Light

1. Light Lecture 9

2. 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

3. 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!

4. 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

5. Speed of Light : Astronomical Measurements 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.

6. 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

7. 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)

8. 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.

9. What is light? A particle or what? diffraction of waves

10. Young’s Double-slit experiment • Light : Is it a particle or what? • Young’s experiment showed that Light is a wave!

11. 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

12. Visible light and beyond William Herschel’s experiment in 1800s invisible form of energy beyond the red end of a spectrum • infrared radiation 1888 : Hertz  radio waves 1895 : Roentgen  X-ray in 1-2 minutes

13. 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) c = frequency × wavelength c = ν × λ low frequency high frequency

15. 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

16. 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

17. Wien’s Law of Blackbody λmax ≈ 1 / Temperature

18. 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)

19. 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.

20. In summary… Important Concepts Important Terms Spectrum Electromagnetic wave Electromagnetic radiation Light visible infrared, ultraviolet microwaves, radio waves X-ray, gamma ray Blackbody • Speed of light (delay in timings of Galilean moons’ eclipses) • Frequency and wavelength • Blackbody Laws (Wien’s and Stefan-Boltzaman) • Chapter/sections covered in this lecture : sections 5-1 through 5-5