MS20 Laboratory Solar Radiation and Light Transmission

Transcription

1 MS20 Laboratory and Light Transmission Introduction The sun produces a huge amount of energy. This energy radiates into space, uniformly in all directions. The Earth, because of its relatively small size and great distance from the sun, intercepts a tiny amount of this energy (roughly 2 parts per 1 billion). Nevertheless, because the total amount of energy is so huge, the flux of energy at the top of Earth s atmosphere is 174,000 terawatts (one terawatt = watts)! Roughly 30% of that energy is reflected back into space, but the remaining flux is still more than 10,000 times the amount of energy used by the entire human population. The solar constant is the average amount of solar energy received by a fixed area at the Earth s surface. Its value is 1366 watts/meter 2, which is equivalent to 2 calories/cm 2 /minute. Since the actual amount of energy must be averaged over a spherical planet, some is lost by reflection and dispersion within the atmosphere, the actual value at the Earth s surface is roughly % of that amount. That energy is transferred to all parts of the planet by the movement of atmospheric winds and ocean currents, to maintain a heat balance over the planet s surface. In this laboratory we will not be measuring solar radiation, since we don't have the instrumentation to mimic the sun, or the equipment to sense the infrared and ultraviolet parts of the energy spectrum. Instead, we will model the sun's radiation with a microscope lamp and measure the light intensity with a "lux meter" -- essentially a light meter that is sensitive to wavelengths that we can detect with the human eye. The lux meter measures the light intensity in units of energy per unit area per unit of time. NOTE: Portions of this lab require work in darkness, with lights out and windows shaded. The lux meters and the microscope lights are expensive and are not waterproof. Do not allow them to get wet. A. Measuring a solar constant Our first task will be to determine the relationship between energy flux, distance and incidence angle. We will do this with a model of the sun-earth system using a globe to stand in for the Earth, and a microscope light on a flexible stand as our sun. Revised on 3/22/2005 Page 1 of 17

2 Figure one. Set-up for the first exercise. The lab jack should be placed under the globe to tilt the rotational axis so that it is vertical (the equatorial plane should be parallel to the table top). The light should be aimed at the center of the globe, on the equator. The 200 cm mark is at the center of the globe (the figure is not to scale, as the front of the globe will be at approximately cm). 1. Set up the microscope lamp on a lab jack at the end of your table near the center aisle. Aim the lamp toward the wall of the room. Using a meter stick, measure four 50 cm intervals along the table from the lamp toward the wall, marking each interval with a small strip of masking tape. 2. Set up your globe at the fourth strip of tape (two meters from the lamp), propping up the base with a lab jack so that the equatorial plane is parallel to the table top (the globe axis should be perpendicular to the table top (see Figure one). 3. Adjust the lamp so that the light is hitting the globe in the center. The light axis should be parallel to the table top (not aimed up or down), and it should be exactly the same height as the equatorial plane of the globe (25-26 cm). There should be an even circular halo of light on the wall surrounding the globe s shadow. The lamp should be set at its maximum brightness, and your lux meter should be set at its highest sensitivity (1X). 4. Once your instructor has turned off the room lights, hold the light sensor of your lux meter 25 cm above the table, pointed toward the light. Take readings of the light intensity at each of the four tape lines on the table (50 cm, 100 cm, 150 cm, and 200 cm). Record these measurements in the appropriate column in table one on the answer sheet. Revised on 3/22/2005 Page 2 of 17

