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Wykład 1. The l igh t. The l igh t. The l igh t. I n the Hypothesis of Light ( 1675 ) Newton stated that light was com - posed of corpuscles [ which were emitted in all directions from a source ] .
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The light In theHypothesis of Light(1675)Newton stated that light was com-posed of corpuscles[which were emitted in all directions from a source]. Newton's argument against the wave nature of light was that waves were known to bend around obstacles [while light travelled only in straight lines]. Newton's theory could be used to predict the reflection of light, but could only explain refraction by incorrectly assuming that light accelerated upon entering a denser medium because the gravitational pull was greater. Newton published the final version of his theory in his Opticks of 1704. His reputation helped the particle theory of light to hold sway during most of the 18th century.
The light Thomas Young's sketch of adouble-slit experiment showingdiffraction. Young's experiments supported the theory that light consists of waves. (approx. 1800)
The light Thomas Young's sketch of adouble-slit experiment showingdiffraction. Young's experiments supported the theory that light consists of waves. (approx. 1800)
The light Water waves and sound waves are examples of mechanical waves. They require a medium to propagate. But light waves are not considered mechanical waves because they don't involve the motion of matter. In particular, light waves differfrom mechanical waves, because they can travel through a vacuum.
The light In 1846 Faraday speculated that light might be some form of disturbance propagating along magnetic field lines. Soon after, Maxwell discovered that self-propagating electromagnetic waves would travel through space at a constant speed. From this, he concluded that light was a form of electromagnetic radiation.
The light It turned out that lightwaves arejus t ONE TYPE ofelectromagnetic wave. Indeed, soon after,Hertz confirmed Maxwell's theory experimentally by generating and detectingRADIO waves in the laboratory. He also demonstrated that these waves behaved exactly like visible light, exhibiting same properties such as reflection, refraction, diffraction, and interference.
Electromagnetic Radiation (wavelength)
Electromagnetic Radiation (frequency)
Electromagnetic Radiation (frequency)
Electromagnetic Radiation Product of wavelength and frequency is a constant (l)(n) = c Speed of Light c ~ 3 x 108 m/s
Electromagnetic Radiation • Electromagnetic radiation, or “light”,is a form of energy. • ER has both electric (E) and magnetic (H) components. • Characterized by: Wavelength (l) Amplitude (A)
A. Red light travels at a greater speed than blue light. B. Blue light travels at greater speed than red light. C. The wavelength of blue light is longer. D. The wavelength of red light is longer.
Black body radiation Black-body radiation is the type of electromagnetic radiation within or surrounding a body in thermodynamic equilibrium with its environment, or emitted by a black body (an opaque and non-reflective body) held at constant, uniform temperature. For a body of any arbitrary material, emitting and absorbing thermal electromagnetic radiation at every wavelength in thermodynamic equilibrium, the ratio of its emissive power to its dimensionless coefficient of absorption is equal to a universal function only of radiative wavelength and temperature, the perfect black-body emissive power.
Black body radiation Comparison of Rayleigh–Jeans law with Wien approxi- mation and Planck's law, for a body of 8 mK temperature.
Black body radiation Planck found that in order to model this behavior one has to envision that energy (in the form of ER) is lost in integer values according to: DE = n·h·n n = 1, 2, 3, ... h = Planck’s constant = 6.626 x 10-34 J.s
Light as Energy In general the relationship between n and “photon” energy is Ephoton = h·n Example: what is the energy of a 500 nm photon? n = c/l = (3 · 108 m/s)/(5.0 · 10-7 m) n = 6 · 1014Hz E = h·n =(6.626 · 10-34 J·s)(6 · 1014 1/s) = 4 · 10-19 J
Waves vs. Particles • We began our discussion by defining light in terms of wave-like properties. • But Planck’s relationships suggest that light can be thought of as a series of energy “packets” or photons.
Compton effect Compton effect relates to an inelastic scattering of a photon by a quasi-free charged particle, usually anelectron. It results in a decrease in energy (increase in the wavelength) of the photon.
Compton effect Clasically the effect would become arbitrarily small at sufficiently low light intensities - regardless of wavelength Thus, light (EMR) must behave as if it consists of particles to explain the low-intensity Compton scattering.
Compton effect Clasically the effect would become arbitrarily small at sufficiently low light intensities - regardless of wavelength Thus, light (EMR) must behave as if it consists of particles to explain the low-intensity Compton scattering.
The Photoelectric Effect In 1887, Hertz observed the photoelectric effect during the production and reception of electromagnetic waves. In 1902, Lenard observed that the energy of individual emitted electrons increased with the frequency (which is related to the color) of the light. This appeared to be at odds with Maxwell's wave theory of light, which predicted that the electron energy would be proportional to the intensity of the radiation. In 1905, Einstein solved the problem by describing light as composed of discrete quanta, now calledphotons (E=hv)
The Photoelectric Effect • Shine light on a metal andobserve electrons that are released. • Find that one needs a mini- mum amount of photon ener- gy to see electrons (no). • Also find that for n ≥ no, number of electrons increases linearly with light intensity.
The Photoelectric Effect • Finally, notice that as freq- uency of incident light is increased, kinetic energy of emitted e- increases linearly. • = energy needed to release e- • (workfunction) Conclusion: light apparently behaves as a particle!
The Photoelectric Effect • For Na with F = 4.4 x 10-19 J, what wavelength corresponds to no? 0 hn = F = 4.4 x 10-19 J hc/l= 4.4 x 10-19 J l = 4.52 x 10-7 m = 452 nm
