Sunlight. 1 radiation.

Transcription

1 Sunlight The eye has evolved to see a narrow range of EM waves which we call 'visible light'. This visible range of frequency is due to the light comes from the Sun. The photosphere of the Sun is a blackbody emitter with a temperature of 5800K. The frequency of this range of radiation is ~1015 Hz. This is beyond what is possible with a tank circuit or magnetron. Electrons motion in atoms product these frequencies of 1 radiation.

2 Sunlight Light from the Sun travels to Earth in about 8 minutes. The 150 million km is traversed in the near vacuum of space at the speed of light (c = 3 x108 m/s). When light travels through a transparent medium, the light slows down. The slowing down of light in a transparent material depends on the material. The ratio of the speed of light in vacuum to the speed in the material is called the index of refraction, n. c n= v Refraction refers to the fact that the direction of the light can change when it goes from one medium to another. For air, the index of refraction is almost 1 (1.0003). Glass has an index of ~

3 Rayleigh Scattering Why is the sky blue? Air (N2 and O2) molecules are much smaller (~ m) than the wavelength of light (~ 5 x 10-7 m). The electric field of the light oscillates ('shakes') the molecule so that it radiates EM waves isotropically. How well the molecule scatters the incident light depends on the wavelength of the light. The molecules are poor antennas for the incident light since they are much smaller than the wavelength of the light. (A good antenna is λ/4) Red light is the poorest antenna and blue light is the 'best' for scattering the incident sunlight. More of the blue light gets scattered than the red light. The blue scattered light makes the sky appear blue. This type of scattering is called Rayleigh scattering. This is is also why sunsets appear red. When sunlight comes in near the horizon, the light passes through a lot more air. Almost all of the colors except red are scattered from the sunlight when the sun is on 3 the horizon.

4 Refraction and Dispersion When light travels from one medium to another some of the light is reflected (~4%). Because the speed of the light in the transparent medium are not the same the light will bend when it goes from one medium to another. This bending effect is known as refraction. It is determined by Snell's law: n1 sinθ1 = n2 sinθ2 Where n1 and n2 are the index of refraction of the two mediums and θ1 and θ2 are the angles with respect to the normal. The frequency of the light stays the same but since v = λν, the wavelength of the light changes as the velocity. 4

5 Refraction and Dispersion Different colors (wavelength) will refract slightly differently. For blue light in crown glass n=1.54 and for red light n=1.52. This effect is called dispersion. It is what causes a prism to break light into a rainbow pattern as well causing rainbows after a rain. Sunlight going through the middle (center) of a round rain drop goes through and does not refract. Light that strikes off to the sid of the drop refracts and because of dispersion is slightly separated into its colors. This dispersed light then reflects from the back of the rain drop. If the angle with the Sun, the rain and the observer are right, we see the rainbow pattern. 5

6 Thin Film Interference The rainbow colors you see on a soap bubble or gasoline on water is an interference effect. The very thin layer must be a fraction of the wavelength of light. When light enters from above, some light is reflected (because of the 'impedance mismatch') with the film's different index of refraction. Some of the light enters the film and refracts unless it enters at 0o. This light then reflects off of the lower interface of the film, travels back through the film and exits through the top surface of the film. The original reflected light (from the top of the film) and the light that reflects from the both of the film interferes. If the light has only one wavelength then you see dark and light bands. But the process depends linearly on the wavelength of the light so for white light you see rainbow patterns in the bright6 bands.

7 Polarization and Sunglasses The transverse nature of light allows it to be polarized. Polarized light is light where all of the light (photons) have their electric field in the same direction. Materials which stop all the light except that light that has its electric field in a particular direction are called polarizers. A polarizer will stop ½ of the light passing through it. Light that reflects off a surface will become partly or completely polarized. Glare on a sunny day is primarily from light reflected from horizontal surfaces. If you have polarized sunglasses, the sunglasses block ½ of the light and most of Glare. They also do not change the natural color of object as most sunglasses do. 7

8 Color Color is an illusion created by the human eye. The retina has cone cells which detect the different colors (wavelengths) of light. Rod cells do no see color. Some cone cells are sensitive to ~600 nm (red). Other cone cells ~550 nm (green) and ~450 nm (blue). This does not result in just seeing three colors. The eye can detect hundreds of colors. If 575 nm light strikes the retina, green and red cone cells are activated so we see yellow. 8

9 Color Red, green and blue are primary colors. Any color can be made using a combination of the primary colors. This is how color TVs and monitors produce color images. Each pixel (tiny dots on the screen) consist of a red, green and blue element. To make yellow, red and green are turned on to produce yellow. Cyan, magenta and yellow are subtractive colors. When a subtractive color is applied to a white (all colors) surface, it removes one of the primary colors. When cyan is applied to a white surface, it subtracts the reflect of red. 9

