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

Chap 16-1

Light Fundamentals

What is Light?

• A transverse electromagnetic wave

What is Light?• A small part of the electromagnetic

spectrum

• The part which stimulates the retina of the human eye

• Visible light has wavelengths 400-700 nmREDLonger λLower fLower E

BLUE Shorter λ Higher fHigher E

How does light interact with matter?

• Light does not require matter (no medium) for transmission

BUT if light does pass through matter

• Light travels in a straight line through a– Vacuum or a– Uniform medium

Ray Model

• A ray is a straight line that represents the path of a narrow beam of light

• ignores the wave nature of light

• a useful model in analyzing reflection and refraction (ray diagrams)

Speed of Light

• Before 17th century: light thought to be instantaneous

• Galileo: thought speed of light was finite but could not measure it

• Ole Roemer (~1700) – through measurements of the period of Io, a moon of

Jupiter,– Calculated that light took 22 min to cross the diameter

of Earth’s orbit– Calculated speed of light at 2.2 x 108 m/s (75% of true

value)

http://en.wikipedia.org/wiki/Speed_of_light#Early_attempts

In 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.

Roemer’s Method for calculating the speed of light:

Albert Michelson

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

Michelson

• Won the Nobel Prize• Successfully measured the speed of light• c = 3.00 x 108 m/s in a vacuum• this is a “defined value” for light• Now objects lengths are determined by how long

it takes for light to travel from one end to the other– Examples:

• Definition of the meter• Light year

Sources of Light

• A luminous body emits light waves.

• An illuminated body reflects light waves produced by an outside source.

• Incandescent: light produced by a hot body.

Luminous Flux

• The rate at which visible light is emitted by a source

• Represented by the letter P

• The unit is the lumen, abbreviated lm

• A typical 100 W light bulb emits 1750 lm

Illuminance• The rate at which light falls on a surface

• Represented by the letter E

• The 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 Intensity

• the 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 π

6. 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?

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

10.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?

11.A public school law requires a minimum illumination of 160 lx on the surface of each student’s desk. An architect’s 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