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On the nature of light - Waves. “Is light a wave or a particle?” This issue was put to rest by James Clark Maxwell (~1873). He studied electricity and magnetism and the nature of waves. On the nature of light - Waves. Happy Birthday Sasha. On the nature of light - Waves.
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On the nature of light - Waves • “Is light a wave or a particle?” • This issue was put to rest by James Clark Maxwell (~1873). He studied electricity and magnetism and the nature of waves.
On the nature of light - Waves • Happy Birthday Sasha
On the nature of light - Waves • Wave - a disturbance that travels from one point to another. It is a way in which energy is transmitted from place to place without physical movement of material from one place to another.
On the nature of light - Waves • How are waves quantified? • Crest and trough • Amplitude - maximum excursion from its undisturbed or relaxed position. • Waves travel at a speed, v. The number of crests that pass at a specific point in space is called a wave’s frequency or f, and is recorded in units of Hertz. • Period - the time it takes for one complete cycle, measured in seconds. This is known as P. • Wavelength - the distance a wave travels during one complete oscillation. • f = 1/P Wavelength() = speed x P or speed/f
On the nature of light - Waves • Waves: • Transverse wave - movement or displacement is perpendicular to the direction of the wave. • Longitudinal wave - movement or displacement is parallel to the direction of the wave.
On the nature of light - Waves • “Is light a wave or a particle?” • The nature of light is that it is electromagnetic. • Electricity and magnetism are two parts of the same electromagnetic force. • What is electricity? • A stream of electrically charged particles.
On the nature of light - Waves • What is an electrically charged particle? These are particles such as electrons and protons. A net imbalance in one charge produces an electrical charge. Atoms gain or lose electrons and become ionized. • Electric force - push and pull between electrically charged particles.
On the nature of light - Waves • Electric field - the force exerted between charged particles. Like charges repel and opposite attract. • This is how electrical force is transmitted through space. • The strength of the electric field decreases with increasing distance from a charge, according to the inverse-square law. By means of an electric field a particle’s presence is felt by other particles.
On the nature of light - Waves • When a charged particle accelerates, change in the electric field moves outward at the speed of light. • F exerted on a charged particle = charge of the particle x the strength of the electric field. • Because an electric field points away from a positively charged particles (or towards a negatively charged particle), F that a second charge feels is either directly towards or away from the first charged particle.
On the nature of light - Waves • An electric field is always accompanied by a magnetic field. • Magnetic force - force between electrically charged particles arising from their motion. This force is also exerted on moving electric charges. • Disturbance produced by moving a charged particle that consists of vibrating electric and magnetic fields, always oriented perpendicular to one another and moving together in space.
On the nature of light - Waves • Maxwell discovered that a change in the motion of a charged particle causes a changing electric field, which causes a changing magnetic field, which causes a changing electric field … • Thus, accelerating charged particles give rise to oscillating electric and magnetic fields that move out in all directions or an electromagnetic wave.
On the nature of light - Waves • As discussed earlier, light is electromagnetic in nature, it is electromagnetic energy.
On the nature of light - Waves • So, information about a particle’s state of motion is transmitted through space via a changing electric field or magnetic field. • The disturbance that is produced in the particle’s electric and magnetic field travels through space as a wave.
On the nature of light - Waves • This means that energy produced in a star, in the form of electromagnetic radiation, moves outward in all directions as a wave or light. • The electromagnetic ripples travel outward as waves. Small charged particles, either in our eyes or in equipment, respond to the field changes or waves by vibrating in tune with the received radiation or wave. • THUS WE CAN SEE LIGHT!
On the nature of light - Spectrum • Light that is spread out, without any interactions with particles, according to wavelength is known as a spectrum of light. The entire range of different wavelengths of light is known as electromagnetic spectrum. • Wavelengths are measured in nm, µm, mm, and cm.
On the nature of light - Spectrum • Because light is electromagnetic in nature, when an object is heated it will produce radiation at higher and higher frequencies, shifting from red to blue. • We will explore this issue in more detail a bit further down the lecture series.
On the nature of light - Particle • Some problems with describing light only as a wave. • Basically the way electrons become excited and release energy occurs in a specific fashion. The amount of light energy absorbed or emitted must correspond precisely to the energy difference between two orbitals or electrons. Thus, energy is absorbed or emitted in distinct packets known as photons.
On the nature of light - Particle • Photon - particles of light or energy of light; packets of electromagnetic radiation each carrying a specific amount of energy moving at the speed of light, thus they are massless (quantum mechanical approach to light).
On the nature of light - Particle • This description of light is related to the wave description by a relationship between the energy of a photon and the frequency or wavelength of the wave. E = hf (h = 6.63 x10-34 J/s - Plank’s Constant) • The higher the f of electromagnetic wave, the greater the energy carried by each photon.
On the nature of light - Spectroscopy • The analysis of the ways in which matter emits and absorbs radiation is spectroscopy. • Atoms can only have discrete energies. They can be at rest or excited. • The lowest-possible energy state is known as the ground state. Allowed states with energies lying above the ground are known as excited states. • Ground state - electrons are not lost or gained, they don’t move orbitals. If they move out of this state, energy is produced.
