It s a wave. It s a particle It s an electron It s a photon. It s light!

Transcription

1 It s a wave It s a particle It s an electron It s a photon It s light!

2 What they expected Young s famous experiment using a beam of electrons instead of a light beam.

3 And, what they saw

4 Wave-Particle Duality Young s famous experiment using a beam of electrons instead of a light beam. Particles (electrons) are behaving like waves and exhibiting interference patterns! This screen is like a TV screen and glows whenever an electron strikes it. simulation/quantum-waveinterference

5 Photons as Particles If light behaved only as a wave, even if it were dim it would fully cover the screen. Because it behaves like a particle, we see partial coverage from a dim source.

6 If particles can behave like waves, can waves behave like particles? YES! Wave-Particle Duality: Waves can exhibit particlelike characteristics, and particles can exhibit wavelike characteristics. Light is a wave, but it is also a particle. Experimental evidence shows that light consists of particles, called photons.

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10 Color and the EM Spectrum EM radiation carries energy through space known as radiant energy. Light waves exhibit all characteristics of waves: Diffraction, refraction, interference. Frequency and wavelength are the only variables for EM waves in a vacuum. We only see a narrow segment of EM.ג spectrum, from about 400nm to 700nm 1nm = 1 x 10-9 m

11 Wavelength vs Energy

12 Relationship between energy and frequency We see things glow a color when hot. Things emitting energy give off radiation and this is sometimes visible. The relationship between temperature, intensity and wavelength of light emitted stumped physicists until the early 1900 s. Along came Max Plank who made the following assumption: Energy can be released or absorbed by atoms only in chunks of some minimum size. Plank gave the name quantum (meaning fixed amount) to the smallest quantity of energy that can be emitted or absorbed as EM radiation.

13 Light of frequency f can be regarded as a collection of discrete packets of energy (photons), each packet containing an amount of energy E: E photon = h f E = energy of a photon in Joules h = Planck s Constant = 6.63 X J s f = frequency of the e-m wave or photon E = N h f for a number, N, of photons. Ex: 1h, 2h, 3h etc Energy is always absorbed or emitted in whole number multiples of h.

14 In other words Energy does not get released in a constant, continuous stream, instead energy is released in packets or quantums at a certain frequency. A laser emits a specific of light which is called monochromatic. Light bulbs emit a range of s and are not.

15 In converting electrical energy into light energy, a 60-watt light bulb operates at about 2.1% efficiency. Assuming that all the light is green light (vacuum wavelength = 555 nm) (a) determine the amount of energy emitted by a single photon, and (b) determine the number of photons emitted per second.

16 And the answer is (a) E = hf = h(c/ ) = =(6.63 X J s)(3 X 10 8 m/s) / (555 X 10-9 m) = 3.58 X J = the amount of energy emitted by one photon (b) light nrg emitted per second (by the 60 W light bulb at 2.1% efficiency) = (60.0 J/s)(.021) = 1.3 J/s (1.3 J/s)(1 photon / 3.58 X J) = = 3.6 X10 18 photons/second = the number of photons emitted per second.

17 Momentum, wavelength and de Broglie de Broglie suggested that all matter could be viewed as having wave properties. All particles -electrons, protons, atoms, baseballs, and even humanshave a wavelength that is related to the momentum of the particle by Wavelength = h / momentum, or = h / p p = h / p = h / =h f/c A particle moving with momentum p can be determined with this relationship. This wavelength is called the de Broglie Wavelength. A bullet of mass 0.02 kg traveling at 330 m/s, for example, has a de Broglie wavelength of h/(mv)=10-34 m. A million million million millionth the diameter of a hydrogen atom. An electron traveling at 2% the speed of light has a wavelength of m, which equals the diameter of a Hydrogen atom. Diffraction effects for electrons are measurable, but diffraction effects for bullets are not.

18 Photoelectric Effect Einstein was awarded the Nobel Prize for determining the photoelectric effect. The photoelectric effect is the ejection of electrons from certain metals when light falls upon them. It was experimentally found that high frequency light, even from a dim source, was capable of ejecting electrons from a photosensitive metal surface; yet low frequency light, even from a very bright source, could not dislodge electrons from that same surface.

