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Chap 16-1 Light Fundamentals
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### Transcript of Chap 16-1 Light Fundamentals. What is Light? A transverse electromagnetic wave.

• Chap 16-1Light Fundamentals

• What is Light?A transverse electromagnetic wave

• What is Light?A small part of the electromagnetic spectrumThe part which stimulates the retina of the human eyeVisible light has wavelengths 400-700 nmREDLonger Lower fLower E BLUE Shorter Higher fHigher E

• How does light interact with matter?Light does not require matter (no medium) for transmission

BUT if light does pass through matterLight travels in a straight line through aVacuum or aUniform medium

• Ray Model A ray is a straight line that represents the path of a narrow beam of lightignores the wave nature of lighta useful model in analyzing reflection and refraction (ray diagrams)

• Speed of Light Before 17th century: light thought to be instantaneousGalileo: thought speed of light was finite but could not measure itOle Roemer (~1700) through measurements of the period of Io, a moon of Jupiter,Calculated that light took 22 min to cross the diameter of Earths orbitCalculated speed of light at 2.2 x 108 m/s (75% of true value)

• Roemers Method for calculating the speed of light:http://en.wikipedia.org/wiki/Speed_of_light#Early_attemptsIn 1675, the Danish astronomer Ole Roemer noticed, while observing Jupiter's moons,that the times of the eclipses of the moons of Jupiter seemed to depend on the relative positions of Jupiter and Earth. If Earth was close to Jupiter, the orbits of her moons appeared to speed up. If Earth was far from Jupiter, they seemed to slow down. Reasoning that the moons orbital velocities should not be affected by their separation, he deduced that the apparent change must be due to the extra time for light to travel when Earth was more distant from Jupiter. Using the commonly accepted value for the diameter of the Earth's orbit, he came to the conclusion that light must have traveled at 200,000 Km/s. This is 75% of the accepted value.

• Albert MichelsonMichelson (1926) made a more precise measurement for speed of light, c with the Michelson-Morely Experiment:

• MichelsonWon the Nobel PrizeSuccessfully measured the speed of lightc = 3.00 x 108 m/sin a vacuumthis is a defined value for lightNow objects lengths are determined by how long it takes for light to travel from one end to the other Examples:Definition of the meterLight year

• Sources of LightA luminous body emits light waves.An illuminated body reflects light waves produced by an outside source.Incandescent: light produced by a hot body.

• Luminous FluxThe rate at which visible light is emitted by a sourceRepresented by the letter PThe unit is the lumen, abbreviated lmA typical 100 W light bulb emits 1750 lm

• IlluminanceThe rate at which light falls on a surfaceRepresented by the letter EThe unit is lumens per square meter,lm/m2 = lux, abbreviated lx

• Consider a 100-W light bulb in the middle of a sphere. What is the illumination (illuminance) of the sphere?

• Note that illumination is proportional to 1/r2

Inverse-Square Relationship:

Light from a point source spreads out over an area proportional to the square of the distance from the source:

• Luminous Intensitythe luminous flux that falls on 1 m2 of a sphere 1m in radius. Unit: candela, cd which is the SI unit of light intensity.Luminous Intensity = P/4

• A lamp is moved from 30 cm to 90 cm above the pages of a book. Compare the illumination on the book before and after the lamp is moved.

• 7. What is the illumination on a surface 3.0 m below a 150 Watt incandescent lamp that emits a luminous flux of 2275 lm?

• A 64 cd point souce of light is 3.0 m above the surfaceof a desk. Wht is the illumination on the desks surface in lux?

• The illumination on a tabletop is 20 lx. The lamp providing the illumination is 4.0 m above the table. What is the intensity of the lamp?

• A public school law requires a minimum illumination of 160 lx on the surface of each students desk. An architects specifications call for classroom lights to be located 2.0 m above the desks. What is the minimum luminous flux the lights must deliver?

• End 16-1