Astronomy The Nature of Light

Transcription

1 Astronomy The Nature of Light A. Dayle Hancock Small 239 Office hours: MTWR 10-11am Measuring the speed of light Light is an electromagnetic wave The relationship between Light and temperature an object The relationship of energy and temperature from an object Wave particle duality Each element has a 'fingerprint' Quantum rules of atoms How atoms emit light How motion of an object changes light (Doppler shift) physics.wm.edu/~hancock/171/ Page 1

2 The Speed of Light Using lanterns and an assistant on a distant hill, Galileo could not tell if light traveled instantaneously or with a finite speed. RØmer in 1676 showed that light had a finite speed. By carefully studying the eclipses of a moon (Io) of Jupiter he showed the timing of the eclipses varied by the distance between Earth and Jupiter. 2

3 The Speed of Light In 1850, Fizeau & Foucault measured the speed of light. They used a rapidly rotating mirror and observed the angle of deflection. With the path lengths and the rotation speed the speed of light can be calculated. The speed of light in vacuum is: c = x 108 m/s 3.0 x 108 m/s Nothing can travel faster than the speed of light. 3

4 Historical views Newton thought light consisted of particles ('corpuscles') Later, Thomas Young ( ) and others showed that light behaves like a wave Interference patterns: light and dark bands Page 4

5 Some General Properties of Waves The waves travel, but the matter does not travel Energy is transported by the waves Page 5

6 Anatomy of a wave AMPLITUDE WAVELENGTH (λ) SPEED (v) FREQUENCY (ν) height of the disturbance distance from peak to peak speed at which any peak travels (For light v = c!) number of full waves passing a point in one second. Page 6

7 Speed, Frequency and Wavelength The speed, frequency and wavelength of a light wave are related by: v=c=λν For an FM radio broadcast the frequency is 100 MHz (1.0 x 108 Hz). As we will see radio waves are a type of 'low frequency light waves'. The wavelength is then: λ = c/ν = (3.0 x 108 m/s) / (1.0 x 108 s-1) = 3.0 m 7

8 Light is an Electromagnetic Wave Electric and magnetic fields are associated with electric and magnetic forces. In the 1860's, James Maxwell showed that light is an electromagnetic wave. Heinrich Hertz later showed the validity of Maxwell's equations by generating and detecting radio waves. The wavelength determines the type of electromagnetic8 wave.

9 Electromagnetic Spectrum The wavelength (λ ) determines the type of electromagnetic wave. The wavelength of radio waves is very long. AM radio wave are about 300 m. Gamma rays can be as short as m. Visible light has a wavelength of nm. The wavelength of visible light determines 9 the color.

10 The Wavelength and Frequency of Light The wavelength of orange light is about 600 nm (6.0 x 10-7 m). Using the relationship between speed, frequency and wavelength: c=λν We have: ν = c/λ = (3.0 x 108 m/s) / (6 x 10-7 m) ν = 5 x 1014 Hz! 10

11 Light from the Sun has many wavelengths mixed together and appears WHITE The wavelengths can be split apart using a prism or diffraction grating Page 11

12 Heat and Temperature Heat is the energy contained in an object because of the (random) motion of the atoms (or molecules). Temperature is a measure of the average kinetic energy of the atoms or molecules. All the atoms or molecules do not move with the same speed. The atoms or molecules have a broad distribution of speeds in random directions. 12

13 Temperature Scales In science, we normally use the Kelvin scale for temperature. 0 K is absolute zero where (almost) all the motion of the atoms or molecules stop. One Kelvin degree is the same temperature change as one Celsius degree. Ice freezes as 0o C, 32o F or 273 K. Water boils at 100o, 212o F and 373 K 13

14 Heat and Radiation An object with a temperature 0 K radiates electromagnetic radiation. The higher the temperature, the more total radiation is emitted. The higher the temperature, the shorter peak in the wavelength. 14

15 Heat and Radiation Some objects (at the same temperature) emit radiation better than others. A perfect emitter is called a 'blackbody'. The object does not have to be black to be a blackbody The Sun is a blackbody emitter of radiation at 5800 K. 15

16 Wein's Law The wavelength of the peak intensity of a blackbody curve is given by Wein's Law λmax = K m / T Where T is in kelvin and the wavelength is in meters. 16

17 Intensity vs Temperature Stefan-Boltzmann Law Higher temperature (T) More energy output Area under curve in the plot F is the energy flux of the object Power = Energy/time F = power/area σ is the Stefan-Boltzmann constant F = σ T4 σ = 5.67 x 10-8 W/m2K4 σ is a lower case sigma 17

18 Energy Flux from the Sun The energy flux (or luminosity) from the Sun at the the Earth Is 1.37 kw/m2 18

19 A Surprise about Light 'Waves' In 1905 along with the famous papers on special relativity, Albert Einstein published a paper on the 'photoelectric effect'. To account for the this effect he proposed that light come in small packets that had many of the properties of a particle! We call these wave packet particles 'photons'. These light particles still have a wavelength and frequency. The travel at 3 x 108 m/s (speed limit for the universe). 19

