# Lesson 1: Light Intro

```Why is the sky blue?
Why are clouds white?
Why are sunsets red?
Optics
______________is
the study of light.
Light is a ____________________________________wave.
transverse electromagnetic
Light can act like a…
Doppler effect
__________–
_________________
wave
diffraction
- _________________
interference
- _________________
particle
photon
…or like a ______________called
a____________
.
all that you can see .
(Visible) light is ________________________
e&m ) ___________________:
spectrum
The electromagnetic ( _____
ROYGB(I)V
The only differences between the different radiations
wavelengths
frequencies
are their _________________
and/or __________________
.
For this reason, the entire ______________________is
e&m spectrum
light
sometimes referred to simply as ___________________
.
Ex. Compare sound and light waves
vibration
type
wave
sound longitudinal
light
transverse
type of
propagation
amplitude
determines:
mechanical loudness
e&m
brightness
Ex: Light waves in a vacuum:
C:
A:
D:
B:
Which light wave is brighter and blue-er? D
Which is dimmer and blue-er? A
Which is brighter and redder?
Which is dimmer and redder?
B
C
frequency
determines:
pitch
color
Ex: Put in order from slowest to fastest in a vacuum:
at the same
All e&m radiation (light) travels ________________
________
________________________:
___________________________.
speed in a vacuum
Ex. Find the wavelength of a red light wave.
Ex: Put in order from slowest to fastest in a vacuum:
at the same
All e&m radiation (light) travels ________________
________
3.00 x 108 m/s
________________________:
___________________________.
speed in a vacuum
Ex. Find the wavelength of a red light wave.
v = fl
3.00 x 108 m/s = (
???
)l
Reference Tables, page 2, top:
The Electromagnetic Spectrum
Are these
frequencies
or wavelengths?
Choose f = 4.0 x 1014 Hz
Ex: Put in order from slowest to fastest in a vacuum:
at the same
All e&m radiation (light) travels ________________
________
3.00 x 108 m/s
________________________:
___________________________.
speed in a vacuum
Ex. Find the wavelength of a red light wave.
v = fl
3.00 x 108 m/s = (
???
)l
3.00 x 108 m/s = l
(4.0 x 1014 Hz)
0.75 x 10-6 m = l
7.5 x 10-7 m = l
 small  little diffraction, normally
Wavefronts propagating from a source
Spherical
wavefronts
plane
wavefronts
Reflection
incident
________________:
A wave in one medium is _____________
boundary
on a surface (__________________
) and returns back
into the same medium
______________________________.
perfectly smooth
Ex. waves incident on a _________________________surface
incident
wave
surface
reflected
wave
Reflection can be understood using Huygens
wavefronts that spread out from each point on
the surface as the incident wave arrives:
incident
wave
surface
reflected
wave
anechoic chambers
a(n) = no
echoic = echo
- very little reflected
sound waves….
Why?
Each time the sound
wave is reflected, it
loses some energy.
wavefronts
For simplicity, this:
direction of motion of wave
…will be replaced by this:
a “ray”
the direction
The ray arrow only shows _______________________
of the wave.
Using only rays:
normal
incident
ray
reflected
ray
qi
qr
surface
incident angle
qi = qr
reflected angle
Law of Reflection
This is called the ___________________________.
with respect to
The angle q is always measured _________________
_____________________
.
the normal.
In the Physics Reference Tables: page 5
Ex: What is the angle of reflection?
600
Draw the reflected ray.
incident
600
600
300
reflected
surface
Ex: What is the angle of incidence? 100
surface
Draw and label the incident ray.
100
incident
reflected
100
How does reflection change a wave?
incident:
vi, fi, li
normal
reflected:
vr, fr, lr
surface
vi = vr
1. Same medium  _________________
fi = fr
2. Same color  ________________
li = lr
l = v/f,  _____________.
3. Since _____
reflected
As a result, the _______________
wave has the same
v, f and l
_______________________
as the incident wave.
always
The Law of Reflection is _____________true…
all waves:
1. …for ______
light
__________
___________
sound
earthquake
water
__________
___________
gravity
__________
___________
etc
2. …and is ______________________ of the v, f or l
of the incident wave. This means that all
different ________________ of light and
_______________ of sound obey the Law.
Ex:
surface
400
________ angle
for any ________
Sound waves obey the same Law of Reflection as light.
How do they “aim” the sound in these concert halls?
always
The Law of Reflection is _____________true…
all waves:
1. …for ______
light
__________
___________
sound
earthquake
water
__________
___________
gravity
__________
___________
etc
2. …and is ______________________
of the v, f or l
independent
of the incident wave. This means that all
colors
different ________________
of light and
pitches
_______________
of sound obey the Law.
Ex:
surface
400
400
same angle
________
for any ________
color
Reflecting telescopes: All colors reflect at the same
angle and meet at the same focus:
white
light
focus
white
light
curved
mirror
mirrors
Why are some surfaces good _______________?
Types of reflection:
Regular, mirror, specular
1. _____________________________:
from smooth, flat
surfaces
diffuse
2. _____________:
from rough surfaces
Regular
________
Diffuse
________
both regular
The Law of Reflection is obeyed for_______________
diffuse reflection.
and____________________________.
Even though the ______________
are in different
normals
angles are equal .
directions, the incident and reflected _________________
Identify regular and diffuse reflections.
3 m Liquid Mirror Telescope (LMT). This
unique telescope used a pool of mercury
spun in a dish at 10 rpm to form the
primary mirror. The main limitation of
the telescope was that it could only
point vertically. The LMT was used to
optically measure the low Earth orbit
(LEO) debris environment. The telescope
was located in Cloudcroft, NM and was
closed in 2001.
These must be smooth
for visible light, whose
l is ~ 10-6 to 10-7 m
The Hubble mirror.
Why
aren’t
these
telescopes
smooth?
Compare radio wavelengths to visible wavelengths:
l = 1-103 m
wavelengths,
the telescopes
"appear"
smooth.
visible:
l = 10-6-10-7 m
Ex: Driving at night:
dry
you
diffuse
reflection
oncoming
traffic
rough surface  more diffuse reflection
 some light scattered back you see road
 less scattered forward  less glare for oncoming
wet
oncoming
traffic
smooth surface  less diffuse reflection
 less light scattered back you can’t see road
 more scattered forward  more glare
for oncoming traffic
Why is the sky blue?
Why are sunsets red?
Why are clouds white?
Water drops and ice crystals of all
different sizes scatter (reflect) all
different wavelengths (frequencies)
of light.
Why are clouds black?
- in shadow of upper clouds
- light cannot penetrate to them.
Why is the ocean greenish-blue?
1. It reflects the sky.
2. It reflect blue wavelengths
better than other colors.
3. It absorbs red wavelengths
better. Lobsters on the ocean
floor appear black because
little red light gets down there
Why are Uranus and Neptune green/blue?
Methane in their atmospheres
absorb orange/red, so the
reflected light is the complementary
color.
Reflection nebulae – young stars, surrounded
by dust that scatter the blue light in our direction.
Ex: Albedo = percentage of incident light that is
reflected back from a planet or moon.
What is the albedo of the Moon? In other words,
of all the sunlight incident on the Moon, what
percentage is reflected back?
Only 0.07 = 7% on average!
out of 100%
only 7% on
average is
reflected back
It seems very bright against a dark sky.
Its brightness is best compared when
1. an "almost full" moon is rising after sunset; or
2. a "just past full" moon is setting at sunrise:
white building
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