Masks with holes
Combining light sources that are not small and masks with various shaped holes – what
will we get?
Materials
 1 light station
 view screen with stands
 small flashlight
 long-filament bulb
 various masks with holes in them, labeled A, B, C
> What are the places from which you can see the light bulb in the light station?
Although the previous question has a perfectly definite answer, it isn’t obvious, just
looking at the light station: you can’t see the region, and have to go look for it. Discuss
what this tells you about light, and about what things you can see.
> A small light bulb hanging from the ceiling is shining on a table. Indicate on the
diagram which regions are in shadow..
Light travels in straight lines! And where the light can’t reach is where the shadow is.
03/27/2015
25- 1
What to do:
1. Obtain a long-filament bulb and mask A (3 mm hole). Place the bulb and the screen
about 50 cm on either side of the mask, but don’t turn it on yet!. Predict what will
appear on the screen when light from the bulb
shines through the hole in the mask.
2. Check your prediction. Explain what happens
3. Suppose you shine a small flashlight next to the long-filament bulb (also 50 cm from
the mask). Predict what will appear on the screen when light from the bulbs shine
through the hole in the mask.
4. Check your prediction. Explain what happens
03/27/2015
25- 2
5. Turn off your light sources. Predict what will appear on the screen when you
replace the mask with a mask that has a triangular hole (using the same arrangement of
lights).
Predict how what appears on the screen will change when you cover part of the long
bulb (for example, by putting some fingers in the way)?
6. Check your prediction (using mask B). Explain what appears, and what happens when
you cover part of the long bulb..
7. Turn off your lights. Predict what will appear on the screen if you use a mask with a
5 cm hole in it.
8. Check your prediction (using mask C). Explain what appears
9. Turn on the light station so that it illuminates the screen. Predict what you will
observe on the screen when you place mask A over the aperture (with the hole positioned
so that light can go through it).
03/27/2015
25- 3
10. Then try the experiment. What do you observe?
11. Repeat activity 10, using a mask with a very small hole in it. What do you observe?
Check: How does the image on the screen (the pattern of light) related to the light
sources and to the hole in the mask?
Sharp and fuzzy shadows
Why are some shadows sharp while others are fuzzy? Sometimes your shadow is very
distinct; other times it is so fuzzy you can hardly see it at all. What decides this?
Materials
 1 light station
 1 diffusing mask for front of light station
 piece plain white paper
 wooden rectangular block, with rod and plastic base
 filament viewer (frosted plastic rectangle)
 view screen with stands
 frosted light bulb
What to do
Put white paper on the table in front of the light
station.
1. Observe the shadow that the wooden rod makes on
the paper. Adjust the wheel on the bottom of the light
station through its full range of motion (it turns about
180). Describe how this adjustment affects the
sharpness of the shadow edges.
2. Using the filament viewer, look briefly at the glowing light bulb through the front
opening of the light station, turn the adjusting wheel and observe how this makes the
filament rotate.
03/27/2015
25- 4
Find the two filament positions shown below, and observe which one makes the
rod’s shadow edges sharp, and which one makes the edges fuzzy. Fill in the blanks.
Filament turned like this
Filament turned like this
makes shadow sides _______________________
makes shadow sides __________________________
Working with the view screen upright
Put the object close to the light and stand
the view screen upright. Notice how
the shadow sharpness on the screen
changes when the filament is turned to
different positions.
3. How does the shadow change when you put in the “diffusing mask” (the piece of
translucent plastic mounted in a frame)?
4. If you have two light bulbs, the same object can have two shadows at once. Find an
example, and then construct some more. Some parts of these shadows are darker than
other parts. Why does that happen?
5. Suppose you were at a fireworks display, and a rocket
has just produced 100 brilliant lights in the sky. What
would your shadow look like?
03/27/2015
25- 5
6. Compare the shadow made by a clear light bulb to the shadow made by a frosted light
bulb. How are they alike, and how are they different?
7. Make shadows of a pencil with the tube light.
pencil?
Why does it matter how you hold the
8. Give rules that determine whether shadows are sharp or fuzzy.
for what you observed in Q1-Q7.
They should account
9. Here is a diagram that shows a light source, an object, and a viewing screen.
Complete the diagram to show where the shadow is.
source
object
viewing screen
The shadow is somewhat fuzzy. How does your diagram explain this?
03/27/2015
25- 6
10. Here is another light source, object, and viewing screen. Complete the diagram to
show that the shadow is less fuzzy when the object is closer to the screen.
