Light . Lecture 9. By reading this chapter, you will learn. 5-1 How we measure the speed of light 5-2 How we know that light is an electromagnetic wave 5-3 How an object’s temperature is related to the radiation it emits
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5-1 How we measure the speed of light
5-2 How we know that light is an electromagnetic wave
5-3 How an object’s temperature is related to the radiation it emits
5-4 The relationship between an object’s temperature and the amount of energy it emits
5-5 The evidence that light has both particle and wave aspects
How can one understand properties of stars (such as temperature, chemical composition, relative speed against Earth, etc.) solely based on star light?
Does light travel instantaneously (i.e., is speed of light infinite)?
Orbital periods of Galilean Moons
When Earth is near Jupiter, we observe eclipses of Jupiter’s moons earlier than expected.
When Earth is far from Jupiter, we observe eclipses of Jupiter’s moons later than expected.Romer’s Observation
In 1676, a Danish astronomer, OlausRomer, found that the moment of Jovian moon’s eclipse gets delayed by up to 16.6 minutes.
timing of eclipses depends on the relative positions of Jupiter and Earth
If light needs time to travel from Jupiter to Earth, this variation of eclipse timings can be naturally explained.
Using the modern distance of 1AU, Romer’s method could yield the correct speed of light. But, the distance (1AU) was not accurately known in his time.
Light does not travel instantaneously!
By knowing the speed of rotating mirror, they could measure the speed of light very precisely
Speed of light now is a constant : c = 299,792.458 km/sec)
It was known that sunlight passed through a prism “creates” a rainbow
however it was believed that the prism somehow adds a color
Newton’s experiment showed that the color of the spectrum is intrinsic to the sunlight.
diffraction of waves
waves (magnetic and electric) packed into one
light = electromagnetic wave = electromagnetic radiation
William Herschel’s experiment in 1800s
invisible form of energy beyond the red end of a spectrum
1888 : Hertz radio waves
1895 : Roentgen X-ray
in 1-2 minutes
Wavelength (λ): the distance b/w successive crests of a wave.
Frequency (ν) : number of waves in a unit time (Hertz = waves / second)
Speed of light (c = 300,000 km/sec)
c = frequency × wavelength
c = ν × λ
Blackbody : an idealized type of object that does not reflect any light. A perfect blackbody absorb all light falling on it ( “black”)
A hotter object emits light more intensely than a cooler object
A hotter object emits radiation at shorter wavelength than a cooler object.
λmax ≈ 1 / Temperature
Flux (brightness of an object) is proportional to Temperature
Flux ≈ Temperature4
Sun : 6000°K, Earth : 300°K
Sun is only 20 times hotter than Earth
But, it is 160,000 times brighter (60004 / 3004)
The concept of dual nature of light both as a particle and wave.
This concept is complicated and is not required in later chapters.
So, you can just skip section 5-5 and Box5-3.
microwaves, radio waves
X-ray, gamma ray