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LIGHTPowerPoint presentation by:

Andyl Joy M. Negro

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PART 11

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EVIDENCE FOR WAVE

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• The topic of light become a topic of debate toward the end of the 1600s as Isaac Newton published the Particle Theory of the light. He believed that the straight-line travel of light could be better explained as small particles of matter that travelled at the great speed from a light source.

• He reasoned that particle should follow a straight line according to law of motion while waves bend.

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• After the same time, Christian Huygens concluded that light is not a stream of particles but rather a longitudinal waves that move through the “ether”. Huygens's Wave Theory proposed that waves created by the light source.

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Huygens's Wave Theory -The wave, according to the theory, did not simply move through the ether. Each point on the wave front was pictured as impacting an “ether molecules”. -Each “ether molecule” produce a small wavelet as a result. Each wavelet was a small hemisphere that spread outward in the direction of the movement. A line connecting all the leading surfaces of the wavelets described a new wave front, which now impacted other “ether molecules,” which made new wavelet, which make a new wave front and so.

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• Even though both theory were advocated during 1700s, majority favored on Newton.

• By the beginning of 1800s, new evidence was found favored to wave theory, an evidence that no other theory could explain but only the wave theory.

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Diffraction

• Newton argue that light is not a wave because if light is a wave, then it must bend on an obstacle, like window. So, Newton argued for a particle nature of light would move straight past an obstacle and makes a sharp shadow.

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Diffraction• In 1665, Francesco Grimaldi reported that there

was not a sharp edge to a shadow moving through a small pinhole and the light formed a larger spot than would be expected by light travelling in a straight line. He made the first description of DIFFRACTION, THE BENDING OF LIGHT AROUND THE EDGE OF AN OPAQUE OBJECT.

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Diffraction

Water wave

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Huygens’s wave theory explains diffraction of light

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InterferenceIn 1801, Thomas Young published evidence of a behavior of light that could only be explained in terms of wave model of light

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• The pattern of bright lines and dark zones is called interference pattern.

• After 15 years later, A.J. Fresnel demonstrated mathematically that diffraction as well as other behaviors of light could be fully explained by the wave theory.

• Red(wavelength): 4 x 10-7m• Violet (wavelength): 8 x 10-7 m

Interference

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EVIDENCE FOR PARTICLE

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• 1850, J.L. Foucault was able to prove that light travels much slower in transparent materials than it does in air. This was in complete agreement with the wave theory and completely opposite of the particle theory explaination

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• By the end of 1800s, Maxwell theoretical concept of electric and magnetic fields changed the concept of the light from mechanical waves to waves of changing electric and magnetic field.

• Further evidence removed the necessity for ether, the “material” supposedly needed for the waves to move through. Light was now seen as ELECTROMAGNETIC WAVE that could move through empty space.

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Photoelectric Effect• Usually, the energy of absorbed light result in

temperature increase such as the warmth that you fell from absorbed sunlight.

• Sometimes, however, the energy from absorbed light result in other effect. In some materials the materials that energy is acquired by electrons, and some of the electrons acquire sufficient energy to jump out of the material. The movement of electrons as a result of energy acquired fro light is known as PHOTOELECTRIC EFFECT.

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e-

light

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Fact!• Low-intensity light caused fewer electrons

to be ejected, and high-intensity light caused many to be ejected.

• All electrons ejected from low- or high-intensity light ideally had the same kinetic energy!. Surprisingly, the KE of the ejected electrons was found to be independent of the light intensity.

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REFLECTION AND

REFRACTION

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Think about this…• When a light wave strikes a

smooth interface between two optical materials such as air and glass or water and glass, the wave is in general partly reflected and partly refracted(transmitted) into the second material.

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ϕ ϕ

Figure 1

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Remember these…With regard to the direction of the incident, reflected and refracted ray…• The incident, refracted and reflected rays

and the normal to the surface all line in the same plane(Law of Refraction or Snell’s Law).

• The angle of the reflected is equal to the angle of incidence for all wavelengths and for any pair of substance(Law of Reflection).

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Remember these…• For monochromatic light and for a given

pair of substance, a and b, an opposite sides of the surface of separation, the ratio of the sine of the angle Φa (between the ray in substance a and the normal) and the sine of the angle Φb (between the ray in substance b and normal) is constant.

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Remember these…• Now let’s consider a beam of monochromatic

light travelling in vacuum, making an angle of incidence Φ0 with the normal to the surface of a substance a and let Φa be the angle of the refracteion into the substance. In this case we call the constant in Snell’s law the index of refraction of substance a, denoted by na.

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Sample problem• In figure 1, material a is water and material

b is a glass with an index of refraction of 1.50. if the incident ray makes an angle of 60o with the normal, find the directions of the reflected and refracted rays.

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Solution• The angle the reflected way makes with the normal

is the same as that of the incident ray, hence, Φr=Φa= 60o to find the direction of the refracted ray, we use the equation:

nasinΦa=nbsinΦb

na=1.33(index of refraction of water), nb=1.50(index of refraction of glass)

Φa=60o.

(1.33)(sin60)=(1.50)(sinΦb),

Φb=50.2o

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• (use powerpoint No.3)

Polarization and Scatteringof light