What makes the color pink?
Black and white TV summary
•
•
•
•
Picture made from a grid of dots (pixels)
Dots illuminated when electron beam hits phosphor
Beam scanned across entire screen ~ 50 times a second
Electron beam created and steered inside a cathode ray tube
– Created using electrostatic forces
– Steered using magnetic forces
Lecture 12
Color TV
Light and the sun
Deflection coils.
Reminders
HW 6 available, Due Monday
No labs this week
Use the time to review…..Midterm 1 week from today
Reading quiz now
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Different color pixels
Color TV
• 3 electron beams,
• 3 control grids (control intensity of each beam)
• 3 sets of phosphor pixels.
• Essentially black & white TV times 3 with colored phosphors.
8 pixels
Black & white
Color
Standard TV screen:
• 525 lines high
• 400 dots across.
Electron
energy
levels in
Phosphor
atom
Energy
- Color of emitted light determined by its wavelength (l)
- Wavelength determined by energy of photon: E = hc / l
- Photon energy determined by electron jump in phosphor atoms
- Electron jump determined by separation of energy levels in phosphor atoms
- Different phosphors produce different color light
Color TV- blend 3 color pixels.
See this
next week:
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3
Why does color TV work? … How does the eye see color?
Different color phosphors
Electromagnetic Spectrum: Visible Light
Black and white TV:
-just one phosphor
-bluish white light.
Oscilloscope:
-one type of phosphor
-green light.
Color TVs:
-3 phosphors
-blue, red, green.
Visible Light: electromagnetic radiation with wavelengths
between 390 and 690 nm.
A Photon: single packet of light energy of a particular wavelength
The Sun: emits photons at all of these different wavelengths
Red photons:
See this
next week:
Color TV emits:
5
Green photons:
Blue photons:
6
1
How does the eye see color?
The eye does not simply measure the wavelength of light to determine color
black and white sensors for night vision (RODS)
3 types of sensors for color detection (CONES)
Cone cell sensitivity:
Each cone responds differently to different color photons:
sensitivity:
CONE 1
CONE 2
CONE 2
CONE 3
CONE 1
CONE 3
Yellow photons will:
a. Stimulate cone 1 much more than cone 2 or cone 3.
b. Stimulate cone 1 and 2 about equally. Cone 3 no stimulation.
c. Stimulate cone 2 much more than cone 1 or cone 3.
d. Stimulate cone 3 only.
e. Cannot see anything because Cone 1 only detects red and cone 2
only detects green and cone 3 only detects blue.
Level of stimulation of Cone 1:
Red photons (650 nm): small to moderate stimulation
Orange photons (600 nm): high stimulation
Green photons (530 nm): small to moderate stimulation
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7
Each cone responds differently to different color photons:
Cone cell
sensitivity: CONE 1
CONE 2
Color TV - making yellow
• Color TV- blend 3 color pixels to make eye see full spectrum
• 3 electron beams, grids control intensity of each beam
- (Negative charge on grid – beam off)
CONE 3
• For yellow light:
- Green electron beam on
- Red electron beam on
- Blue electron beam off
8 pixels
Back to color TV. We have red, green, and blue photons coming out.
How can we make our BRAIN see yellow?
a.
b.
c.
d.
e.
Red, green, and blue in equal amounts.
Red and blue in equal amounts. No green.
Red and green in equal amounts. No blue.
Green and blue in equal amounts. No red.
Some other combination of red, green, and blue.
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9
Color demo
What will happen if we spin this colored
disk very fast?
a) We will see blurry green and red
b) We will see black
c) We will see white
d) We will see blue
e) We will see yellow
10
Two Useful Sims
• Color Vision
http://www.colorado.edu/physics/phet/simulations/colorvision3/colorvision3.jnlp
12
2
Seeing white light
What is white? The Spectrum of the Sun
Cone cell sensitivity:
Emits slightly more blue/green
CONE 2
CONE 3
CONE 1
Emits slightly less red
Cones of eye see combination of strength of colors given off by
sun as “white” … high and equal stimulation of all three cones
White light
If less light, but still equal stimulation …
darker shade … grey instead of white.
Sensor
level
Cone 1
Cone 2
Cone 3
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14
What about pink?
Cone cell
sensitivity:
CONE 1
CONE 2
CONE 3
a. pink photon has more energy than a yellow photon
b. pink photon has less energy than a yellow photon
c. pink photons do not exist.
Red 656 nm
Blue 486nm
Violet 434 nm
15
Finish TV’s – some recent buzzwords:
16
Light
Digital TV:
• New way of transmitting information to the TV. (CRT inside TV is unchanged)
• Send signals about brightness of each pixel digitally. (pixel 218 x 111
red-brightness 12, pixel 218 x 111 green-brightness 2, etc )
• Electromagnetic spectrum
• Where does light come from
• Light interacting with matter
HDTV (high definition TV):
• More pixels per inch.
• Already do both (digital and more pixels) in computer monitors.
– Colors of objects – absorption and reflection
– Scattering of light
– Refraction and reflection
Flat screen monitors-- completely different technology!
• see physics 2000
• Light comes from back. Liquid crystal light valves at each pixel open and shut
to allow desired amount of red, blue, green through.
Plasma TV – Yet another different technology
• Each pixel made of tiny R, G and B florescent lights
• Applied voltage accelerates electrons → collide with noble gas atoms → emit
UV photons → excites phosphor → emits colored visible light
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17
3
Electromagnetic Spectrum
What is light?
