L 31 Light and Optics [1]
• Measurements of the speed of light:
186,000 miles per second (1 foot per nanosecond)
• light propagating through matter – transparent vs.
opaque materials
• colors, why is an orange orange?
• The bending of light – refraction
• dispersion - what makes the pretty colors?
• total internal reflection- why do diamonds sparkle?
• how are rainbows formed
• Atmospheric scattering
• blue sky
• red sunsets
Measurement of the speed of light
• Speed of light in vacuum = c =
300,000,000 m/s = 186,000 miles/s
• the moon is 239,000 miles from the earth, so it
takes 239,000 mi/186,000 mi/s =1.3 s for light
from the moon to get to the earth
– 8 minutes from the Sun to Earth
– 24 minutes across the solar system
• Galileo was the first to consider whether the
speed of light was finite or infinite
• He attempted to measure it by stationing himself
on one mountain and an assistant on a nearby
mountain
Galileo and the speed of light
Galileo’s
assistant
Galileo
D
• Galileo turns on his flashlight and starts his clock
• When his assistant sees Galileo’s light, he turns
on his flashlight
• When Galileo sees the light from his assistant’s
flashlight, he notes the time
Galileo’s result
• “FAST”
• Suppose D = 2 miles, then the time delay
would be t = D/c = 5 millionths of a sec.
• It is not surprising that Galileo was not
able to measure this!
• Modern measurement of the speed of light
using fiber optics Î we can do this!
v(speed in medium)
= c(speed in vacuum) / n
The speed of light inside matter
• The speed of light c = 300,000,000 m/s in
vacuum
• In any other medium such as water or
glass, light travels at a lower speed.
• The speed of light in a medium can be
found by using v = c/n, where c is the
speed in vacuum (300,000,000 m/s) and n
is a number (greater than 1) called the
index of refraction.
MEDIUM
INDEX OF
REFRACTION
(n)
SPEED OF
LIGHT (m/s)
(v)
300,000,000
Vacuum
air
water
Exactly 1
1.000293
1.33
glass
1.52
197,368,000
diamond
2.42
123,967,000
225,564,000
1
light propagating inside matter, e.g., glass
• when light is transmitted through matter, some of the
electrons in the matter are forced into vibrations
• when the electrons vibrate they reemit the light waves Î
this happens in transparent materials
• this is a resonance effect, like the effect of one tuning
fork making a nearby tuning fork vibrate
• in some materials the energy that the electrons receive
from the light wave is passed on to a neighboring atom,
thus giving up the energy as heat Î this is what
happens in opaque materials
• the delay between receiving the light energy and
reemitting the light energy is way causes light to travel
more slowly in a material compared to in vacuum
glass blocks both ultraviolet and
infrared light, but is transparent to
visible light
Glass
ultraviolet
visible
infrared
VISIBLE LIGHT
COLOR
• Any color can be made by combining primary
colors Æ Red, Green and Blue
• A color TV uses mixtures of the primary colors to
produce “full color” images
• Perceived color is a physiological effect Æ
depends on how our eyes work
Color Î WAVELENGTH OR FREQUENCY
Wavelength × Frequency = c
RefractionÆ the bending of light
• One consequence of the fact that light
travels more slowly in say water compared
to air is that a light ray must bend when it
enters waterÆ this is called refraction
• the amount of refraction (bending) that
occurs depends on how large the index of
refraction (n) is, the bigger n is, the more
bending that takes place
Reflection and refraction
at a surface
Incident
Light ray
Normal line
reflected
Light ray
refracted
Light ray
2
Refraction of light
Normal incidence
• If the ray hits the interface at a right angle
(we call this normal incidence) there is no
refraction even though the speed is lower
• The wavelength is shorter, however
Incident
ray
refracted
ray
Water n= 1.33
Glass (n=1.5)
The refracted ray is bent more in the glass
Refraction from air into water
normal
n = 1.0
n = 1.33
Refraction works both ways:
water into air
When a light ray
goes from air
into water, the
refracted ray is bent
away toward the
normal.
water
Some interesting effects
due to refraction
• Underwater objects appear to be closer to
the surface than they actually are
• Total internal reflectionÎ fiber optics
• Seeing through a window
• Dispersion Î rainbows
normal
n = 1.0
n = 1.33
When a light ray
goes from water
into air, the refracted
ray is bent away
from the normal.
water
What does it mean to “see” something?
• To “see” something, light rays from the object must get
into your eyes.
• unless the object if a light bulb or some other luminous
object, the light rays from some light source (like the
sun) reflect off of the object and enter our eyes.
BEER!
3
Where is the fish? Closer
than you think!
Total internal reflection
Apparent location
Of the fish
n2
n 1 > n2
When the incident
angle is too big, the
refracted ray disappears
and the incident ray is
totally reflected back.
fish
Fiber optics
ÎA fiber optic cable is a bunch (thousandths) of
very fine (less than the diameter of a hair) glass
strands clad together.
Î The light is guided through the cable by
successive internal reflections.
Seeing through the window
A light ray is offset slightly
When it passes thru a pane
Of glass. The thinner the
Glass, the smaller the offset.
fiber optic communications
• can carry more info with less distortion
over long distances
• not affected by atmospheric conditions or
lightning and does not corrode
• copper can carry 32 telephone calls, fiber
optics can carry 32,000 calls
• takes 300 lbs of copper to carry same info
as 1 lb of fiber optics
• downside Î expensive
Seeing thru a window
ref
lec
t
ed
en t
incid
When the angle of incidence
Is small, most of the incident
Light passes thru the glass,
Only a small amount is
reflected
ted
smit
tran
4
Windows behaving as mirrors
When the angle
Of incidence is
Large (grazing
Incidence) more
Light is reflected,
The window is
Like a mirror
in
c
id
en
t
ted
lec
ref
transmitted
5