain
your thinking.
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10. Test your prediction. You will need to borrow two additional maglites from a neighboring group.
Make sure the card is closer to the screen than it is to the maglites. Sketch your observations by
shading in the screen to the right.
Suppose you had lots and lots of maglites, with their tips very, very close together, all aligned one above
the other. What kind of shadow pattern would you expect to see on the screen? Explain your thinking.
Share your thinking with one or more neighboring groups. What predictions and ideas do they have for
what would happen with lots of maglites?
Share maglites with a few other groups and hold as many maglite tips as you can, close together and
above each other. What does the shadow look like? Is it what you had expected?
11. Go to your Lab Notebook. Based on the evidence you gathered in this experiment develop a response
to the following questions:
Question #1: How does light travel from a source to a screen?
Question #2: In what direction or directions does light seem to travel after leaving a small
light source? (Develop an idea that could be used to address this. Name this idea.)
Question#3: How does the number and distance of light sources affect the brightness of
light on a screen?
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What is the Relationship Between Intensity and Distance?
Inventing a Mathematical Relationship
Materials: mini-maglite, maglite holder, meter stick, 1-ft x 1-ft whiteboard, white board markers, 5-in
x 7-in index card with a 1 inch square hole in the middle.
PROBLEM: Determine a mathematical relationship between the intensity of the light on a
screen and the distance of the light source from the screen.
1. Using the white board for a screen, place one maglite and the index card with the 1-inch square
hole in it, in such a way that you get a sharp square of light in the upper left hand corner of the
white board. This square of light should be approximately one to two inches on a side. Using the
blue white board marker, trace over the outline of the square on the white board.
2. In order to find the relationship between the intensity of the light hitting the screen and the
distance of the light source from the screen, it is necessary to keep the amount of light coming
through the hole constant. Explain why this is necessary and how we are accomplishing it.
3. To ensure that the amount of light passing through the hole is constant, keep the maglite the
same distance from the index card. Move the white board so that it is twice as far away from the
index card. Adjust the vertical position of the index card so that the blue square on the
whiteboard is in the upper left hand corner of the new square of light that is shining on the white
board. See the diagram below. Using the red white board marker, trace over the outline of the
new square on the white board.
Blue
Square
Red
Square
4. Explain how the intensity of the light hitting the screen now compares to the intensity of the light
hitting the screen in step 1 above.
Hint: Compare the area of the squares.
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5. Keeping the maglite the same distance from the index card. Move the screen so that it is three
times farther away from the index card. Adjust the vertical position of the index card so that the
original blue square on the whiteboard is in the upper left hand corner of the new square of light
that is shining on the white board. Trace the outline of the new square of light with the green
white board marker. How does the intensity of the light on the screen at this distance compare to
the intensity of the light hitting the screen in step 1 above?
6. Determine a mathematical relationship between the intensity of the light on the screen (white
board) and the distance the light source is from the screen. Explain your procedure for doing
this both in words and with diagrams. Include any measurements that you use to find the
relationship. (Put your data in a data table.)
7. Share your conclusions with the rest of the class and, as you listen to other groups, record any
results that were different than your own.
8. Your teacher will now demonstrate with the computer simulator how the intensity of the light
changes with distance. Record distances and values from the intensity meter that are displayed
on the computer screen. Does your mathematical relationship correctly predict these values?
Show your work below.
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Overlapping Color Lights
The Light Box
Predictions:
Overlapping color lights
Resulting color
Red + green
Red + blue
Blue + green
Red + green + blue
Observations:
Overlapping color lights
Resulting color
Red + green
Red + blue
Blue + green
Red + green + blue
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How do Colored Objects Appear
Through Colored Filters?
Imagine you were wearing a pair of blue-tinted sunglasses. How would a blue ball look?
How would a yellow book look? What would a stained-glass window look like?