On the nature of light - Spectroscopy • Once an atom is in an excited state it will decay from that state to a lower-lying energy state by getting rid of some of the extra energy. • The process of loosing energy occurs at once, it is not a gradual process. Thus, a photon is released that contains exactly the amount of energy lost by that atom as it jumps states. This process is known as emission. • The energy state or level that an atom is within determines the wavelength of radiation or light that can be emitted (or absorbed).
On the nature of light - Spectroscopy • The bright, single-colored feature in the spectrum is referred to as an emission line. • The dark feature seen in the spectrum is termed an absorption line. • Absorption is when an atom captures the energy of a passing photon.
On the nature of light - Spectroscopy • Kirchhoff’s Laws - • 1. A luminous solid or liquid, or a sufficiently dense gas, emits light of all wavelengths and so produces a continuous spectrum of radiation. • 2. A low-density hot gas emits light whose spectrum consists of a series of bright emission lines. These lines are characteristic of the chemical composition of the gas. • 3. A cool thin gas absorbs certain wavelengths from a continuous spectrum, leaving dark absorption lines in their place superimposed on the continuous spectrum. These lines are characteristic of the composition of the intervening gas, they occur at precisely the same wavelengths as the emission lines produced by that gas at higher temperatures.
On the nature of light - Spectroscopy • Temperature is the measure of the energetic nature of particles. The more energetic or the more they move, the hotter the temperature, or the slower the colder. • When material is dense enough for atoms to move other atoms they will emit light simply because of their temperature. This sort of “radiation” is termed thermal radiation.
On the nature of light - Spectroscopy • The radiation from an object will change as it heats or cools because the movement of atoms changes. • When an object emits thermal radiation it is also luminous or has luminosity. The hotter an object gets, the bluer the radiation or light will become. • Luminosity - the amount of energy radiated, or Watts.
On the nature of light - Spectroscopy • Why blue? Objects that emit radiation in an area where it is also absorbed, in equal proportions, is said to be a blackbody. Thus the thermal radiation emitted is equal to what is absorbed. Blackbody spectrum or Planck Spectrum
On the nature of light - Spectroscopy • Why blue? • Plank spectrum shows that the increase in luminosity is proportional to the fourth power of the temperature or luminosity T4 (Stefan’s law). • Stafan’s law states F = T4 • F = flux, • = Stefan-Boltzmann constant (5.67 x10-8 W/(m2K4) • Thus, hotter means more luminous!
On the nature of light - Spectroscopy • Why blue? • The total amount of light emitted is = the flux times by the surface area of the filament. • 100W = A x T4 solving A = 100W/ T4 A = 4.5 x 10-5 m2 • Basically slight changes in temperature equate to LARGE changes in brightness.
On the nature of light - Spectroscopy • Why blue? • As temperature increases, the peak of the Plank spectrum shifts towards shorter wavelength, meaning that the average energy of the photon gets shorter. • We can use Wien’s law to determine that the peak wavelength of a blackbody is inversely proportional to its temperature. Thus, we can use this law to calculate the temperature of an object. peak = 2,9000 µmK/T
On the nature of light - Spectroscopy • Brightness - the amount of electromagnetic radiation or light that arrives at some location. Thus, brightness is a measure of how much light falls per square meter per second. • Twice as far = one fourth as bright
On the nature of light - Spectroscopy • We can use what we learned above to determine how hot an object in space is or the distance to that object. • Finally, in the case of planets, some light is reflected, known as albedo, and some of the energy is absorbed (1 - albedo). Thus, the amount of energy emitted or absorbed by a planet can be calculated.
On the nature of light - Speed • In the 1670’s Ole Romer was studying the moons of Jupiter. He discovered that at the farthest distance between the Earth and Jupiter the moons of Jupiter appear 16.5 minutes later than predictions. When the Earth was at the closest to Jupiter, then moons appearance made up the lost time. • 2.99792458 x 108 m/s in vacuum
On the nature of light - Speed • Maxwell’s research discovered a “flaw” in Newtonian physics. “The ball in the car issue”. • The laws of Newtonian physics that govern the above issue however, do not apply to the speed of light. • Enter our hero, stage right; Albert Einstein. • Either light is a wave that does not move or Newton was wrong when it comes to light.
On the nature of light - Speed • Special relativity: Einstein focused his attention onto events. In relativity, an event is a particular location in space and time. • Snap your fingers while a car moves in front of you. Snap again, time has gone by and you have not moved. To the person in the car, according to Newton, only time has gone by, they are still in the car. But to the person watching the car both time and distance have changed with the second snap.
On the nature of light - Speed • But…the passage of time, at the speed of light, actually depends on the observer’s frame of reference. Einstein discovered space-time, four-dimensional universe. • Time between two events is t2-t1 = 2l/c to a stationary observer. But if the observer, who is also stationary in perspective, is actually moving…