10 Quantum Mechanics and Wave-Particle Duality In the early part of the 20th century, Niels Bohr and others explained the basic structure of the atom but at the expense of revising many aspects of Newtonian physics. To understand how atoms work and electrons move in solids, we need to understand some concepts from quantum mechanics. In the same year (1905) that Einstein published his first papers on special relativity, he published a paper concerning the photoelectric effect. This effect had been discovered in the 1880s and was something of a mystery give the known wave nature of light. Consider an evacuated glass tube with two electrodes connected to a potential difference. If you shine one color (blue) of light on the positive electrode, a current will flow. If you shine red light on the electrode, no current will flow. 10

11 Photoelectric Effect The mystery was it did not depend on the intensity of the light, only the color. For a classical wave, the intensity should cause more current to flow. Einstein explanation was that the light come in packets or photons particles of light. Only the blue light had enough energy to dislodge the electrons from the electrode. 11

12 Quantum Mechanics and Wave-Particle Duality The energy of the photons is given by: E=hν Where E is the energy, ν is the frequency of the phone and h is Planck's constant (6.624 x J s). Planck was the one who worked out the blackbody spectrum using the trick of dividing the energy of the emitter into quanta (or packets) of energy. In 1923 a Louis de Broglie proposed that electrons could behave like waves. This idea was quickly confirmed. Particles can behave like waves and waves can behave like particles. This idea is know as wave-particle duality. The wavelength of an electron is given by λ = h/momentum 12

13 More Light Bulbs 'Neon' signs emit particular colors because of the atoms are excited by a high voltage and the electrons 'jump' between different energy levels in the atoms. Actually, the bright redorange is due to neon. Other gases give different colors. Electrons travel from the negative electrode through the low pressure gas and excite the neon atoms which emit the redorange light. The potential difference between the positive and negative electrodes is ~ 10 kv. This type of light source(like fluorescence bulb) is called a discharge tube. Discharge tubes are 6-10 times more efficient at producing visible light than a heated filament (incandescent bulb). 13

14 More Light Bulbs For greater intensity, street lights use denser gas. In a mercury street light (the blueish colored lights), a much higher pressure is used than in a fluorescent bulb. The mercury still emits in the UV (245 nm) but the density of the gas is large enough that the UV is re-absorbed by the mercury vapor. Eventually this radiation trapping has so many atoms excited that transitions from other energy levels which emit in the visible spectrum. High pressure sodium street lights (yellow) use sodium vapor. The main wavelength of the sodium 'D' lines is ~589 nm. With the high pressure and heat in the light bulb, the spectrum of the emitted light is thermally broadened. The thermal motion of the atoms causes the wavelength to smear out over a wider range. 14

15 Atoms Quantum mechanics explains the periodic table of the elements. Bohr first proposed that electrons were stable in certain energy 'orbitals' and emitted light when they jumped between orbitals. This explained a simple atom like hydrogen. In more complicated elements, the orbitals are filled one orbital at a time starting from the with the lowest energy level. Each orbital can only contain one electron because of the Pauli exclusion principle. Particles that obey the exclusion principle are called fermions. Actually two electrons can go in each energy level because of a property called spin. Since the electron has spin ½, two electrons can be in one orbital (one spin up and one spin down). In addition, angular momentum is 'quantized'. Each energy level in an atom has a very fine structure which is determined by both the orbital angular momentum (denoted by l) and the 15 sub-state (denoted by m).

16 Atoms The labels for the angular moment are relics of early work in atomic spectrometry. The letter s denotes no orbital angular momentum, p = 1 quantum of angular momentum etc. For a given energy level (n) the value of l=n-1. There are 2l+1 sub-states for each l value. The quantized energy and angular momentum accounts for the periodic table of the elements are these levels are filled up from the bottom with a spin up and spin down electron 16 in each level.

