Experiments with a Laser Pointer and The Optics Suitcase

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Experiments with a Laser Pointer and The Optics Suitcase
Supplement to be used with the theme packets from Optics Suitcase and other materials supplied.
Support provided by APS / OSAF / SPIE and Tessera Technologies, Inc.
By T. M. Pfuntner and S. D. Jacobs
01/18/10
Supplement Supplies
• quantity 12 ea., 650 nm (red) laser diode pointer w/ warning label
• quantity 1 ea.,
plastic sheet-diffractive optical element (DOE) mounted on wood frame
• quantity 11 ea., plastic sheet-DOE mounted on fold-out, white paper frame
• quantity 12 ea., white "screen": medium card stock-semi-smooth finish, paper 8.5" by 11"
• quantity 12 ea., glass bottle w/ screw cap
• quantity 12 ea., plastic, disposable, transfer pipet
Warnings Regarding Laser Pointers
These are class IIIa lasers with output power <5 mW
http://en.wikipedia.org/wiki/Laser_safety#Class_IIIa].
"The US Food and Drug Administration has advised
that laser pointers are not toys and should not be
used by minors except under the direct supervision
of an adult…….. a brief 0.25-second exposure to a
<5 mW laser such as found in red laser pointers does
not pose a threat to eye health. On the other hand
there is a potential for injury if a person deliberately
stares into a beam of a class IIIa laser for few
Fig. 1
seconds or more at close range. Even if injury
occurs, most people will fully recover their vision. "
[ref:
Introduction
This supplement provides ideas and materials for simple experiments that may be done with the
laser pointer and the Optics Suitcase during calendar year 2010, as part of LaserFEST celebrating 50 years of Laser Innovation (www.laserfest.org). The goal is to show middle school
students some of the unique attributes of the laser in comparison to other light sources.
Experiments described below explore "collimation and divergence", "monochromaticity",
"diffractive optical elements", "polarization", "scatter", "total internal reflection" and "heat from
light". Many of these experiments utilize materials from the three theme packets in the Optics
Suitcase [see Educational Outreach Section at http://osarochester.org/]. Some of the ideas for
these experiments come from resources cited at the end of the supplement.
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Advanced Preparation
Read over the entire supplement and estimate how many of the seven experiments will fit into
the time allotted. Assemble and prepare all supplies ahead of time. If you choose to break up
these experiments over several days, remember that the Optics Suitcase theme packets are
necessary for many of them. Since theme packets are meant to be taken home by the students,
you may need to hold onto these until all experiments are completed. The laser pointers should
be collected at the end of each presentation; they are not meant to be given away.
To Begin
One laser pointer is intended to be used by a group of 2 to 3 students. You, the presenter,
should be the judge of how much direct supervision is required to carry out the
experiments described below, based on the ages of those in your audience.
[http://web.princeton.edu/sites/ehs/LabPage/laserpointersafety.htm]
First, go over laser safety issues:
1) "Never look directly into the laser beam."
2) "Never point a laser beam at a person, even someone far away, in a car, for example."
3) "Do not aim the laser at a reflective surface like a mirror."
Per directions on the white box, pull the white tab out of the back of the pointer if it is still
inserted. Examine the pointer with the students. Note the warning labels. A laser pointer is not a
toy. "Laser pointers have become consumer novelty products and promoted as toys," says
Jerome Dennis, a consumer safety officer for the FDA's Center for Devices and Radiological
Health (CDRH). "They're hazardous as toys and shouldn't be used frivolously."
-[http://www.fda.gov/Radiation-EmittingProducts/RadiationSafety/AlertsandNotices/ucm153548.htm]
This laser pointer has two "on" buttons, located between the warning labels (see Fig. 1). The
silver button operates a white light-emitting-diode (LED). It is not a laser. When this button is
pressed, two bright, white light beams are emitted from the device. The "red" button activates
the "red" diode laser, and the laser beam exits from the case from the side opposite the key ring
(see red arrow in Fig. 1). [Note: Power cell replacement is discussed at the end of the
supplement.] Although each experiment is given a number, they may be done in any order, and
some may be omitted if time is a problem. One exception: experiments #4 and #5 should be done
together.
Experiment #1: Collimation and Divergence
Materials required for each group of students:
• one flashlight from the Rainbow Peephole theme packet
• one "screen"- white card stock, folded in half
• one laser pointer
Take the flashlight from the Rainbow Peephole theme packet.
Erect a folded piece of white paper as a "screen" (see Fig. 2), and
shine both the flashlight and the laser pointer onto it. Move both
light sources closer and further away from the screen. The laser
beam is naturally collimated. This means that the size of the beam
expands in size (e.g., diverges) very slowly with distance. As a
consequence of collimation, laser beams may be directed to
distant targets with high accuracy. This can be good (laser
communications, laser ranging - distance to the moon), or bad
(laser beam sent into the cockpit of a commercial jet to
Fig. 2
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temporarily blind the pilot). With care, the laser beam may be directed toward the ceiling or to a
distant wall within the classroom where it will remain visible.
