MS 20
Solar Radiation and Light
Transmission
Solar Energy
• Earth receives 2ppb (parts/billion) of the
total energy produced by the sun.
• Solar constant = average amount of solar
energy received by a fixed area at the
Earth’s surface = 2 calories/cm2/minute
Measuring a “solar constant”
• We will use a “lux meter” to measure light
intensity in units of energy/unit area/time
Set to highest sensitivity (1X).
• Measure four 50 cm intervals
• Set the globe two meters from the lamp
– Equatorial plane parallel to table top
– Axis perpendicular to table top
• Set light (highest intensity) to hit the globe
at the center (look for an even halo on wall
behind the globe)
• Take light intensity readings at 50 cm, 100
cm, 150 cm, and 200 cm.
• Record readings in Table 1.
Table 1 (page 9)
Distance from
light source
Light intensity
Measured
50 cm
100 cm
150 cm
200 cm
Theoretical
Light intensity decreases
proportionally to the square of
the distance
100 cm is twice as far from the light as 50 cm, therefore
the light is ¼ as bright at 100 cm.
Relative to 50 cm, what is the brightness at 200 cm?
Equation 1
Intensity light X =
Intensity light Xo
Distance light X / Distance light Xo
Xo
=
Intensity at a standard distance (100 cm)
X
=
Distance (50 cm, 150 cm, or 200 cm)
Calculate and record “theoretical” values in Table 1 (at 50 cm,
150 cm, and 200 cm).
Table 1 (page 9)
Distance from
light source
Light intensity
Measured
50 cm
100 cm
150 cm
200 cm
Theoretical
Effect of latitude and season on
incoming solar radiation
•
•
Hold the lux meter parallel to the surface of the globe
Take light measurements at
–
–
–
–
–
•
23.5° S (the Tropic of Capricorn);
at the equator;
at 23.5° N (the Tropic of Cancer);
at 45° N; at 66.5° N (the Arctic Circle); and
at the North Pole.
Record your measurements in Table 2 on the answer sheet (p. 11).
Table 2
Latitude
Light intensity
Axis vertical
90o (north pole)
66.5 N (arctic circle)
45 N
23.5 N (T of Cancer)
0 (equator)
23.5 S (T of Capricorn)
Axis tilted
•
•
Remove the jack stand so that the base of the globe lies flat on the table top.
Take light measurements at
–
–
–
–
–
•
23.5° S (the Tropic of Capricorn);
at the equator;
at 23.5° N (the Tropic of Cancer);
at 45° N; at 66.5° N (the Arctic Circle); and
at the North Pole.
Record your measurements in Table 2 on the answer sheet (p. 11).
Table 2
Latitude
Light intensity
Axis vertical
90o (north pole)
66.5 N (arctic circle)
45 N
23.5 N (T of Cancer)
0 (equator)
23.5 S (T of Capricorn)
Axis tilted
Albedo
Equation 2
Albedo is affected by color, texture, and composition of
the surface struck by the incident radiation.
•
•
Place the aluminum pan inside a plastic tray
• For support
• To protect the table from spillage
Set the microscope light as shown
• White pan first (mimics a snow-covered surface)
• Measure incident radiation once (it will be the same for all subsequent
experiments)
• Measure reflected radiation (orient lux meter as shown)
NOTE:
Take all of your readings from the same spot.
Make all measurements from the same height above
the pan (at least 5 cm above the surface).
Hold the lux meter so that you do not cast a shadow
over the area you are trying to measure.
• Repeat the experiment using the pan with the black
bottom.
• Record this measurement in the Table 3.
• Calculate the albedo from equation 2.
• Fill the black pan with 2 cm of water and repeat the
experiment.
• Record this measurement in the Table 3.
• Calculate the albedo from equation 2.
• Fill the black pan with ice (this mimics an ice-covered
sea).
• Record this measurement in Table 3.
• Calculate the albedo from equation 2.
•
•
•
•
•
Turbidity and secchi disk
measurements
Fill an aquarium 10 cm mark with tap water.
Affix a lux meter "shield" on the far end.
Measure the white light intensity (in LUX) over the 60 cm path length
of the aquarium.
Place the mini Secchi disk in the aquarium and slide it from the light
source toward the far end.
Record the point at which the Secchi disk is no longer visible (note:
without sediment in the water it will not disappear).
•
•
•
•
•
•
•
Measure out three 1.00 gram samples of the powdered clay/silt into
three separate weigh boats.
Add a little calgon from the squeeze bottle to the first 1.00 gram
sample.
Add the powdered clay/silt to the aquarium, and mix thoroughly (Be
careful not to splash the microscope light or the lux meter!). This
amount of sediment equals approximately 50 milligrams/liter after
mixing.
Mix the sediment completely with a ruler and wait exactly 10 seconds
for the aquarium to "quiet."
Measure the light passing through the 60 cm path with the lux meter.
Take a mini Secchi disk reading.
Record the data in Table 4.
Table 4
Sediment concentration
0 grams
1 gram (50 mg/liter)
2 grams (100 mg/liter)
3 grams (150 mg/liter)
Light intensity Visibility limit
(lux)
(cm)
60 cm
•
Wet the second silt/clay sample with calgon,
• add it to the aquarium,
• mix,
• measure and
• record the light attenuation and "depth of visibility.“ in Table 4.
•
Wet the third silt/clay sample with calgon and repeat as above.
Plot the data on the graph.
• Do not plot the first point at zero sediment concentration.
• Use two different symbols: X for light attenuation and O for depth
of visibility.
• Connect the points with straight lines.
Light attenuation and settling
rates
•
The amount of time the suspended sediment stays in the water column
is a function of the sediment size and settling rate.
•
Like inorganic sediment, phytoplankton are affected by "sinking“. If
phytoplankton sink out of the photic zone, productivity ceases. To
combat sinking, some species have developed adaptations to retard
their settling rates (i.e., oils to reduce bulk density, appendages to
increase surface area, etc.).
•
The aquarium still has 150 milligrams per liter of suspended sediment
from the previous exercise. Turn on the microscope light, and resuspend the sediment by stirring with the ruler.
•
Wait 10 seconds for the surface of the water to stabilize,
•
Record the light intensity (lux) through the aquarium (60 cm) at one
minute intervals in Table 5. (The amount of time the suspended
sediment stays in the water column is a function of the sediment size
and settling rate.)
Table 5
Time
0 minutes
1 minute
2 minutes
3 minutes
4 minutes
5 minutes
6 minutes
7 minutes
8 minutes
9 minutes
10 minutes
Light intensity
(lux)
Suspended
sediment (mg)
•
Fill in the suspended sediment column by estimating the correct values
from the light intensity curve you prepared in the previous exercise.
•
Plot the suspended sediment concentrations as a function of time on
the graph.