Dec 9, 2009
photometer, a meter
stick, and standard
luminosity source (for comparison).
The basic idea is to
place a light bulb (of known luminosity ) and the sun on opposite sides
of a wax photometer (a flux-comparing
device) and vary the distance from the photometer
to the bulb until the bulb flux at the photometer matches the solar
flux at the
photometer.
light bulb
READING: Universe 17(2); how a pinhole
camera works (see web page)
socket,
cord, and clamp
wax
photometer ( = 2 slabs of wax separated by opaque foil)
meter
stick; fancy transparent rulers
pinhole camera
OBSERVATIONS
unsafe behavior on
the roof will result in a grade of 0 for the entire group
(the
sun-earth distance is assumed to be known in this lab and can be looked
up.)
1) Clamp the bulb onto
something stable, such as the
fence around the observing platform.
How/where
I'm sure your well-labeled
diagram will show
all.... remember
lab journal expectations....
3) a) One partner should
be the flux judge and decide
where the photometer should be held
such
that
the sun and bulb have equal fluxes on the two photometer faces. This person
should
then hold the photometer steady at this distance while the other
partners
insure
proper alignment, measure and
b) Repeat
steps above until each partner in the group estimates the position of
balanced
flux (sun
vs. light bulb) at least
three times.
4) Individually
(and without consulting with any of your partners) record observations
that
YOU make about the
colors of the light
observed in the two halves of the photometer.
Of course, this should be done with the
bulb on and
with the photometer at the distance
such that the fluxes of bulb and sun match.
5) Use the pinhole camera
to measure the
size of the sun's image as accurately as
possible.
RESULTS
1) Using the average
value of your group's measured
distances (of balanced solar and bulb
flux),
calculate the luminosity of the sun. Make sure that you
start with the condition that
you made happen on the roof.
In any lab where you are measuring/determining
something that has a standard known
value, it is expected
that
you will compare (% difference) your value to the standard.
2) Explain your
(individual) color
observations using appropriate laws and standard
temperatures of the light sources.
Both
calculations and word explanations are
appropriate
here,
right?
3) Give one excellent
reason
that would explain why your calculated value of the solar
luminosity
value might
('Human errror' never
with an accepted value.)
4) Repeat question 3 with
'higher' replaced by 'lower.'
Questions 3 and 4 will be
graded according to 3
criteria:
a) importance of the reason
offered (i.e., how likely your reason is to produce a significant
discrepancy between the calculated and the expected
answers);
b) correctness of
the reason (does it actually result in 'higher' or 'lower' values that
you said
it did?);
c) a convincing explanation for why the effect results
in 'higher'
or 'lower'
values.
5) Mars is about 1.5
times farther away from the sun
than Earth. If you repeated this
experiment
away from your bulb (as compared to Earth)
so that the
two sides of the photometer
were illuminated with equal flux?
The answer should NOT depend on the value of
the luminosity of
the sun!
6) Use your
measurements (& the distance of the sun) to
determine the radius of the
sun.
7) Determine the surface temperature of the sun from your measurements.
(As a general guide, I would think maximum
differences between your measurements
and standard values would be 50%, 15%, and 10%,
respectively for L, R, and T.