17 Electrons in Solids In a solid the nuclei are fixed in position. In some elements (metals) the electrons can more around (conductors), in other elements the electrons are more or less fixed to their atoms (insulators) and some elements are in between (semiconductors). Because of wave-particle duality the electrons in solids form energy bands. These bands are standing waves patterns. Electrons are fermions and obey the Pauli exclusion principle. If the solid was at absolute zero (0k) then the electrons would fill up all of the lower levels to a certain level. This level is called the Fermi level. (The electrons below this level are said to be in the Fermi sea). Thermal motion in the solid means a few electrons may be jump to a higher level and will eventually fall back to the lower level. 17

18 Electrons in Solids If the material is an insulator, there is a large energy difference between the conduction band and the valence band (where to electrons are 'stuck' to their atom). In a conductor the conduction band and valance bands overlap and the electrons can easily be excited (millivolt or less) into the conduction band and move throughout the sold. Semiconductors have a small band gap (1.1 volts in silicon). Apply a few volt potential difference (voltage) across a semiconductor and a current will flow i.e. electrons can move into the conduction band and a current will flow. 18

19 Diodes So far, we have been discussing very pure materials. If you add a small amount of the right element as a impurity ('dope') to a semiconductor, the properties of the semiconductor can be changed. If the impurity creates a few empty valence levels ('holes') the material is called a p-type semiconductor. If the impurity adds a few extra electrons to the conduction band the Semiconductor is called n-type. Since the impurities and the semiconductor atoms are all neutron, the material has no net charge. The most common semiconductor is silicon (Si). The most common dopant for p-type silicon semiconductors is boron (b) used in silicon integrated circuits. Phosphorus is commonly used in silicon for n-type. 19

20 Diodes If you place an n-type and p-type semiconductor together, something interesting happens. The extra electrons in the conduction band of the n-type fill in the holes in the p-type. This creates a depleted region in the between the p-type and n-type and no current can flow through the semiconductor. 20

21 Diodes If a negative voltage is applied to the n-type and a positive potential to the p-type, the extra electrons will fill in the depleted region and a current will flow. Reverse the potential (p-type negative and n-type positive) and the depleted region will grow. No current will flow. A diode conducts in one direction and does not conduct in the opposite direction. 21

22 Light Emitting Diodes (LEDs) Silicon is not transparent and will not transmit light. Other types of semiconductors (gallium, indium and other) can transmit light. It a GaAs (gallium arsenide) diode is biased (large enough potential difference) so that an electron falling from the conduction band to the valence band losses the right amount of energy results in a photon in the visible spectrum. A 1.9 volt bias results in a 1.9 ev photon (1 ev = 1.6 x J) which with E = hν and v = λν give a photon with a wave length of 650 nm (red). Other semiconductors with different band gaps can produce infrared through blue wavelengths. 22

23 Lasers Lasers are used for many things. Lasers are used to read information from DVDs. Lasers are used for surgery, for surveying, for distance measurements and even for classroom pointers. Lasers action has even be observed in interstellar gas clouds. Recently they have been installed on Navy ships as anti-missile weapons. The original idea for a laser was developed by Einstein in The first practical devices were devices were developed in the 1960s. 'Laser' is an acronym for 'Light Amplification by Stimulated Emission of Radiation'. Before practical lasers were developed, there were devices like lasers in the microwave region of the spectrum know as 'masers' (m microwave). Lasers light is one pure wavelength (monochromatic), very intense and coherent. 23

24 Lasers A laser requires a 'population inversion'. The atoms in the laser medium are excited into some level above the ground state. Normally the atom would decay back to the ground state (with the spontaneous emission of a photon). If the excited state is temporarily stable (or some intermediate state) then 'stimulated emission' can occur between the excited state (upper laser level) and a lower laser level. 24

25 Lasers The photons which are emitted by stimulated emission coherent. This means the electric and magnetic fields of the EM wave all oscillate together. Creating the 'population inversion' can be done by exciting the laser medium with a flash lamp, with an electrical current, or by a chemical reaction. Once a photon is spontaneously emitted it travels through the laser medium and cause stimulated emission of many more photons in the laser medium. This greatly amplifies the number of coherent photons. In a real device mirrors at each end of the laser medium reflect all or some of the photons back through the laser medium. A continuous lasers (cw) has 25 a partially silvered mirror to allow some of the light to escape.

26 Lasers Pulsed laser can be made to emit one very intense and short pulse of light ( less than seconds). A 'Q' switch is used in a pulsed laser. A 'Q' switch is a component of the laser medium which absorbs the light. When the population inversion reaches a high level, the Q switch changes to a transparent state. When the Q switch changes, the laser light is amplified and output in one very short and intense beam of light. Such beams are very useful to 'take snapshots' of very fast events like atoms in a chemical reaction. 26

27 Lasers Diodes A laser diode works much like a light emitting diode (LED). A thin (less than 1 µm) p-type n-type junction is heavily doped. When a large current is passed through the semiconductor, a population inversion is established in the diode. The semiconductor can then amplify the spontaneously emitted photon to produce laser light. The ends of the diode can be coated with a reflective coating to act as a mirror. 27