Lab04_light
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Santa Barbara City College *
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101
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Astronomy
Date
Apr 3, 2024
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docx
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Lab 4: Light, Color, and Temperature First:
Explain, in one sentence, what is meant by the Electromagnetic Spectrum. Part 1: Mixing primary colors of light. You now know from the lecture that the primary colors of LIGHT (
not paint, which is a pigment!
) are RED, GREEN, and BLUE, and that these three colors, in equal proportions, make WHITE light. Whatever your kindergarten teacher told you about the primary colors being red, yellow, and blue was WRONG. They were mixing up primary colors and primary pigments. Changing the proportions of RED, GREEN, and BLUE light, relative to the others, can make any color you want. This is how our eyes see color, and this is how color is created on
your computer and television and smart phone screens. Use the interactive color mixing demonstration HERE (
https://www.physicsclassroom.com/Physics-Interactives/Light-and-Color/RGB Color-Addition/RGB-Color-Addition-Interactive
) Try to make the following colors: 1. turquoise, 2. sea green, 3. pale orange, 4. medium brown, 5. gray, and 6. your favorite color To show how you made each color, fill in the following table with the percentages of Red, Green, and Blue for each color you made.
% Red % Green % Blue
Turquoise
59
100
100
Sea Green
45
75
62
Pale Orange
100
75
0
Medium Brown
60
44
17
Gray
39
44
52
Favorite Color
purple
100
59
100
Did anything surprise you? I don’t think that anything suprised me as I understood that more warmer colors took more red and green and cooler colors took more blue and green
Part 2: Understanding Color and Temperature Sunlight was first split into the rainbow of colors by…guess who? Sir Isaac Newton! In addition to figuring out his three laws of motion and how gravity works, he was the first person (in the west, in recorded history, that is) to split sunlight into the rainbow of colors and recombine the
colors back into white sunlight. We will need to understand a LOT about light in this course. Since we can’t play with lights and
spectroscopes in class, we will play with simulations. Goal: In this activity you will gain an understanding of how the spectrum of radiation given off
by a heated body (star, light bulb, planet, living being) is dependent on its temperature. 1. Look up the definition of the term BLACK BODY RADIATION. Write it down: Black-body radiation is the thermal electromagnetic radiation within, or surrounding, a body in
thermodynamic equilibrium with its environment, emitted by a black body
2. Go to this URL: https://phet.colorado.edu/sims/html/blackbody-spectrum/latest/blackbody
spectrum_en.html
This simulation allows you to change the temperature of an object and see the spectrum of the
light it puts out. The peak in the spectrum – the wavelength where the intensity of the light is maximum – tells you the temperature of the surface of that object.
Click on “Graph values” and “Labels” to turn on the labels. 1. The simulation opens up to the Sun. a. What is the temperature of the surface on the Sun, as indicated by the thermometer on the
right side of your screen? 5800K
b. At what color is the peak of the light spectrum? green
c. The horizontal scale tells you wavelength. The authors of this simulation have chosen micrometers (µm) for the unites. 1 µm = 10
-6 m = 10
3
nm. What is the wavelength where the peak of the light output is? 0.5um
2. Lower the thermometer to the temperature of a light bulb, using the slider. a. What is the blackbody temperature of a light bulb, as read on the thermometer? 3000K
Use the + and – symbols for the vertical and horizontal axes to change the scale, so that you can
see the peak of the spectrum better.
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b. At what part of the spectrum does the peak output of a lightbulb occur – visible, infrared
(longer wavelength than red) or ultraviolet(shorter wavelength than violet)? longer wavelength
c. What is the wavelength of the peak in radiation, approximately? Read this on the horizontal
axis. 0.9um
3. Lower the thermometer to the temperature of the Earth. a. What is the blackbody temperature of Earth? 250K
Use the + and – buttons on the horizontal and vertical axes until you can see the spectrum. b. Where is the peak of the Earth’s spectrum? there is no peak as it is a straight line on the graph
c. What is the wavelength of the peak of the Earth’s black body spectrum?
I believe it is longer wavelength because it is a straight line 4. Lastly, move the slider on the side of the thermometer up to Sirius A. Sirius A is a white
dwarf. a. What is the surface temperature of Sirius A? 10000K
b. Where is the peak of its spectrum – visible, infrared, or ultraviolet? shorter wavelength
c. What is the wavelength of the peak of the black body spectrum of Sirius A?
0.5um
Test your understanding by answering the following questions, based on your observations in these simulations. 1. Which is hotter, a blue star or a red star? blue
2. Is a star ever too hot to see at all, even if the peak of its spectrum is in the ultraviolet? Explain your reasoning based on what you observe in the simulation most of the time the hotter the star is the more lght they emmit meaning they would been more visable
3. The surface temperature of an average mammal like you is around 300 Kelvin. Slide the temperature slider so that the thermometer reads 300K. What is the peak wavelength in microns? there is no peak because it is a straight line
What portion of the electromagnetic spectrum is this in? none because it is a straight line at the very bottom
4. Move the temperature slider to 3000K, the temperature of the surface of a red giant or red
dwarf star. In what portion of the electromagnetic spectrum is the peak of its spectrum? red
What is the peak wavelength?
0.9um
What color would the star appear (look at the color of the star at the top of the screen).
orange
5. What is the color of a star with a surface temperature of 7000K? At what wavelength
blue
and spectral color is the peak of its blackbody spectrum?
Blue/Purple
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