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# Light - PowerPoint PPT Presentation

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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### Light

Lecture 9

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

• 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!

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

Orbital periods of Galilean Moons

• 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.

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!

Jupiter

Sun

Figure 5-1

Earth

Earth

Jupiter

Fizeau Jupiter’s moons earlier than expected.-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 Jupiter’s moons earlier than expected.

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? Jupiter’s moons earlier than expected.

diffraction of waves

Young’s Double-slit experiment Jupiter’s moons earlier than expected.

• Light : Is it a particle or what?

• Young’s experiment showed that Light is a wave!

Maxwell : light = electromagnetic wave Jupiter’s moons earlier than expected.

• 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 Jupiter’s moons earlier than expected.

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

Wave : wavelength, frequency, speed Jupiter’s moons earlier than expected.

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

low frequency

high frequency

All object emits EM Jupiter’s moons earlier than expected.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 Jupiter’s moons earlier than expected.

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

 Stefan-Boltzman Law

A hotter object emits radiation at shorter wavelength than a cooler object.

 Wien’s Law

Wien’s Law of Blackbody Jupiter’s moons earlier than expected.

λmax ≈ 1 / Temperature

Stefan- Jupiter’s moons earlier than expected.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 Jupiter’s moons earlier than expected.

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… Jupiter’s moons earlier than expected.

Important Concepts

Important Terms

Spectrum

Electromagnetic wave

Light

visible

infrared, ultraviolet