3 5. The cone of light produced by the source (your lamp in this case) spreads out in a linear fashion. Geometrically, the radius of the circle of illumination increases proportionally to the distance from the source. Since the area of the circle is proportional to the square of its radius, the light intensity should decrease proportionally to the square of the distance from which you take your measurement: (1) where Light (X 0 ) is the light intensity at a standard distance, Light (X) is the light intensity at distance X, and (X/X 0 ) is the ratio of the two distances. 6. We will use the lux value you have measured at a distance of 100 cm as our standard value. Write this value in the column labeled theoretical in table one. Using equation 1, calculate the theoretical lux values for 50 cm, 150 cm, and 200 cm and record these in the third column in table one. B. Effect of latitude and season on incoming solar radiation The sun is obviously brighter when it is directly overhead, around the middle of the day, and dimmer at sunset. Some of this is due to the fact that the light has to pass through more atmosphere at sunset (and at sunrise). More important is the fact that when the sun is lower in the sky its light is spread over a larger area of the Earth s surface. Our next task will be to determine the relationship between light intensity and incidence angle. 1. Hold the lux meter parallel to the surface of the globe (Figure Two). Take light measurements at the following latitudes: 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. Figure two. Set-up for the second exercise. For each measurement be sure that the light sensor on the lux meter is parallel to the surface of the globe at the latitude of interest. Revised on 3/22/2005 Page 3 of 17

4 2. Remove the lab jack supporting the globe so that the base lies flat on the table top, with the northern axis pointed away from the light (Figure Three). Take light measurements at the same set of latitudes: 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. Figure three. Set-up for the second part of exercise B. C. Albedo A portion of the incident solar radiation that enters Earth s atmosphere is reflected back into space by clouds, by dust particles in the atmosphere itself, or by the Earth s surface. This reflected light has no effect on the temperature of the Earth, since it is not absorbed in the surface or into the atmosphere. The percentage of light reflected varies depending on the properties of the material (a mirror, for instance, reflects 100% of incident light). The average reflected radiation divided by the average incident radiation is called the albedo: (2) The albedo is affected by the nature (color, texture, composition) of the surface that the incident radiation strikes. This experiment compares the albedo values of common earth materials and water. 1. The black and white aluminum pans will be used to simulate the earth materials over which the albedo is measured. The experimental setup is shown in Figure four. Place the aluminum pan inside a plastic tray to give it support and protect the table from spillage! Revised on 3/22/2005 Page 4 of 17

5 2. Set the microscope light up as shown in Figure Four, fixed at a 30 degree angle from the vertical, 50 cm from the surface of the pan. Figure four. Set-up for the albedo experiment. 3. Set up the white pan first (this pan mimics a snow-covered surface). Measure the incident radiation by holding the lux meter above the surface of the pan, aimed toward the light, and record this measurement in the table on the answer sheet. You need only measure the incident radiation once; it will be the same for all of the subsequent experiments. 4. Turn the lux meter over, aimed at the pan, and measure the reflected radiation. Record this measurement in the table on your answer sheet and calculate the albedo from equation 2. NOTE: Take all of your readings from the same spot. Make all measurements from the same height above the pan (at least 5-10 cm above the surface). When measuring reflected radiation, hold the lux meter so that you are not casting a shadow over the area you are trying to measure. 5. Repeat the experiment using the pan with the black bottom. Record this measurement in the table on your answer sheet and calculate the albedo from equation Fill the black pan with approximately 2 cm of water and repeat the experiment. Record this measurement in the table on your answer sheet and calculate the albedo from equation 2. Revised on 3/22/2005 Page 5 of 17

6 7. Finally, fill the black pan with ice (this mimics an ice-covered sea). Record this measurement in the table on your answer sheet and calculate the albedo from equation 2. C. Turbidity in Seawater One of the physical properties oceanographers measure in seawater is its turbidity. There are various factors that cause water to be turbid or "cloudy;" the most prevalent are suspended sediment or plankton. Early measurements of seawater turbidity were estimated by measuring the "depth of visibility" of disks lowered from the side of a boat (figure five). Since the "depth of visibility" depends somewhat on the size of the disk, a disk with a diameter of 20 cm was chosen as a standard (Secchi disk). The problem with Secchi disk measurements is that the results depend not only upon the turbidity of the water, but also on the sea state, altitude of the sun, eyesight of the observer, etc. In order to make the turbidity measurement more quantitative, a turbidity meter was developed that uses a standard light source and a photocell detector to determine the amount of light absorption and scattering over a fixed path length. These turbidity meters can be calibrated by measuring the light attenuation while at the same time comparing the turbidity value to water where the weight of suspended matter per liter is known. Unfortunately, calibration is not simple; and depends not only on the mass of the suspended material, but also on its size and composition! Figure five. Measuring turbidity in seawater. Turbidity and Secchi disk measurements 1. Fill an aquarium to the 10 cm mark with tap water. Affix a lux meter "shield" on the far end as shown in figure six. 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 (figure six). Record the point at which the Secchi disk is no longer visible (note: without sediment in the water you will see it all Revised on 3/22/2005 Page 6 of 17