Materials: a set of six color filters—red, green, blue, yellow, cyan and magenta
In the first column of the table below are six colored squares. You are to predict what color (or
black) you would see if you were to hold each of the six colored filters listed along the top row
over the colored square. Discuss each prediction with your group and explain your reasoning in
terms of the Consensus ideas for cycle IV. For simplicity, confine your prediction to one of the
following choices: RED (R), GREEN (G), BLUE (B), YELLOW (Y), CYAN (C), MAGENTA (M),
WHITE (W), BLACK (Bk). Record your predictions by typing in the appropriate word in each cell
in the table. After you have made all the predictions along a row (looking at the same square
through each of the filters), then test your prediction and record your observations below your
predictions. To simplify things, assume the color you actually observe can be classified as one of
the colors mentioned above. (Ignore slight differences).
Colored Square
Red
(R)
Green
(G)
Blue
(B)
Yellow
(Y)
Cyan
(C)
Magenta
(M)
Red
Filter
Green
Filter
Blue
Filter
Yellow
Filter
Cyan
Filter
Magenta
Filter
Predict
Observe
Predict
Observe
Predict
Observe
Predict
Observe
Predict
Observe
Predict
Observe
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Light Problems
1. What is the frequency of yellow light, λ = 556 nm?
2. One nanosecond (ns) is 10-9 s. Laboratory workers often estimate the
distance light travels in a certain time by remembering the approximation
“light goes one foot in one nanosecond”. How far in feet, does light
actually travel in exactly one nanosecond?
3. Modern lasers can create a pulse of light that lasts only a few
femtoseconds.
a. What is the length of a pulse of violet light that lasts 6.0 fs?
b. How many wavelengths of violet light (λ = 400 nm) are included in
such a pulse?
4. The distance to the moon can be found with the help of mirrors left on the
moon by astronauts. A Pulse of light is sent to the moon and returns to
Earth in 2.562 s. Using the defined speed of light, calculate the distance
from Earth to the moon.
5. Use the correct time taken for light to cross Earth’s orbit in 16 minutes and
the diameter of the orbit, 3.0 x 1011 m, to calculate the speed of light using
Roemer’s method.
6. A lamp is moved from 30 cm to 90 cm above the pages of a book.
Compare the illumination on the book before and after the lamp is moved.
7. What is the illumination on a surface 3.0 m below a 150-watt incandescent
lamp that emits a luminous flux of 2275 lm?
8. Draw a graph of the illuminance from a 150-watt incandescent lamp
between 0.5 and 5.0 m.
9. A public school law requires a minimum illumination of 160 lx on the
surface of each student’s desk. An architect’s specifications call for
classroom lights to be located 2.0 m above the desks. What is the
minimum luminous flux the lights must deliver?
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Reviewing Concepts
1. Sound does not travel through a vacuum. How do we know that light does?
2. What is the range of wavelength, from shortest to longest, that the human eye
can detect (red, 4.6 x 1014Hz blue 7.5 x1016Hz)?
3. What color of visible light has the shortest wavelength?
4. What was changed in the equation υ = ƒ λ in this chapter?
5. Distinguish between a luminous body and an illuminated body.
6. Look carefully at an ordinary, frosted, incandescent bulb. Is it a luminous or an
illuminated body?
7. Explain how we can see ordinary nonluminous classroom objects.
8. What are the units used to measure each of the following?
a. luminous intensity
b. illuminance
c. luminous flux
9. What is the symbol that represents each of the following?
a. luminous intensity
b. illuminance
c. luminous flux
10. Distinguish among transparent, translucent, and opaque objects.
11. Of what colors does white light consist?
12. Is black a color? Why does an object appear to be black!
13. Name each primary light color and its secondary light color.
14. Name each primary pigment and its secondary pigment.
15. Why can sound waves not be polarized?
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Applying Concepts
16. What happens to the wavelength of light as the frequency increases?
17. To what is the illumination of a surface by a light source directly proportional?
To what is it inversely proportional?
