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!

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

Does light travel instantaneously (i.e., is speed of light infinite)?

Speed of light

Orbital periods of Galilean Moons

Speed of Light : Astronomical Measurements

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.

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.

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!

Romer’s Observation

Figure 5-1JupiterSun

EarthEarth

Jupiter

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.

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)

Fizeau-Foucault Measurement

It was known that sunlight passed through a prism “creates” a rainbowhowever 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.

Newton’s experiment

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

William Herschel’s experiment in 1800s

invisible form of energy beyond the red end of a spectrum

infrared radiation

1888 : Hertz radio waves1895 : Roentgen X-ray

Visible light and beyond

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)

Wave : wavelength, frequency, speed

Speed of light (c = 300,000 km/sec)

c = frequency × wavelength

c = ν × λ

low frequency

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 : 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

Blackbody Curves

Wien’s Law of Blackbody

λmax ≈ 1 / Temperature

Stefan-Boltzman LawFlux (brightness of an object) is proportional to Temperature

Flux ≈ Temperature4

Sun : 6000°K, Earth : 300°K

Sun is only 20 times hotter than EarthBut, 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 ConceptsSpeed of light (delay in timings of

Galilean moons’ eclipses)Frequency and wavelengthBlackbody Laws (Wien’s and

Stefan-Boltzaman)

Important TermsSpectrumElectromagnetic waveElectromagnetic radiationLight

• visible• infrared, ultraviolet• microwaves, radio waves• X-ray, gamma ray

Blackbody

Chapter/sections covered in this lecture : sections 5-1 through 5-5