7 the way back to the end of the aquarium) Enter the light attenuation value in Table four on the answer sheet. Figure six. Measuring turbidity in seawater. 2. Measure out three 1.00 gram samples of the powdered clay/silt into three separate weigh boats. Into the first 1.00 gram sample add a little Calgon from the squeeze bottle, then add the powdered clay/silt to the aquarium, and mix thoroughly (Be careful not to splash the microscope light or the lux meter!). At a water depth of 10 cm, 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." Then measure the light which passes through the 60 cm path with the lux meter. Also take a mini Secchi disk reading. Record the data in Table four. 3. Wet the second silt/clay sample with Calgon then add the sample to the aquarium, mix, and again measure and record the light attenuation and "depth of visibility." (This is equivalent to 100 mg/liter). 4. Wet the third silt/clay sample with Calgon and then add it to the aquarium, mix and repeat the experiment. 5. Plot the data on the graph provided on the answer sheet. 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. C. Light attenuation and settling rates Turbidity is a physical property of seawater that changes with time. For example, if a severe coastal storm causes flooding, river systems almost immediately transport suspended sediment into the estuaries, thereby raising the turbidity of the seawater. The amount of time the suspended sediment stays in the water column is a function of the sediment size and settling rate. Revised on 3/22/2005 Page 7 of 17

8 Phytoplankton are also affected by "sinking," which is a function of the size and density of the particular species. If the phytoplankton sink out of the photic zone, productivity ceases. To combat sinking, some phytoplankton species have developed adaptations to retard their settling rates (i.e., oils to reduce bulk density, appendages to increase surface area, etc.) 1. The aquarium still has 150 milligrams per liter of suspended sediment from the previous exercise. Turn on the microscope light, and re-suspend the sediment by stirring with the ruler. Wait 10 seconds for the surface of the water to stabilize, then begin recording the light intensity (lux) along the 60 cm path length at one minute intervals in the table on the answer sheet. 2. Fill in the suspended sediment column by estimating the correct values from the light intensity curve you prepared in the previous exercise. 3. Plot the suspended sediment concentrations as a function of time on the graph provided. Revised on 3/22/2005 Page 8 of 17

9 Name Lab Section Date. MS20 Laboratory: and Light Transmission Answer sheet: record all data using appropriate metric units (centimeters, grams, etc.). Remember to use significant figure rules and to indicate appropriate units (if the scale reads 13.4 g, your answer is not 13.4, but 13.4 g (or 13.4 grams). A. Measuring a solar constant Table one: Distance from light source Light intensity Measured Theoretical (equation 1) 50 cm 100 cm cm 200 cm Are your measured values of light intensity similar to your calculated (theoretical) values? List several possible sources of error. As stated in the introduction to the lab, the accepted value for the Earth s solar constant is 1366 W/m 2 (watts per square meter) or approximately 2 calories/(cm 2 x minute). The planet Venus is 72% of the distance from the Earth to the sun, and the planet Mars is 152%. Ignoring clouds and reflection (albedo), how much more sunlight might you expect to receive at the surface of Venus? How much less light might you expect to receive at the surface of Mars? Revised on 3/22/2005 Page 9 of 17