Experiment #2: Monochromaticity
Fig. 3
Materials required for each group of students:
• one flashlight from the Rainbow Peephole theme
packet
• one Rainbow Peephole from the Rainbow
Peephole theme packet
• one "screen"- white card stock, folded in half
• one laser pointer
Take the flashlight and shine it through the Rainbow
Peephole and into the eye, duplicating the
experiment done as part of the Optics Suitcase
presentation. The Rainbow Peephole is called a
diffractive optical element (DOE -more in
Experiment #3 below). The complex arrangement of bumps on the plastic film of the Rainbow
Peephole (see atomic force microscope image on the packet) diffracts, or redistributes, the white
light from the flashlight according to color. The light from the flashlight consists of a spectrum
of colors. It is polychromatic.
Now, direct the laser beam through the Rainbow Peephole and onto the white "screen" (see Fig.
3). This laser is a monochromatic light source. It has only one color - red. Monochromaticity
means only one color, and it is a property of lasers. The monochromatic property of lasers is
advantageous for the treatment of skin cancers, for eye surgery, for fiber-optic relay of
information in telecommunications, and for chemical research (initiation of color-specific
chemical reactions).
Experiment #3: Diffractive Optical Element
Fig. 4
Materials required for each group of students:
• one "screen"- white card stock, folded in half
• one DOE mounted on a white paper frame
• one laser pointer
Set up the DOE and place the "screen" behind it (see
Fig. 4). Direct the laser beam through the DOE.
How many distinct patterns can be observed? (ans: 17
to 21, depending on how closely one looks (don't forget
to shine the laser through the central region of the
plastic foil). DOEs are called pattern generators for
laser light. They transform a single laser beam into
various simple or complex patterns. They redistribute
the energy in the laser beam. The type of pattern is
determined by the micron-scale profile embossed onto
each square in the DOE. Other DOEs find applications
in imaging systems as thin lenses or lens arrays. A form of DOE called a "hologram" may be
used as an anti-theft device. Find the hologram on your parent's credit card.
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Experiment #4: Polarization
Materials required for each group of students:
• one linear polarizer from the Magic Stripes theme packet
• one "screen"- white card stock, folded in half
• one laser pointer
• white correction fluid or a permanent marker pen
This experiment is a follow-up activity to the lesson on polarization and
color from white light in the Optics Suitcase.
Observe room light or sunlight reflected from any shiny surface
while holding up one of the linear polarizer squares. You must
be looking at an angle of observation that is ~30° to 40° up from
horizontal (e.g., the table top, see Fig. 5a). Rotate the polarizer.
"Glare" is light that has become linearly polarized in a
horizontal direction after reflection from a shiny surface. The
angle is called the "polarizing angle", or "Brewster's angle",
after Scottish physicist David Brewster (1781-1868). With this
simple experiment you may mark
the linear polarizer to indicate the
horizontal polarization direction
of light that is "passed" by the
polarizer. [Because the Magic Stripes
Fig. 5a
polarizer may not be marked to indicate the polarization "pass" direction, you
may need to scrape off the white line and re-apply it with white correction fluid
or a permanent marker pen (see Fig. 5a).] Polarizing sunglasses are
Fig. 5b
manufactured with a vertical polarization "pass" direction, to
block glare reflected of the highway (See Fig. 5a).
Now, erect the white screen. Shine the laser pointer on the screen
and insert the marked polarizer in the beam. Rotate the polarizer
to maximize the amount of laser light hitting the screen. Unlike
the light from other light sources (direct sunlight, the flashlight),
most laser radiation is polarized. The output beam from this laser
pointer is "mostly" linearly polarized parallel to its broad, flat
surface (see Fig. 5b). Show that it is not completely polarized.
Experiment #5: Scatter, Polarization and Total Internal Reflection
Materials required for each group of students:
• one flashlight from the Rainbow Peephole theme packet
• one (correctly marked) linear polarizer from the Magic Stripes theme packet
• one "screen"- white card stock, folded in half
• one laser pointer
• one disposable, plastic transfer pipet
• one 20 ml glass bottle with screw cap [you will need to provide some low fat or nonfat milk]
Before carrying out this experiment, the presenter should fill each bottle
with about 10-15 ml of tap water to the mark on the side.
