Download Life in the Universe

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
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
Light: a tool to understand the Universe
 Early 1800s, the French philosopher (founder of sociology), Auguste Comte claimed
that humanity will never know about the nature and composition of stars because
stars are so far away…
 Only thing we can “see” from stars is their light.
How can one understand properties of stars (such as temperature, chemical
composition, relative speed against Earth, etc.) solely based on star light?
 By understanding basic properties of light, we can do!
Speed of light
Does light travel instantaneously (i.e.,
is speed of light infinite)?
 Galileo tried to measure the speed of light




on two hilltops with a known distance
shuttering a lantern at the flash of light
using his pulse as a timer
measuring the time b/w his opening the
shutter and seeing the light from his assistant
 by changing distance b/w him & his assistant,
he tried to see an increasing time span with
distance
failed to see the positive relation b/w time
and distance
speed of light is too high to measure
Speed of Light : Astronomical Measurements
Orbital periods of Galilean Moons
Io
1.77 days
Europa
3.55 days
Ganymede
7.16 days
Callisto
16.69 days
 Jupiter and its 4 largest moons over 7 hours
of observation
 We can see eclipses of moons, and their
periods (i.e., one eclipse to next) are
constant.
Romer’s Observation
In 1676, a Danish astronomer, Olaus
Romer, 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.
When Earth is
near Jupiter, we
observe eclipses
of Jupiter’s moons
earlier than
expected.
Sun
Earth
Figure 5-1
Jupiter
Earth
Jupiter
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!
When Earth is far
from Jupiter, we
observe eclipses
of Jupiter’s moons
later than
expected.
Fizeau-Foucault Measurement
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)
Newton’s experiment
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.
What is light? A particle or what?
diffraction of waves
Young’s Double-slit experiment
 Light : Is it a particle or what?
 Young’s experiment showed that Light is a wave!
Maxwell : light = electromagnetic wave
 Maxwell later showed that light is in fact two
waves (magnetic and electric) packed into one
 light = electromagnetic wave = electromagnetic radiation
Visible light and beyond
William Herschel’s experiment in 1800s
in 1-2 minutes
invisible form of energy beyond the red end of a
spectrum
infrared radiation
1888 : Hertz  radio waves
1895 : Roentgen  X-ray
Wave : wavelength, frequency, speed
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)
low frequency
c = frequency × wavelength
c=ν×λ
high frequency
All object emits EM wave (light)
 The appearance of a heated bar of iron changes with temperature.
• As the temperature increases, the bar glows more brightly because it radiates
more energy.
• The color of the bar also changes because as the temperature goes up, the
dominant wavelength of light emitted by the bar decreases
Blackbody Curves
Blackbody : an idealized type of object
that does not reflect any light. A
perfect blackbody absorb all light
falling on it ( “black”)
Blackbody radiation
• A hotter object emits light more
intensely than a cooler object
 Stefan-Boltzman Law
• A hotter object emits radiation at
shorter wavelength than a cooler
object.
 Wien’s Law
Wien’s Law of Blackbody
λmax ≈ 1 / Temperature
Stefan-Boltzman Law
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)
Skip section 5-5
The concept of dual nature of light both as a particle and wave.
Photo-electric effect!
This concept is complicated and is not required in later chapters.
So, you can just skip section 5-5 and Box5-3.
In summary…
Important Concepts
Important Terms
 Speed of light (delay in timings of
Galilean moons’ eclipses)
 Frequency and wavelength
 Blackbody Laws (Wien’s and
Stefan-Boltzaman)
 Spectrum
 Electromagnetic wave
 Electromagnetic radiation
 Light
• visible
• infrared, ultraviolet
• microwaves, radio waves
• X-ray, gamma ray
 Blackbody
Chapter/sections covered in this lecture : sections 5-1 through 5-5