10 In our model, the light source is 200 cm from the globe, and the globe has a radius of 15 centimeters. A flea sitting on the globe is 15 cm closer to the light source at noon than he is at sunrise or sunset. How much does the light intensity drop as our flea is rotated from noon (position A) to sunset (position B; see figure)? The sun is 150 million kilometers from the Earth, and the Earth s equatorial radius is 6378 km. How much would you expect the sun s intensity to drop, based only on this additional distance? Revised on 3/22/2005 Page 10 of 17

11 B. Effect of latitude and season on incoming solar radiation Table two: Latitude of measurement Axis vertical Light intensity Axis tilted 90 N (north pole) 66.5 N (arctic circle) 45 N 23.5 N (T of Cancer) 0 (equator) 23.5 S (T of Capricorn) How do the measurements compare between latitudes when the tilt of the globe is changed? What day(s) of the year are represented by the globe with its axis vertical? What day(s) are represented by the globe with its axis tilted?. Compare the decrease in light intensity from the equator to 23.5 N with the decrease between 45 N and 66.5 N. Since the changes in latitude are similar, why is the change in intensity so much different? Revised on 3/22/2005 Page 11 of 17

12 In the northern hemisphere winter, Earth is near perihelion (closest approach to the sun), at 147 million km, while during the northern hemisphere summer Earth is at aphelion (furthest from the sun) at 152 million km. Why are the summers hotter than the winters in the northern hemisphere? C. Albedo Table three: Surface White Black Water Ice Reflected Radiation Albedo (equation 2) If you had a lava flow made of fresh, black basalt partially covering a light-colored granite surface, which rock would have the greater albedo? What happens to the radiation that is incident to the surface but is not reflected back into the atmosphere (hint: think about walking across an asphalt parking lot in bare feet in the summer.)? Revised on 3/22/2005 Page 12 of 17

13 Which surface was more reflective, the water or the ice? If continental glaciers and sea ice were to begin to expand on the Earth at the expense of rock and ocean, what effect might this have on global temperatures? Currently, as glaciers melt and sea ice retreats further into the arctic regions each year, what effect are we likely to see on global temperatures? C. Turbidity in Seawater Table four: Sediment concentration Light intensity (lux) Visibility limit (cm) 0 grams 60 cm 1 gram (50 mg/liter) 2 gram (100 mg/liter) 3 gram (150 mg/liter) Revised on 3/22/2005 Page 13 of 17

14 Which is the more accurate measure of turbidity: measuring light loss with the lux meter, or using the mini Secchi disk? Explain! Revised on 3/22/2005 Page 14 of 17

15 Suppose rather than fine grained clay/silt sized particles, you had added the same concentrations (50 mg/liter, 100 mg/liter, and 150 mg/liter) of sand sized particles to the aquarium. Would you expect the turbidity (i.e., light attenuation) to be the same, greater, or less? Explain! C. Light Attenuation and settling rates Table five: Time Light intensity (lux) Suspended sediment (g) 0 minutes 1 minutes 2 minutes 3 minutes 4 minutes 5 minutes 6 minutes 7 minutes 8 minutes 9 minutes 10 minutes Revised on 3/22/2005 Page 15 of 17

16 On the diagram below, sketch the earth's axis at the winter and summer solstice, and indicate at what latitude the solar constant is at maximum during these periods. Would the earth's albedo be higher or lower in the southern hemisphere compared with the northern hemisphere? Explain! (hint: consider the distribution of land mass/water in the two hemispheres) Revised on 3/22/2005 Page 16 of 17

17 From your laboratory experiments, can you tell if water with higher suspended sediment concentrations would heat up faster than clear water? Explain! (hint: consider the light absorption experiments, not the albedo!) Examine the Sediment Concentration vs. Time Graph below. This settling graph is similar to your laboratory experiment. Which of the two curves (A or B) represents the coarser sediment? Explain! Primary productivity is enhanced by light. The depth of the photic zone is measured by the depth of light penetration. Can productivity affect the depth of the photic zone? Explain! Revised on 3/22/2005 Page 17 of 17