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8 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

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Dr. Baron

Physics


Ch. 16:

Fundementals of Light

Illumination


Ray Model of Light


Light travels as wave with an overall straight
path


Rays change direction if:


Reflected




Refracted

Light Sources


The Sun or a light bulb is a Luminous
Source


The Moon or a car reflector is an
Illuminated Source


Media are:


Opaque (no transmission, some reflection)


Transparent (allow transmission)


Translucent (transmit and reflect)



Flux and Illuminance


Luminous Flux = Rate at which light is
emmited from luminous source. The
SI

unit of luminous flux is the
lumen

(lm).


Illuminance = rate at which light falls on
surface. In
SI

derived units
, these are both
measured in
lux

(lx) or lumens/square cm.



Inverse Square Law


A point source with a luminous flux of P
gives illuminance of E at a distance r from
the source such that:




E

=


P





4
π

r
2



Units: P is in Lm E is in lx and r is in m

Speed of Light


Galileo first said speed is constant


He also used speed = distance/time


Later measurements showed c = 2.2 x 10
8

m/s


Michelson used time of flight of light on 2
California mountains to give: 2.998 x 10
8


Light year is distance of light travelled in 1
yr (9.46 x 10
12

km in one year

Difraction and Waves


Grimaldi found edge of shadows are
“fuzzy”


Huygens said light was in little waves
which overlap after passing an edge

Spectrum of Visible Light

Color Wavelengths


RED




7 x 10


7 m


VIOLET



4 x 10


7 m


Wavelengths & Photons

Red Photon

Blue Photon

Green Photon

Yellow Photon

Prism

Spectrum

Particles of light, called photons, each have a wavelength.

Adding Color Lights

Stream of

red & green photons

looks
same as

yellow photons

(metamerism)

Theatrical lighting

or

Eye to

Brain

Notice overlap of
red
,
green
, &
blue

is seen as
white

light

Additive Color Wheel

Spectral

Colors

There are
No

Photons
of These
Colors

R

Y

B

G

M

C

Red

Yellow

Green

Cyan

Blue

Magenta

Color by Subtracting


Color can be changed by absorption and
reflection


Dyes

(usually minerals) are molecules that
absorb and reflective certain wavelengths


Pigments

(usually plants or insects) do the same


White light on Red dyes and pigments absorb
blue and green and refect red


Blue light on Red dyes and pigments mostly
absorb blue and appear black (no reflections)


Color by Subtracting (cont)


Primary Pigments absorbs
1

and reflect
2

with white light.


Secondary Pigments absorbs
2

and reflect
1

with white light.



CMYK color model


In the printing industry, to produce the varying colors the
subtractive primaries

cyan
,
magenta
, and
yellow

are
applied together in varying amounts.

Light Polarization

Polarized Sunglasses

Reduce glare off the roads while driving

Linear Polarization

Polarization Photography

Reduce Sun Glare

Reduce Reflections

Darkens Sky

Increase Color Saturation

Reduce Haze

Polarization Photography

Without Polarizer

With Polarizer



Provides better Color Saturation




Darkens the sky

Polarization Photography

Without Polarizer

With Polarizer

Polarization Photography : Scattering

De
-
hazed

Haze

Polarization Photography : Reflections

Reduce

Reflections

Polarization Photography : Reflections

Reduce

Reflections

Polarization Photography : Underwater



Underwater pipelines


and communication



Offshore structures



Underwater ROV/AOV



Offshore drilling rigs



Vessel inspection



Recreational photography



Marine archaeology



Marine biology



Underwater mapping

Crossed Polarizers

Law of Malus

Amplitude:

Intensity = Const . (Amplitude)^2

Law of Malus

In words…..

The Intensity of light coming out of a
second polarizing filter is equal to
the intensity of polarized light from
the first filter times the cosine,
squared, of the angle between the
polarizing axes of the two filters

The Speed of a Light Wave


Obeys wave equation (velocity = distance/time):

C =
λ
/T


Where c = speed of light (constant at 3 x 10
8

m/s




λ

= wavelength (m)




T = period (secs)


Other way of writing it uses f = 1/T so:

C =
λ

f


where f is the frequency

Light (cont)


Relative Motion and Light


The Doppler Effect also can be found in
light waves BUT light is different than
sound:



does not need a medium



has two vibrations so we use only part
of

wave on axis between source and
observer

Doppler
Frequency

Eqn. for Light


f
o

= f
s

[1
+

(v
s



v
o
)]


where f
o

is frequency observed




f
s

is frequency of source




v
s

is velocity of source




v
o

is velocity observed

Use + for moving away,
-

for moving towards

Doppler
Wavelength

Eqn. for Light

Since
λ

= 1/f


1/
λ

o

= 1/
λ

s

[1
+

(v
s



v
o
)]


where
λ

o

is wavelength observed




λ

s

is wavelength of source


Rearrange:

λ

s

=
λ

o

[1
+

(v
s



v
o
)]





Use + for moving away,
-

for moving
towards

Doppler
SHIFT

Eqn. for Light

Doppler shift: =
λ

o

-

λ

s

Use + for moving away (called a red
shift),


Use
-

for moving towards (called a blue shift)

1932 Edwin Hubble


Before 1919:Universe was only as large as the Milky
Way and constant size.


1919, Hubble found stars further than the Milky Way


Our galaxy was just one of many galaxies


Hubble saw mostly redshifts in the light wavelengths


Also Hubble saw that galaxies were moving away from
each other at a rate constant to the distance between
them. (Hubble's law)


These findings signaled that the universe is expanding


Laid the foundations for the Big Bang theory


states that the universe exploded into existence from a single
point or a very small region in time and space and has been
expanding ever since.




1989 NASA named the Hubble Space Telescope after
him

Doppler Effect: frequency shifts due to moving source/observer

If the speed of the source is much less than the speed of light





f
d

= f
s

(1+ v
s
/c)
or




f
d

= f
s

(1
-
v
s
/c)


Where f
d

is the frequency detected



fs


frequency of the source


c

velocity o f the light wave


v
s

velocity of the source

Assume coordinate system such that positive means

source moving away from observer (or detector)






Dopler “Shift”


Changing Dopler Frequency equation to
Wavelength gives:




λ

=
+

v
λ
/c