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# Properties of light: 1. Propagation within a uniform medium is along straight lines. 2. Reflection occurs at the boundary of a medium.

## Presentation on theme: "Properties of light: 1. Propagation within a uniform medium is along straight lines. 2. Reflection occurs at the boundary of a medium."— Presentation transcript:

Properties of light: 1. Propagation within a uniform medium is along straight lines. 2. Reflection occurs at the boundary of a medium.

3. Refraction may occur where a change of speed is experienced. 4. Interference is found where two waves are superposed. 5. Diffraction takes place when waves pass the edges of obstructions.

Two theories can explain the properties of light: the particle theory (corpuscular theory) and the wave theory. Particle theory: Isaac Newton (Laplace) Wave theory: Christian Huygens (Robert Hooke)

CORPUSCULAR THEORY 1. Rectilinear propagation: particles moving at great speed would curve very little due to gravity (or other forces). How could waves travel in straight lines?

2. Reflection: Elastic particles striking a surface would bounce off in a regular way.

3. Refraction: The rolling ball model. Water attracts light particles the same way gravity attracts a rolling ball. Requires speed of light in water to be faster than in air. (This was not measured until 123 years after Newton’s death). Jean Foucault found in 1850 that the opposite was true.

WAVE THEORY Huygen’s Principle: Each point on a wave front may be regarded as a new source of disturbance. Wave supporters could satisfactorily explain reflection and refraction, but not rectilinear propagation. (the basis for Newton’s rejection).

The discovery of the interference of light in the early 1800’s and its subsequent use to explain diffraction imply a wave character. These phenomena can’t be explained very well by a particle theory.

The final blow to the corpuscular theory came in 1850 with Foucault’s measurement of the speed of light in water compared to air.

Michael Faraday developed the principle of the electric generator; he postulated tubes of force between charged bodies.

James Clerk Maxwell developed a series of mathematical equations predicting that heat, light, and electricity all move in free space at the speed of light as electro-magnetic disturbances.

Electromagnetic theory states that the energy of an electromagnetic wave is equally divided between an electric field and a magnetic field, each perpendicular to each other, and both perpendicular to the direction of the wave.

Electromagnetic wave a periodic disturbance involving electric and magnetic forces.

Heinrich Rudolf Hertz- experimental confirmation of the theory by 1885.

Many believed that all significant laws of physics were now discovered. Hertz himself soon discovered an important phenomena which would create problems for wave theory. This discovery set the stage for quantum physics.

Electromagnetic Spectrum 10 Hz to 10 25 Hz constant speed of 3 x 10 8 m/s v = fλ λ range is 3 x 10 7 m to less than 3 x 10 -17 m

Eight major regions: Hard gamma rays, Gamma rays, X rays, Ultraviolet radiation, Optical spectrum, Infrared radiation, Radio waves, Power frequencies.

The intensity of light follows the inverse square law, as did sound intensity.

About the only thing left for scientists to explain involved EM radiation and thermodynamics. Specifically, the glow of objects at high temperature.

Hot objects do not perform the way classical mechanics predicts. Classical theory predicts that as the wavelength of light approaches zero (frequency becomes greater), the amount of energy being radiated should become infinite.

Experimental data shows that the energy reaches a peak, and then approaches zero along with the wavelength. This contradiction is called the ultraviolet catastrophe, because the disagreement occurs at the UV end of the spectrum.

Negatively charged zinc plates lose their charge when illuminated by UV radiation. Positively charged plates are not discharged by similar illumination.

PHOTOELECTRIC EFFECT the emission of electrons by a substance when illuminated by electromagnetic radiation. These electrons are called photoelectrons.

First law of photoelectric emission: The rate of emission of photoelectrons is directly proportional to the intensity of the incident light.

Work must be done against the forces that hold an electron in a piece of metal to make the electron escape the surface of the metal. This work is called the work function.

If photoelectrons acquire less energy than the work function, they will not escape. If photoelectrons acquire more energy than the work function, the excess is kinetic energy and appears as velocity.

Photoelectrons emitted from various atom layers below the surface will be emitted at various velocities ranging up to a maximum value (electrons at the surface).

If the collector plate potential is made increasingly negative (repelling the electrons emitted), the current decreases until it reaches zero.

At this point all electrons emitted are being repelled back to the emitter. This is the stopping, or cutoff, potential V CO.

This measures the photoelectrons with the highest kinetic energy. This cutoff potential is the same for all intensities of light.

Second law of photoelectric emission: The kinetic energy of photoelectrons is independent of the intensity of the incident light.

Robert A. Millikan (American) found that the cutoff potential had different values for various frequencies. The cutoff potential depends only on the frequency of the incident light.

Therefore, the maximum kinetic energy of photoelectrons increases with the frequency of the light illuminating the emitter.

Also, for each kind of surface there is a characteristic threshold or cutoff frequency f CO below which the photoelectric emission of electrons ceases regardless of the intensity.

Only a few elements demonstrate the photoelectric effect with visible light. (alkali metals)

Third law of photoelectric emission: Within the region of effective frequencies, the maximum kinetic energy of photoelectrons varies directly with the difference between the frequency of the incident light and the cutoff frequency.

First law of photoelectric emission doesn’t conflict with the EM theory, because the magnitude of the photoelectric current is proportional to the light intensity. However, the velocity of the electrons emitted is not raised with an increase of intensity, as the wave theory suggests.

Also, light of any frequency should cause emission if it is intense enough. But there are cutoff frequencies below which emission does not occur, even at high intensity.

The wave theory suggests that given enough time a weakly illuminated electron could “soak up” enough energy to be emitted, but no such lag time exists.

In case you haven’t noticed, the wave theory looks pretty sick right now. It can’t explain this new evidence (while the particle theory does), but the particle theory can’t explain the old observations.

Max Planck suggested that the energy emitted by a source is equal to a constant multiplied by the frequency of the light emitted. He suggested that light is emitted and absorbed in indivisible energy packets, or quanta.We now call these packets photons.

The energy in a photon is determined by the frequency of the radiation. The relationship is expressed in this equation: E = hf

E = hf f is the frequency in hertz, h is Planck’s constant (h = 6.63 x 10 -34 Js) E is the energy of the photon expressed in joules.

This led Einstein to publish a simple explanation to the photoelectric effect.

Quantum theory - the transfer of energy between light radiations and matter occurs in discrete units called quanta, the magnitude of which depends on the frequency of the radiation.

When a photon is absorbed by a emitter surface, its quantum of energy hf is transferred to a single electron. If hf is equal to the work function, w, the electron has just enough energy to escape the surface (cutoff frequency).

If hf is greater than w, the electron leaves with the excess energy being expressed as kinetic energy and therefore as velocity.

The maximum kinetic energy is expressed as: ½ mv 2 max = hf - w

Einstein’s photon hypothesis has no problems explaining all experimental evidence about electromagnetic energy.

If ½ mv 2 max = 0, then 0 = hf - w, and it follows that: hf CO = w this is the cutoff frequency

The modern view of the nature of light recognizes its dual character: Radiant energy is transported in photons that are guided along their path by electromagnetic waves.

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