3.2c Nature of Light - Lecture notes 3 PDF

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Fundamenals Of The Physical World notes...

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Nature of Light Newton who studied light in detail concluded that light is made up of tiny particles, and he called them corpuscles. The properties of rectilinear propagation, reflection and refraction of light are consistent with those of fast moving particles. But light being a part of electromagnetic waves have frequency and wavelength. The behavior of light is also consistent with properties of waves. Does it mean that light is both particles and waves? In 1801 Thomas Young published evidence of a behavior of light that could be explained only in terms of the wave model of light.

Young’s double slit experiment Young allowed a monochromatic beam of light to pass through two closely spaced slits and the split beam was allowed to fall on a screen.

Intensity of light on the screen.

http://www.colorado.edu/physic inger/two-slit2.html

On the screen he noticed alternative dark and bright fringes. The only explanation for this observation is that when light arrive on the screen from two separate slits, they either combine to give more light, less light or no light at all. This is possible only if light is a form of wave. Two waves superimposed on each other can produce a larger wave or no wave depending on the phase difference between them. At points on the screen where the waves from both slits arrive in phase, there is more light due to constructive interference. At points where they meet out of phase, there is no light due to destructive interference.

Polarization of light. Light is a form of electromagnetic waves with vertical and horizontal vibrations. Polarized light waves are light waves in which the vibrations occur in a single plane.

The process of transforming unpolarized light into polarized light is known as polarization. The most common method of polarization involves the use of a Polaroid filter. Polaroid filters are made of a special material that is capable of blocking one of the two planes of vibration of an electromagnetic wave.

When unpolarized light is transmitted through a Polaroid filter, it emerges with one-half the intensity and with vibrations in a single plane

http://www.olympusmicro.com/prim er/java/polarizedlight/3dpolarized/

Unpolarized light can also undergo polarization by reflecting from nonmetallic surfaces. A person viewing objects by means of light reflected off nonmetallic surfaces will perceive a glare if the extent of polarization is large. Light reflected off water is partially polarized in a direction parallel to the water's surface. Fisherman know that the use of glare-reducing sunglasses with the proper polarization axis allows for the blocking of this partially polarized light. By blocking the plane-polarized light, the glare is reduced and the fisherman can more easily see fish located under the water.

Polarization can also occur by the refraction of light.

The refracted beam acquires some degree of polarization perpendicular to the surface.

Photoelectric Effect The interference and polarization of light can be explained only using the wave nature of light. This situation changed in 1905 when Einstein extended the photon picture to explain another phenomenon of light which could not be accounted for in the conventional wave picture - the photoelectric effect. When light is shone on a metal, electrons are released. Einstein came up with an explanation of the photoelectric effect which used Planck's photon hypothesis. In this theory Einstein assumed that photons of frequency f carry an amount of energy given by: Here h is Planck’s constant.

E = hf.

When these photons hit the metal, they could give up some or all of their energy to an electron .

hf. Higher intensity light has more photons, and so will knock out more electrons.

However, if the frequency of the light is such that a single photon is not energetic enough to release an electron from the surface, then none will be ejected no matter how intense the light.

Dual nature of Light Is light a wave or a particle? Your shadow when you stand in the sun suggests that light consists of particles moving in straight lines. Particle theory could explain reflection refraction, and photoelectric effect. If two thick glass plates are pressed together with a little water in between, fringes caused by interference of light could be seen. Wave theory could explain the interference and polarization of light which particle theory could not.

Neils Bohr (1928) offered his 'complementarity principle': that particle theory and wave theory were equally valid. Scientists should simply chose whichever theory worked better in solving their problem. The currently accepted theory of light is the quantized electromagnetic field theory or quantum electrodynamics (QED) This theory merges particle and wave theory into a single theory....