Check: discuss your rule and diagrams with an instructor.
How big is a shadow?
Your shadow can be many different shapes, but also different sizes. What determines
how big a shadow is? Let's find out!
Materials
For each group of four students
 1 light station
 1 view screen with 2 stands
 1 wooden cylinder
 1 wooden rod and object stand
 metric grid paper
 2 clips to attach grid paper to screen
 meter stick or tape
 ruler
What to do




Set up the light station and view screen about one meter apart.
Put one of the screen supports flat on the table holding the bottom of the screen.
Clip the grid paper to the screen.
Adjust the wheel on the bottom
of the light station so the sides of
the cylinder’s shadow are sharp.
03/27/2015
25- 7
1. Explore shadow size. How does the position of an object affect the size of its
shadow?
2. Using the wood cylinder (standing vertically, like the diagram shows), determine how
the width of a shadow depends on how close the object is to the screen. Measure (using
the metric grid paper or a ruler) the width of the cylinder’s shadow for about 6 to 8
different locations of the cylinder, covering the full range of positions between the screen
and light. Record each measurement in the table.
3. Make a line graph of your data showing how the shadow width depends on the
position of the object.
03/27/2015
25- 8
4. Interpret your graph. That means to explain what the graph tells you about the
relationship between your dependent and independent variables.
03/27/2015
25- 9
Each group should hand in one copy of this page at the end of class
Group:
Names of group members present:
5. Explain how to use your graph to infer where you would need to place the cylinder in
order to make a shadow that is 10 cm wide.
6. Why did the instructions say to observe the width of the shadow and not the height?
7. Louise has a great idea how to decorate her living room for when she hosts the bridge
club next week: she will draw hearts and spades on the light bulbs, so that they project
these pictures onto the lampshade and the ceiling. Easy to do, cheap, and after the party
she can just wash the pattern off. Her neighbor Margie objects, "If that’s going to work,
why doesn't it say '100 Watts' on the ceiling already?" Is Margie right, that this won't
work? Or can you tell Louise how to rescue the scheme?
03/27/2015
25-10
Checks
Masks with holes
Light travels in straight lines, starting from every part of the light source and going in all
directions. When the light arrives at an opaque part of a mask, it stops; only light that
goes through the holes can continue on to the viewing screen.
When the light source is small and the hole is large, what appears on the screen is the
shadow of the mask – a pattern of light that is the same shape as the hole.
When the light source is large and the hole is small, this gives an image of the light
source on the viewing screen
The image is upside down (the top of the
source produces the bottom of the image).
When the light source and the holes in the mask are about the same size,
Sharp and fuzzy shadows:
5. Each firework is a light source, and a different place in the sky. Each gives you a
shadow, but these are in slightly different places. The middle parts overlap and give a
dark shadow; the differences are at the edges, which now will be indistinct.
8. You get a sharp shadow if the light source is small compared to the other distances
and sizes. The light is coming (almost) from one place, and the object either blocks it or
not, giving a shadow with sharp edges. When the light source is larger than the object,
there can be a shadow on a screen right behind the object, but not when the viewing
screen is far away.
How big is a shadow?
6. Why did the instructions say to observe the width of the shadow and not the height?
The light source in the light station is not really small. But if it is turned the right way, it
is very narrow, so that the shadows of a vertical edge are sharp. The shadow of the top
and bottom edge is not as sharp, which would make it hard to measure it.
03/27/2015
25-11
7. Louise has a great idea how to decorate her living room for when she hosts the bridge
club next week: she will draw hearts and spades on the light bulbs, so that they project
these pictures onto the lampshade and the ceiling. Easy to do, cheap, and after the party
she can just wash the pattern off. Her neighbor Margie objects, "If that’s going to work,
why doesn't it say '100 Watts' on the ceiling already?" Is Margie right, that this won't
work? Or can you tell Louise how to rescue the scheme?
Of course, Louise should use an unfrosted light bulb. But even then, for this to work the
light source has to be small compared to the diagram Louise draws on the light bulb.
But the light filament is a centimeter long, so the diagram will have to be pretty large.
Then the shadow on the wall will be enormous.
Using a large shadow-maker that is farther from the light source will work better.
The light bulb company writes the “100 W” on clear bulbs in faint brown, and even then
there is a brown smudge on the ceiling – a very fuzzy shadow. With frosted bulbs the
sign is printed in dark black letters, because there isn’t going to be a shadow anyway.
03/27/2015
25-12