18. A point source of light is 2.0 m from screen A and 4.0 m from screen B. How
does the illumination of screen B compare with the illumination of screen A?
19. You have a small reading lamp 35 cm from the pages of a book. You decide
to double the distance. Is the illumination on the book the same? If not, how
much more or less is it?
20. Why are the insides of binoculars and cameras painted black?
21. The eye is most sensitive to yellow-green light. Its sensitivity to red and blue
light is less than ten percent as great. Based on this knowledge, what color would
you recommend that fire trucks and ambulances be painted? Why?
22. Some very efficient streetlights contain sodium vapor under high pressure,
They produce light that is mainly yellow with some red. Should a community
having these lights buy dark-blue police cars? Why or why not?
23. An apple is red because it reflects red light and absorbs blue and green light.
Follow these steps to decide whether a piece of transparent red cellophane
absorbs or transmits blue and green light:
a. Explain why the red cellophane looks red in reflected light.
b. When you hold it between your eye and a white light it looks red, Explain.
c. Now, what happens to the blue and green light?
24. You put a piece of red cellophane over one flashlight and a piece of green
cellophane over another. You shine the light beams on a white wall. What color
will you see where the two flashlight beams overlap?
25. You now put both the red and green cellophane pieces over one of the
flashlights in problem 27. If you shine the flashlight beam on a white wall, what
color will you see? Explain.
26. If you have yellow, cyan, and magenta pigments, how can you make a blue
pigment? Explain.
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Plug and Chug
27. Convert 700 nm, the wavelength of red light, to meters.
28. Light takes 1.28s to travel from the moon to Earth. What is the
distance between them?
29. The sun is 1.5 X 108 km from Earth. How long does it take for the sun’s light
to reach us?
30. Radio stations are usually identified by their frequency. One radio station in
the middle of the FM band has a frequency of 99.0 MHz. What is its wavelength?
31. What is the frequency of a microwave that has a wavelength of 3.0 cm?
32. Find the illumination 4.0 m below a 405-lm lamp.
33. A screen is placed between two lamps so that they illuminate the screen
equally The first lamp emits a luminous flux of 1445 lm and is 2.5 m from the
screen, What is the distance of the second lamp from the screen if the luminous
flux is 2375 lm?
34. A three-way bulb uses 50, 100, or 150W of electrical power to deliver 665,
1620 or 2285 lm in its three settings. The bulb is placed 80 cm above a sheet of
paper. If an illumination of at least 175 lx is needed on the paper, what is the
minimum setting that should be used?
35. Ole Roemer found that the maximum increased delay in the disappearance
of lo from one orbit to the next is 14 5.
a. How far does light travel in 14 s?
b. Each orbit of Io takes 42.5 h. Earth travels the distance calculated in
part a in 42.5 h. Find the speed of Earth in km/s.
c. See if your answer for his reasonable. Calculate Earth’s speed in orbit
using the orbital radius, 1.5 X 108 km, and the period, one year.
Equations
V = d/t
ν=λf
E = P/ 4πr2
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Video Quiz –Hewitt - Light Waves
1. Give three examples of where you might find a complete color spectrum.
a. _____________________
b. _____________________
c. _____________________
2. Light does not change ______________________ when it reflects.
3. Diagram how the spectrum can be seen in a gas spill on blacktop after a good
rain.
4. The sun gives off ________________ light.
5. When you take one color away from white light, you are left with its
_______________ color.
6. On a camera, the lens cancels out what color? _______________
Why?_______________________________________________________
____________________________________________________________
7. Hewitt uses picket fences and a rope (wave) to form an analogy to explain
the__________________ of light.
8. Hewitt uses ______________ diagrams to explain how components of the light
are cancelled with polarized film.
9. Draw the polarized glasses you would want to use to eliminate glare as you drive
on a sunny day.
10. Why would you choose these glasses?
____________________________________________________________
____________________________________________________________
____________________________________________________________
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