Transcript Class 4

Physics 1230: Light and Color
Ivan I. Smalyukh, Instructor
Office: Gamow Tower, F-521
Email:
[email protected]
Phone: 303-492-7277
Lectures:
Tuesdays & Thursdays,
3:30 PM - 4:45 PM
Office hours:
Mondays & Fridays,
3:30 PM – 4:30 PM
TA: Jhih-An Yang
[email protected]
Class # 4
Textbook Reading & Our Plans
Read Chapter #1
Reading is necessary to do well in
solving HW and exam problems;
Today – finish material of Chapter #1;
Next class – Chapter #2
2
When does an electric field
exert a force on a charge?
A. Always
B. Sometimes (changing field)
C. Sometimes (depends on the charge)
D. Sometimes (depends on many
things)
E. Something else
Speed, wavelength, frequency
Wavelength (lambda), or
how wide each oscillation
is (meters)
Speed = “c” in a vacuum
v=fλ
Units:
m/s
Either
wavelength
or frequency
will tell you
the color of
light
Frequency, or oscillations
per second (Hertz). f
Clicker questions
•
•
•
Which of the light
waves has the longest
wavelength?
Which of the light
waves is brightest?
Which of the light
waves has the highest
speed in empty space?
a) b) c)
d) They all have the
same speed
A
B
b)
c)C
6
Let’s practice:
Electromagnetic spectrum sheet
v=fλ
Clicker Question
An FM radio station transmits at a frequency
of: f = 100 MHz = 108 Hz
then the wavelength is :
A)1 m B) 0.3 m C) 3 m D) 100 m
E) None of these.
c=lf
f=c/l
l=c/f
l = [3 x 108 m/s] / [108 1/s] = 3 m
8
Clicker Question
Your microwave oven operates at a
frequency of: f = 3 GHz = 3x109 Hz
then the wavelength is :
A)1 cm B) 0.3 cm C) 3 cm D) 30 cm
E) None of these.
c=lf
f=c/l
l=c/f
l = [3 x 108 m/s] / [3x109 /s]
l = 0.1 m Or 10cm
9
How does an ordinary incandescent
light bulb work?
Gas
Filament with current of electrons
which hitinto atoms causing light to
be emitted
Atom
Atom
Electron
s
Atom
Light emitted
Electrode leading to
the other side of
the wall plug
Electrode leading to one
side of the wall plug
We see only some of the emitted light:
The hotter the source the more bluish the light.
The cooler the source the more reddish the light
These are curves of the intensity of
each of the wavelengths present in an
"off-white" light as the temperature
of the filament in the bulb increases.
Fluorescent lamp
(based on fluorescent mineral coating)
Fluorescent coating absorbs UV
photons from mercury gas discharge
and emits visible light photons with
high efficiency.
Calcium fluoride is fluorescent.
Lifetime:
Energy used:
Cost:
Demo: fluorescent minerals
8-15 times longer than incandescent
3 – 5 times less
3 – 10 times more
12
Sources of light
•
•
•
•
•
•
•
Flame (atomic emission)
Incandescent
Gas discharge (neon or mercury atomic emission)
Fluorescent (mercury gas discharge + phosphor)
Plasma TV (phosphors have color)
Light emitting diode (LED)
Laser
All sources involve moving electrons !
13
Intensity
Intensity
Color temperature of "white" can be understood by
mixing just three lights of different intensities
Off-white
in center is
slightly
reddish.
Low color
temp
Off-white
in center is
slightly
bluish.
High color
temp
From small to large WAVELENGTH
So now we can understand these different images as being due to
different wavelengths and frequencies of light emitted and detected
How big is 500 nm?
• 500 nm = 0.5 mm
• Human hair is 17 – 180 mm in diameter.
A 50 mm hair is 100 wavelengths.
• There are 2000 wavelengths of green light per
millimeter.
• A “light” microscope cannot see things smaller
than about 0.5 mm.
18
A prism
spreads out
the overlapping
wavelengths
in white light
into different
spatial
locations
where they
can be seen
as colors.
We see color when waves of different
wavelengths enter enter our eyes!
Light with wavelength of 650 nm
appears red when it enters a viewers eye
Light with wavelength of 520 nm
appears green when it enters a viewers ey
Light with wavelength of 470 nm
appears blue when it enters a viewers eye
The speed of light in empty space is the same for all wavelengths
Clicker question
•
What does Alex see
when the wave at left
with wavelength 650 nm
goes by him?
a)
b)
c)
d)
e)
Red
Blue
Green
White
Nothing
What happens when two or more waves
with different wavelengths reach your eye?
Light with both wavelengths 650 nm and
520 nm appears yellow when it enters
a viewers eye
Light with only wavelength 580 nm
ALSO appears yellow when it enters
a viewers eye (A DEEPER YELLOW
THAN FOR THE CASE ABOVE)
What is white light?
Light which is a mixture of
650, 520 and 470 nm
wavelengths (and possibly
more wavelengths) appears
WHITE when it reaches your
eye
No single wavelength (monochromatic) wave appears
white
when it reaches your eye!
Different wavelengths scatter
differently from really tiny particles
• Blue and green scatter more strongly (“Raleigh scatte
– They have a small wavelength, so are better
scattered by small particles smaller than the
wavelength of light, so blue light comes at us from
everywhere overhead.
Sunset
So why does the sunset part look red?
• All the blue has been removed from scattering
in the looong distance that the light has to
travel from the horizon, leaving red behind
What can happen to incoming light
Transmitted
Through something opaque
Reflected (including scattering)
From something opaque
Opacity, transparency, and reflectivity
depend on the light’s wavelength
Infrared camera
Opacity of atmosphere
31
Transmission of glass
UV is
blocked.
No
sunburn
indoors.
IR is
blocked.
Opacity is the opposite of transmission.
0% transmitting is 100% opaque.
32
For THz radiation, skin is
opaque and clothing is
transmitting.
For x-ray radiation,
bones are opaque and
skin is transmitting.
Terahertz
33
Sunlight and the earth
• Sunlight is constant at 1000 W/m2
(UV and x-ray output varies)
• Earth’s temperature is a balance between
sunlight in and infrared radiation out
• Carbon dioxide blocks infrared going out and
has a warming effect.
34
Properties of light (Ch. 1)
1. Light travels in vacuum.
Sound travels in air (no sound in vacuum).
2. Light carries energy. (Sunlight warms, generates electricity.)
3. Light moves with a particular speed in vacuum,
but moves less rapidly in other materials (water, glass).
4. Light travels in vacuum in straight lines (rays).
Rays can be bent by materials.
5. Light has amplitude (intensity).
35
Review (Chapter 1)
• Light is an electromagnetic wave
• EM waves have both traveling electric and magnetic
fields
• EM waves are created by accelerating charge.
• EM waves can make other charges move.
• EM waves travel outward like waves in a pond, with
electric and magnetic vectors perpendicular to the
direction of motion.
• EM waves carry energy
36
Review (Chapter 1)
• c = 300,000 km/s =3 x 108 m/s
• c = fl, units are Hz for f and meters for l
• wavelength of light spans 400 – 700 nm
(blue to red)
• Atmosphere transmits light and radio,
is opaque to UV, x-rays, some IR
• Amplitude of a wave is the half-height
• Wavelength is distance from crest to crest
• Period is the time for the wave to go up and down
once at some location.
37