Spectrum: All EM waves. Complete range of wavelengths.
Wavelength (l) = distance (x) until wave repeats
Frequency (f) = # of oscillations per second at X
• Part of electromagnetic spectrum
– Wavelengths: 390-690nm
– Photon energies: 3-5×10-19J
• Behaves as both electromagnetic
wave and particle of energy (photon)
• What is waving?
– Wave consists of oscillating
electric and magnetic fields
l
Blue light
l
LONG
SHORT
Red light
19
Speed of light ( and all other EM waves)
Energy in light
All EM waves (all colors of light) travel at same speed in vacuum
Speed = speed of light (c) = 3 x 10 8 m/s.
• Light comes in little particles of energy, called photons
• Each wavelength has own size energy particle:
Green
Energy of one photon = h x speed of light = h × f
l
Blue
h = Plank’s constant = 6.6x10-34 J s
c = speed of light =3.0x108 m/s
Red
Total energy
speed = wavelength x frequency
meters
oscillation
Photon picture: Etotal = number of photons x energy per photon
Wave picture: More energy means bigger amplitude oscillations of E
and B fields
oscillations
sec
c=l×f
22
How is light (and rest of EM spectrum) created?
a)
b)
• Almost all light on earth comes from sun.
An oscillating electric field
An oscillating magnetic field
e
• Ultimate source of nearly all energy on earth
B
But that is just an EM wave!
-
b)
e.g. Discharge lamp, LED
Discreet wavelengths emitted
Free electrons with lots of thermal energy that
are whizzing around and bumping into stuff
-
e.g.Hot light bulb filament, Surface of sun
BB radiation emitted
• Hot ball of atomic/sub atomic soup
- atoms (73% hydrogen, 25% helium)
- free electrons, protons and neutrons
• Different layers at different temperatures:
- Core – hottest (14 million K)
Powerhouse of Sun
- Photosphere (5800K)
Where visible light comes from
Energy
Where do we find accelerating or oscillating charges?
a)
Electrons in an atom or solid moving from one
allowed energy level to another
The Sun
E
Accelerating (oscillating) electric charges create:
• Power output: 4*1026 W (400 YottaWatts)
- V. large power plant 1*109W (1GW)
++
+
+
24
4
Where does the sun’s energy come from?
Light from the the Sun
– Sun’s energy from nuclear fusion, like in a hydrogen bomb.
– Fusion - creation of larger nuclei from smaller ones
- mass difference released as energy
p
p
Various
steps
+
p
p
4 hydrogen
nuclei
pn
np
helium
nucleus
1. Get electrons hot (5500 C) with nuclear fusion
2. Electrons have lots of kinetic energy. Shaking (oscillating) around
with a range of frequencies
n
+
+
+
3. Emit electromagnetic radiation (light!) at those frequencies
n
2 neutrinos
2 positrons
Lots of
energy
- Called “Black body” or “thermal” radiation.
- Emitted light depends only on temperature
++
• Hydrogen “burning” in core produces helium, and energy to power the sun
• Helium nuclei fuse to form larger elements (up to iron) more slowly
• Released energy goes into thermal energy
 means atoms and electrons in the sun are VERY HOT and have
LOTS of kinetic energy– they are moving and shaking a lot!
25
Blackbody spectrum of the sun
+
+
Free electrons and protons
26
A useful simulation
• Blackbody radiation
INTENSITY
What if sun was hotter?
Sunlight would look:
a. same as now,
b. redder
c. bluer
WAVELENGTH
http://phet.colorado.edu/simulations/sims.php?sim=Blackbody_Spectrum
27
Power, Energy, Intensity
28
Light interacting with matter
• What’s the difference between
1 Joule of energy used in 1 second
and 1 Joule of energy used in 1 year ?
Power = Energy / Time
• 1. Color of objects (absorption/reflection of different l)
• 2. Scattering
• Watts = measure of power (Joules / second)
– Why the sky is blue
– Why sunsets are red
• What about how much area that Power goes into?
• 3. Refraction and reflection at interfaces
Intensity = Power / Area = Energy / (time x area)
– Underwater objects
– Twinkling stars
Units: Watts/m2
29
5
Color of objects
1. Color of objects
• Types (colors) of photons absorbed depends on atomic energy levels in
material.
• Level separation must match photon energy for absorption.
• Leaf contains several different molecules absorbing red, yellow and blue
photons.
It’s all about what the electrons in the atoms of the object are doing:
 What colors do they absorb?
 What colors do they reflect?
Electron
energy
1. Leaves look green to us. What colors do they absorb? What
colors do they reflect?
A
a.
b.
c.
d.
Absorbs green. Reflects reds, yellows, blues
Absorbs blue. Reflects reds, yellows, greens.
Absorbs reds, yellows, blues. Reflects greens.
Absorbs reds, yellows. Reflects greens and blues.
B
C
The diagrams above show energy levels in 3 types of leaf molecule.
One absorbs red, one yellow and one blue light.
Which absorbs blue light?
31
White objects
Look at paper in bright sunlight, it looks white.
This means that:
a. paper is absorbing all colors of light from the sun equally
b. scatters only green and yellow
c. scatters all colors from sun equally
d. absorbs all but red
e. scatters white photons.
6