19 Photoelectric Effect The photoelectric effect is the name given to the observation that when light is shone onto a piece of metal, a small current flows through the metal. The light is giving its energy to electrons in the atoms of the metal and allowing them to move around, producing the current. mulation/photoelectric

20 Einstein explained the photoelectric effect by thinking of light in terms of photons. One, and only one, photon is completely absorbed by each electron ejected from the metal. It was the energy per photon that mattered, not the brightness (# of photons) or intensity of light. The critical factor is the frequency (or color) of light. Only high frequency photons have the concentrated energy to pull loose an electron. (red vs. blue) The photoelectric effect suggests that light interacts with matter as a stream of particle-like photons. The number of photons in a light beam controls the brightness of the whole beam, whereas the frequency of the light controls the energy of each individual photon. Light travels as a wave and interacts with matter as a stream of particles.

21 Photoelectric effect If photon has more energy than needed to eject an electron then extra energy goes into KE of electron. hf = KE + W 0 hf = photon energy W 0 = work function or energy needed to eject electron. (work done by photon to remove electron)

22 hf = KE + W 0 Higher-frequency photons have more energy, so they make the electrons come flying out faster. If you leave the frequency the same but crank up the intensity, more electrons should come out (because there are more photons to hit them), but they won't come out any faster, because each individual photon still has the same energy.

23 Energy Emission & Absorption.

24 Compton Effect and the momentum of a photon Like two billiard balls colliding on a pool table, when an incident photon strikes an electron at rest, the electron will have a recoil velocity and the incident photon will be deflected off at an angle with a new, smaller frequency corresponding to the energy transferred to the electron. He noticed the difference between the two frequencies depended on the angle at which the scattered photon leaves the collision.

25 See pg 952 in text

26 Support for the photon model of em waves Compton Scattering X-rays scatter from electrons, changing direction and frequency in the process only explained by the photon model of em waves Suppose an x-ray photon is moving to the right. It has a collision with a slow moving electron. The photon transfers energy and momentum to the electron, which recoils at a high speed. The x-ray photon loses energy, and the photon energy formula E=hf tells us that its frequency must decrease. Very similar to two particles colliding.

27 1. When the x-ray photon scatters from the electron, A) its speed increases B) its speed decreases C) its speed stays the same. 2. When the x-ray photon scatters from the electron, A) its wavelength increases B) its wavelength decreases C) its wavelength stays the same 3. When the electron is struck by the x-ray photon, A) its de Broglie wavelength increases B) its de Broglie wavelength decreases C) its de Broglie wavelength stays the same

28 1. When the x-ray photon scatters from the electron, A) its speed increases B) its speed decreases C) its speed stays the same. 2. When the x-ray photon scatters from the electron, A) its wavelength increases B) its wavelength decreases C) its wavelength stays the same 3. When the electron is struck by the x-ray photon, A) its de Broglie wavelength increases B) its de Broglie wavelength decreases C) its de Broglie wavelength stays the same 1. C (v = c) 2. A (E, f, ) 3. B (f, )

29 Check it out. oelectric kbody-spectrum owaves

30 Bright line Spectra The fingerprints of hydrogen (top) and iron when they emit light after being energized by heating. Iron has many more lines because it s a much more complicated element than hydrogen with 26 protons and 26 electrons compared to hydrogen s one proton and one electron.

31 Bohr Model & Emission Bohr hypothesized there can only be certain values for energy emitted corresponding to the specific energy needed to change (drop) energy levels (orbits) for an electron. Electrons don t radiate energy while in a stationary orbit E i E f = hf can find from this. Lowest level = ground state any higher levels electrons jump to = excited state.

32 Bright Line Spectra Cont: So bright line spectra can be used to identify elements since each element has a unique electron configuration. Pictured on the left are high pressure sodium vapor lamps

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34 Big Screen Projection TV s

35 Adding Light Blue light + Green Light = CYAN Red light + Blue Light = MAGENTA Green light + Red light = YELLOW Red light + Blue light + Green light = WHITE

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39 Additive & Subtractive Primaries

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