Frederick Herschel was actually studying the sun when he noticed that different amounts of heat seemed to pass through the different colored filters that he was using on his telescope. Imagine his surprise when he discovered that the biggest change in temperature seemed to be caused by light that isn't even visible.
Herschel had discovered the existence of light rays that are not visible to the human eye. He called them calorific rays. Today they are referred to as infrared radiation - that is, radiation that is just beyond the red end of the spectrum. Herschel's discovery led to an entirely new branch of astronomy called infrared astronomy. In this field, scientists use special instruments to observe light from distant objects that is not visible to the naked eye. Temperature and Visible Light
In this activity, you will explore how different colors of light shining on a thermometer affect the temperature that it reports. The Gizmotm shows a tabletop upon which lies a thermometer and 16 cm ruler. At the left edge of the table is a prism, which separates white light into its component colors. Clicking Play ( ) causes sunlight to shine through the prism and onto the tabletop.
1. In the Gizmo, click Play to start the experiment. Drag the thermometer into the blue-green area of the spectrum where Position is about x = 8 cm. Note that every time you move the thermometer the Elapsed time at current position resets. Also notice that the rate of time passage in the Gizmo is faster than real time. 1. What do you observe about the temperature as the thermometer remains in this position? 2. Observe the temperature until the Elapsed time at current position reaches about fifteen minutes. Is the temperature still changing? What is the final temperature at x = 8 cm? Click Pause ( ). 3. Move the thermometer to a position slightly to the left of the original blue- green position, to about x = 7 cm. Click Play. What happens to the temperature at this position? What is the final temperature in this light- blue region? Which has a higher final temperature, x = 7 cm (light blue) or x = 8 cm (blue-green)? 4. Move the thermometer to a position slightly to the right of the original blue-green location, to about x = 9 cm. If necessary, click Play. What happens to the temperature at this position? What is the final temperature in this green region? Which has a higher final temperature, x = 9 cm (green) or x = 8 cm (blue-green)? 2. On a sheet of paper, draw a table like the one shown below. Record the Temperature in shade at the top of the table. Click Play. (Important: Do NOT click Reset ( ) during this activity!) Place the thermometer at about six or seven different positions within the spectrum of visible light and wait until the temperature stops changing in each location. Record the position, the color, and the final temperature for each position.
1 1. Which position has the highest final temperature? Which has the lowest? 2. Which color has a higher final temperature, red or blue? Is this what you would have guessed? Explain why. 3. Pick a new position between two adjacent positions that are included in your table. Based on your earlier findings, estimate what the final temperature will be at this position. Without clicking Reset, move the thermometer to this position and test your hypothesis. What did you find? Temperature and Light Beyond the Visible Spectrum
In this activity, you will test the regions just beyond the ends of the visible spectrum for the ability to affect temperature.
1. If the Gizmo is running, click Pause and then click Reset. Note that the Temperature in shade changes when the Gizmo is reset. Record the value for Temperature in shade in a table like the one shown below. After this, do NOT click Reset again until you complete this activity.
2. Place the thermometer in the green part of the spectrum, at around x = 10 cm, and click Play. Wait until the temperature stops changing then record the position of the thermometer, the color at that position and final temperature in your table. Repeat the process with the thermometer within the yellow part of the spectrum and then within the red part of the spectrum.
2 3. Now move the thermometer just beyond the red region of the visible spectrum, to about x = 13 cm. 1. What happens to the temperature at this position, just beyond the red region? Do you think Herschel was surprised when he observed this in his original experiment? Why or why not? Wait for the temperature to stop changing and record the final temperature for this position in your table. 2. What do you expect the temperature to be at about x = 14 cm? Move the thermometer to this position and observe the results. Is the final temperature what you expected it to be? Record this data in your table. 3. Repeat the process one more time with the thermometer at about x = 15 cm. How does the temperature at this position compare with the temperature in the shade? Suggest an explanation for this. 4. Repeat the data collection process for a position in the blue part of the spectrum, and then for a position in the violet part of the spectrum. Add your findings to your table. 5. Now move the thermometer just beyond the violet region of the visible spectrum, to about x = 3 cm. 1. Is the final temperature higher or is it lower than the final temperature in the violet region? How does the temperature at this position compare to the temperature in the shade? 2. Repeat the process one more time with the thermometer even further beyond the violet end of the spectrum, at about x = 2 cm. How does the temperature at this location compare to the temperature in the shade? Suggest an explanation for this. 6. Collect data for three more points in the middle of the visible spectrum and record your findings in the table. Then plot a graph with temperature on the vertical axis and position on the horizontal axis. Connect the points that you plotted with a smooth curve. 1. Describe the graph in the region that represents visible light. 2. What does the graph look like beyond the blue end of the visible spectrum (x < 4 cm)? 3. What does the graph look like beyond the red end of the visible spectrum (x > 12 cm)?
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