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Erect the "screen" and use it as a
Fig. 6a
backdrop for this experiment. Direct the
laser through the bottle of water. It is
not possible to see the laser beam inside
the water, because the water is
transparent. Using the disposable,
plastic transfer pipet, add 5 drops of
nonfat or no fat milk to the bottle, seal
and gently agitate (you do not want to
create bubbles). The liquid is now cloudy. Shine the laser through the bottle. The milk in the
water scatters the light sideways, making the beam easily visible (see Fig. 6a). Hold the
flashlight up to the side of the bottle and note the difference in divergence for this incandescent
light source compared to the laser light source.
The following experiments with polarized light assume that both the
Optics Suitcase Magic Patch experiment and Experiment #4 (above) were completed earlier.
Hold the laser between thumb and index finger as shown (see Fig. 6b). The laser light is now
vertically polarized. Observe how the intensity of the sideways scattered laser light is diminished
as a linear polarizer (from the Magic Stripes theme packet) is rotated from vertical to horizontal
(see Fig. 6c). The sideways scattered light is vertically polarized. Repeat this experiment with the
flashlight. The light from the flashlight is unpolarized (how can you show this?), but the
sideways scattered light is still vertically polarized. Polarized lasers are needed for satellite
communications; polarized lasers are used in CD readers.
Fig. 6b
Fig. 6c
Now, tip the bottle on its side and shine the laser beam in from the end of the bottle. Vary the
direction into the end of the bottle from perpendicular (see Fig. 6d) to oblique (see Fig. 6e).
When the angle of the laser beam at the water/air interface inside the bottle exceeds a critical
angle, the beam is totally internally reflected within the water. Fishermen may take advantage of
this phenomenon to sneak up close to the edge of a brook, staying invisible to the fish. Pretend to
be a fish in a swimming pool where the water's surface is smooth. What can you see? The critical
angle is a key to the sparkle of cut diamonds.
Fig. 6d
Fig. 6e
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Experiment #6: Heat from Light
Materials required for each group of students:
• one flashlight from the Rainbow Peephole theme packet
• one liquid crystal mood patch from the Magic Patch theme packet
• one laser pointer
Take the magic patch and hold it between thumb and index finger so the color-changing side
faces up. Turn on the flashlight and shine it onto the patch from the rear, while gradually
decreasing the distance between the two. How close does the flashlight bulb have to come to the
patch in order to observe a color change (see Fig. 7a)? Move the patch over the flashlight bulb.
Can you always find the location of the flashlight bulb? Do the same experiment with the laser
pointer (see Fig. 7b). Think about how useful lasers are for industrial cutting and welding
applications, and for laser surgery.
Fig. 7a
Fig. 7b
Resources
• Safety and Laser Pointers: L. Bren, "Illuminating Facts About Laser Pointers," FDA
Consumer Magazine (May-June 2005) http://www.fda.gov/fdac/features/2005/305_laser.html;
and "Illuminating the Hazards of Powerful Laser Products," FDA Consumer Health
Information / U.S. Food and Drug Administration (June 2009)
http://www.fda.gov/downloads/ForConsumers/ConsumerUpdates/UCM167564.pdf
•
•
•
Brewster's angle and "glare": M. G. Raymer, The Silicon Web, Taylor & Francis p. 254
(2009).
Total internal reflection: J. O'Connell, "Optics Experiments Using a Laser Pointer," The
Physics Teacher, vol. 37, p. 445 (Oct., 1999).
Scatter: R. E. Benenson, "Light Polarization Experiments with a Diode Laser Pointer," The
Physics Teacher, vol. 38, p. 44 (Jan., 2000).
Acknowledgments
We gratefully acknowledge the OSA Foundation for the provision of 500 laser diode pointers;
APS, OSA and SPIE for a LaserFEST Grant to assemble and distribute this supplement with The
Optics Suitcase; Tessera Technologies, Inc., for the provision of 500 DOEs; and Stephanie Bloch
/ Bruce Smith -members of the Technical Council / Rochester Section / OSA for suggesting the
use of DOEs and where to obtain them. Finally we acknowledge the Laboratory for Laser
Energetics of the University of Rochester for providing logistics support for the assembly and
distribution of this Laser Pointer Supplement and The Optics Suitcase.
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Addendum - Laser Pointer Battery Replacement
The light output of the pointer will diminish with
time. The two batteries that power the pointer
may be replaced with fresh ones by removal of
six, tiny screws. Use a small, jeweler's Phillips
head screwdriver. Once disassembled, the
pointer consists of 3 parts (see Fig. 8): top cover
with warning labels, middle portion with buttons,
and base with circuits, laser diode, and LEDs.
The two batteries are located under the metal clip
at the bottom of the base. [In Fig. 8, one battery
has been removed, while the other remains in
place.] Remove and carefully dispose of the two
"lithium" cells, model # CR1220 3V. Replace
with two fresh cells. These should be inserted
with the (+) side facing down. During reassembly of the unit, carefully replace the key
ring over the metal knob on the base.
Fig. 8
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