20 Light is also a particle a wavepacket' a PHOTON is a particle of light a quantum of light Energy carried by a photon hc E= λ Planck's constant extremely tiny h = x J-s Energy from light is quantized in chunks of hc/ λ Page 20

21 Spectra An Important Tool in Astronomy

22 The Light from different Elements Heat up an a chemical substance and 'split' the wavelengths with a prism and a series of spectral lines will appear. This bright emission lines identify the chemical elements. Helium was discovered on the Sun from its spectral lines before it was found on Earth. 22

23 Each Element has a 'Fingerprint' 23

24 Types of spectra A. Continuous Spectrum Made from opaque thermal objects obey Wien & Stefan-Boltzmann laws apply Kirchoff's law 1 B. Emission spectrum C. Absorption spectrum Fingerprint of an element emitted directly from a gas (here- hydrogen gas) Kirchoff's law 2 Continuous spectrum absorbed by a gas (here hydrogen gas) Kirchoff's law 3 Page 24

25 Examples of Spectra Emission line spectra of several elements (their 'fingerprints') Emission line spectrum of the Orion nebula (UV) Page 25

26 The Suns Spectrum We can learn a tremendous amount about the chemical composition in the solar atmosphere by the characteristic spectral patterns 26

27 Blue Sky and Red Sunsets The molecules of O2 and N2 that make up the atmosphere are 1 nm. Visible light has a wavelength of nm. When a wave scatters from a much small object, the shorter wavelengths scatter more. This is know as Rayleigh scattering. Thus blue light scatters better than red light in the atmosphere. This blue scattered light makes the sky blue. So much light can be scatter at sunset that the sunset can appear red. 27

28 Atoms and Light All of the known matter is made of atoms. In the Rutherford model, atoms consist of a very small nucleus with 'orbiting' electrons. The nucleus contains protons ( + charge) and neutrons ( no charge). The protons electrostatic repulsion is overcome by the 'strong force' or nuclear force. The number of protons (atomic number) determine the number of electrons to make the atom neutral. The number of electrons determine the chemistry of the element. The number of 28 neutrons and protons determine the atomic weight.

29 Periodic Table of the Elements Chemical properties of elements are similar in each column 29

30 Bohr's Model of Hydrogen Niels Bohr in 1913 proposed a model of the atoms where the electrons 'orbit' only at certain radii labeled n=1, n=2, n=3 etc. Each orbit corresponds to the electron having a certain energy. The electron can only 'orbit' at certain radii. The model was 'ad hoc' and violated ideas from classical physics. It only worked well with the simplest atom (hydrogen). It did explain atomic spectra of simple atoms and was the first step to quantum mechanics. 30

31 Bohr's Model of Hydrogen When a photon of light with the correct wavelength (and energy) is absorbed by a hydrogen atom, it can cause an electron at the correct radius (an energy) to 'hop' to a higher energy level (and large radius) 31

32 Bohr's Model of Hydrogen When an electron is in an energetic excited state (large radius), it can fall or 'hop' down to a lower energy and smaller radius. In the process, the atom emits a photon of light. 32

33 Spectra of Hydrogen The visible spectrum of hydrogen is know as the Balmer series. By trail and error, Balmer worked out the wavelengths wavelengths as: =R ( 2 ) λ 4 n Where R is the Rydberg constant (R = x 107 m-1) and n is an integer (1, 2, 3, etc) 33

34 Bohr formula Balmer, Lyman & Paschen Bohr's model gives the correct formula for the Lyman spectrum (ultraviolet with N =1), Balmer spectrum (visible with N=2 and Paschen spectrum (infrared with N=3) =R ( 2 2 ) λ N n 'ev' is an energy unit 1 ev = x joules 34

35 The Doppler effect: Moving source of waves Page 35

36 Perceived Doppler shift For sound Shorter wavelength is higher pitch Receding is longer wavelength Approaching is shorter wavelength Longer wavelength is lower pitch For light Approaching is shorter wavelength Shorter wavelength is bluer Receding is longer wavelength Longer wavelength is redder 36

37 Doppler shift For a moving wave source V 0 VR 0 VR is the relative velocity How fast it is coming or going from you V is the velocity of the wave Speed of sound or speed of light (c) λo is the original (lab or stationary) wavelength λ is the observed wavelength Δλ is the change in wavelength For Light (c=v) (observed original) c 0 VR VR c 0 Page 37

38 Example: Doppler shift used to measure velocity A green laser has a wavelength of about 510nm If we see a green laser from the Andromeda galaxy it would have a wavelength of nm How fast are we moving relative to Andromeda? Δλ = λ λ0 = nm 510nm = 0.5nm vr = Δλ/λ c = 0.5nm/510nm m/s = m/s = 294 km/s Useful units: 109 nm = 1 m & 106 μm = 1 m a nanometer is the size of a molecule What does the sign mean? Page 38

39 Doppler shift and a rotating body l 39

40 The effect of temperature on spectral lines: Broader = hotter Temperature records amount of random motion in the molecules Different Doppler shifts for different emitting photons Some towards you Some away from you Cold cloud in space Makes lines broader Hot clouds usually are too hot for molecules Hot cloud in space Only atoms is a clue also Page 40