Transcript Invisible visible

Makeing invisible visible
Gorazd Planinšič
Faculty fot Mathematics and Physics,
University of Ljubljana, Slovenia
Experiment should convince
• Outcomes of experiments should be as evident
as possible even for the student in the last row
• All senses (not only sight) should be used to
make outcomes of the experiments perceptible
and convincing
We should also tell students that in practice often huge technological efforts
and new scientific knowledge is required to achieve small increase in
sensitivity of detectors
Two types of approaches
• Observation by magnification (amplification):
object or change is too small to be observable.
Optical or other methods are used to make the
object or change perceptible.
• Observation via agent: property of an object or
part of a space is invisible or very hard to be
seen. The property in question is made visible or
amplified indirectly by an agent.
Observation by magnification
• Rotation of a beam
• Projection
• Refraction (angular magnification)
Rotation of a beam
R
L

L  R 
Large
displacement
Large
radius
Small angle
Rotation of a beam
Cavendish experiment
light spot  2 mirror
Rotation of a beam by rolling
Thermal expansion
2r

x
x

2r
Rotation of a beam by rolling
Deflection of a wall
x
2r

L
2L
x

r
m => cm
D L MacIsaac, M Nordstrand, Demonstrating and measuring the flexure
of masonry wall, TPT 41(2003)74.
Shadow magnification
by a point light source
Shadow magnicifaction
by water drop
syringe
laser
Shadow magnification
by water drop
2d  1 
M
1  
r  n
x  r
Shadow magnification
by water drop
Cyclops (Cyclops
strenuus)
Mosquito larva
(Anopheles species)
Water flea shadows magnification
G Planinsic, Water drop projector, Phys. Teach. 39 (2001) 18-21
Observation via agent
• Electric and magnetic field: reorientation
of small particles (iron filings, salad seeds),
oscillations of compass needle...
• Gas flow and liquid flow: streamers
• Temperature distribution: IR photography,
LCD thermometers
• Stress distribution: light polarization
Observation via agent
• Small vibrations: waves, ripples,
bouncing ping-pong balls..
• Presence of gases
Eddy currents
Iron filings
Al plate
Magnet
Electro motor
Eddy currents
Top
view
Eddy currents
Side
view
Small vibrations
Singing wine glasses
G Planinsic, More fun
with wine glasses, Phys.
Teach. 37 (1999) 33-35.
Presence of invisible substance:
how to show presence of gases?
What makes them different from air?
• Speed of sound
• Breakdown voltage
• Index of refraction
Let’s go to work