light and continuous spectra. energy production from the sun: the sun dominates the energy...
TRANSCRIPT
Light and Continuous Spectra
Energy Production from the Sun:
The Sun dominates the energy ‘budget’ of the solar system
• How much energy does the Sun produce?• How does the energy reach us?
• How does it produce that energy?
Light properties
• Energy• Power• Intensity• Wavelength• Frequency• Speed
Which of the following is NOT a unit of energy?
1. Joule2. Kilowatt3. Kilowatt-hour4. Electron volt
Watt’s the difference between energy and power?
• Consider the units: a Joule (J) is a unit of energy, and a Watt (W) is a unit of power
• 1 W is defined as 1 J/s (Joule per second)• Thus, power is a rate at which energy is
delivered• Example: your “power” bill for electricity is
based on the number of kiloWatt-hours you consume. So is the utility charging you for power or for energy?
“Power” bills
• So when a utility charges you for a kW-hr:• 1 kW = 1000 W; 1 hr = 3600 s• A kW-hr can be converted into an equivalent
in Watt-seconds• Recall that 1 W is defined as 1 J/s, so 1 J = 1
Ws• In other words, your utility is charging you for
energy usage, not power. It’s not a power company
Intensity
Intensity is a power density.Units: W/m2 = W m-2
A
Area
PowerIntensity
Solar IntensityThe Solar Energy Output is 4 x 1026 W.
How much of that hits us?
When the Sun is directly over head, it delivers the equivalent of 22 × 60 watt light bulbs over each square meter (m2) of ground!!!
This amount, 1340 W m-2, is known as the solar constant
How is solar energy delivered from the Sun to the Earth?
As light!!!!
Is the intensity of light reaching the surface always 1340 W/m2?
1. Yes2. No
Electromagnetic Wave: propagating wave of electric and magnetic fields that oscillate perpendicular to each other and the direction of propagation
Electromagnetic Wave
Electric field
Magnetic field
In a vacuum, wave propagates with speed = 3 x 108 m/s(cosmic speed limit)
Wave Properties
frequency (f or ν (nu)): number of peaks that pass a location in a given time (units: Hertz (Hz) = 1/s = s-1)
speed (v): how much distance the wave moves per unit time(for an EM wave v = c = 3 x 108 m/s)
Wave Propertieswavelength (): distance between two consecutive peaks
(units: km, m, cm, mm, m, nm…)
amplitude: height of the wave (or depth of the trough); related to intensity but we won’t use it
Wave Propertiesspeed (v): how much distance the wave moves per unit time
(for an EM wave v = c = 3 x 108 m/s)
frequency (f): number of peaks that pass a location is a given time (units: Hertz (Hz) = 1/s = s-1)
wavelength (): distance between two consecutive peaks (units: km, m, cm, mm, m, nm…)
These three properties are related:
c
f
If wavelength is 10 m and frequency is 100 Hz (oscillations / seconds), what would be the speed of the wave?
1. 10 m/s 2. 1 m/s 3. 1000 m/s 4. 100 m/s
If wavelength is 10 m and frequency is 100 Hz (oscillations / second), what would be the speed of
the wave?
m/s 1000
m 10s 100 1-
fv
vf
The PhotonLight behaves like both a particle and a wave!
Photon: smallest bundle of light energy (i.e., a particle of light)
Photons carry light energy:1. A photon’s energy is proportional to frequency
(Eph f).• A photon’s energy is inversely proportional to wavelength (Eph
-1).
hc
hfEph Plank’s constant (h) = 6.602 x 10-34 Js
Matter actually a wave too!
• All matter exhibits particle and wave properties (Louis de Broglie, 1921)
• For ordinary objects, the wave nature of matter is much too small to measure– The wavelength of a baseball
moving at 80 mph would be about 10-34 meters
• But for small particles, this is wave nature of matter is measurable– The wavelength of an electron is
about 10-10 meters
Electron diffraction pattern showing its wave nature
How is a difference in the frequency or wavelength of light observed?
The Visible Spectrum:
For visible wavelengths COLOR
How does a prism work?• Dispersion: Speed of light in
the prism (glass or plastic) depends on the frequency (color)
• Refraction: Change in speed of light causes a change in its direction
• Result: Blue changes direction most since its speed is the lowest inside the prism. And red changes direction least since its speed is highest inside the prism.
Red R Orange O Yellow Y Green G Blue B Indigo IViolet V
Herschel Thinks Outside the Box:In 1800 William Herschel made a discovery when he tried to determine the
temperature of light.
• He noticed that a thermometer recorded energy from the Sun`s spectrum even when placed beyond the red end of the visible rainbow.
•He called this emission Calorific Rays and it was the first discovery that light had colors invisible to the human eye.
•These rays are known today as infrared light.
Herschel’s work color is associated with a temperature
Visible light is just a small part of the electromagnetic (EM) spectrum
Why are we spending so much time discussing the electromagnetic spectrum?
We rely on remote sensing of EM radiation.
Tells us the temperature and composition
This gives us important clues to the origins of these objects.
Not easy to visit astrophysical objects (the Sun, planets, other stars) and make direct in situ measurements
When we look at the spectrum of the Sun, we see a distinct distribution of colors.
The Solar Spectrum:
Other stars have similar patterns as do most hot objects. The main difference is where the peak color is.
Gustav Kirchoff (1862) called this kind of emitter a ‘blackbody’.
Ideal Blackbodies Blackbody: an object that radiates energy into space in a manner that is
characteristic only of the temperature of the radiator.
Wavelength (nm)
Characteristics of Black Bodies1. Characteristic I vs. curve (shown left)
(all blackbodies radiate in more than one color)2. Curve is related to T only3. Increasing T, increases total intensity4. As T increases, peak moves to lower 5. Wavelength of peak intensity related only to T. Wilhelm Wien (1893) describes the relationship mathematically (now called Wien’s Law), where is measured in nanometers and T is measured in Kelvins:
Calculating The Sun’s Temperature
So how well does this work?
Given Tsun = 6000 K
Which matches the observed solar output curve pretty well.
Ideal Blackbodies
Wavelength (nm)
Amount of light radiated by an object is given by the Stefan-Boltzmann Law (Josef Stefan, 1879; Ludwig Boltzmann, 1884):
Where L is the luminosity (power) emitted by the object (in Watts), T is its temperature (in Kelvins), and is the Stefan-Boltzmann constant, which has the value 5.67 x 10-8 Wm2K4
Visible InfraredUV
Stars may be considered to act like blackbodies
If star A has a surface temperature of 9000 K and star B has one of 3000 K, what is the ratio of the power output of the stars (PowerA/PowerB)?
1. 812. 23. 94. 35. 16
SolutionIf star A is 9000 K and star B is 3000 K, what is the ratio of the power output of the stars (PowerA/PowerB)?
If star A has a surface temperature of 6000 K and star B has one of 3000 K, which star has a longer peak wavelength output?
1. Star A2. Star B
Solution
Since
The colder star, star B, has the longer wavelength.
If star A is 6000 K and star B is 3000 K, which star has a longer peak wavelength output?