12/5/2011
Final review
Discharge lamp summary
1) Free electrons
speeding down
tube bash atom
Physics 1010: Dr. Eleanor Hodby
2) Electrons in atom
jump to higher energy
(Excited Atom)
A
3) Electrons in atom jump
back to lower energy state,
giving off photon of light.
Ephoton = hc/l
A
A
• Electrons in atom have just a few allowed energy levels
• Photon energy = energy difference between levels
 An atom emits just a few different photon energies (colors)
Day 29:
Finish fluorescent lights
Review
Reminders:
HW 11 due tomorrow at midnight
Final: Monday 4.30-7pm, G1B30. MANDATORY.
Review class tomorrow (wed) 7pm, G1B31
FCQ Thursday at end of lecture
• Need white light which contains every visible wavelength
 add phosphor to atomic discharge lamp
• Fluorescent light: Mercury discharge lamp (emits UV photons)
+ Mixed phosphor coating (converts UV to variety of visible photons)
The human eye
Why does fluorescent light look white?
Modern fluorescent
light
Electron transitions in different phosphors
Retina
Cones:
fovea
UV
from
Hg
cornea
lens
• Combination of different phosphor molecules produces
several colors of light
900nm
• Like atoms, phosphor molecules have allowed energy
levels and jumps, so emit just a few photon energies (colors).
Vitreous
humor
Rods:
900
300nm
300
• However this is not a continuous spectrum of colors
like in sunlight.
Sun
Light from an object (eg, the tree) is focused onto your retina, where special
cells turn these photons into electrical signals that go to your brain.
• The rod cells are really sensitive, but do not distinguish colors.
- That’s why you see in black and white at low light levels
• There are three different types of cone cells, each of which responds to a
different wavelength (color) range
• So why fluorescent light look white?
Cone cell response
Seeing color
This cone picks up red light best.
This cone picks up green light best.
Suppose your cones detect two
yellow photons.
Your brain sees one signal in a red
cone, and one signal in a green cone,
and interprets this (correctly) as
yellow.
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12/5/2011
Seeing color
Seeing color
• Because of the’crude’ way we detect color, a small collection of colors (3)
can trick our brains into ‘seeing’ all colors.
• Equal, strong stimulation of all 3 cones = white light
Your brain sees one signal in a red
cone, and one signal in a green cone,
and interprets this as yellow light,
too!
Suppose your cones detect one
orange photon and one green
photon.
You can ‘see’ yellow light without any yellow photon entering your eye!
300nm
900nm
Which of these spectra look white to us?
- LCD screens, color TV only produce red, green, and blue, in
different combinations.
- LED ball demo. White LED lights
- Spinning colored disk
Recycling your old CFLs
E: All of them
B
A
300
900
C
D
intensity
900
350
• Examples:
300
600
l (nm)
900
• All fluorescent lights including CFLs contain mercury.
• Mercury is nasty stuff: do not ingest or inhale the vapor
• See EPA guidelines for cleaning up a broken CFL
• Do not put a used CFL in trash: recycle it for free
- Western Disposal Services Waste Transfer Station (www.westerndisposal.com)
- Ace hardware
- McGuckins
Fluorescent light summary
Midterm 1 topics - Motion
V
•
•
•
e-
A
A
+
Discharge lamps (e.g. Sodium, Hydrogen, Neon)
- Very efficient (80%)
- Limited number of electron transitions in atom  Produce colored light
Fluorescent lamp (60% efficient)
- Low pressure Hg discharge lamp (UV photons at 254nm)
- Coating of variety of phosphors
- Phosphor molecule gets excited by absorbing UV photon
- Reradiates energy as lower energy visible photons
- By using a variety of phosphor molecules (and hence electron
transitions) fluorescent bulb emits photons of several colors
Eye has only 3 color detectors (red, green and blue).
- Equal stimulation of all 3 tricks brain into ‘seeing’ white light
•
Position, velocity and acceleration
– Definitions, Units
– Scalars and vectors
– Graphs of x, v, a vs time and relationships between graphs
– Equations of motion and how to use them:
Constant velocity:
x = x0 + vt
Constant acceleration: v = v0 +at
x = x0 + v0t + ½ at2
•
Forces
– Definition, units, vector
– Fgravity = mg downwards
– Ffriction  0.3 × weight in direction opposing motion
– Fspring = -kx in direction opposing extension/compression
– Fnet = ma  If a = 0, Fnet = 0
– Free body diagrams and finding Fnet
2
12/5/2011
Midterm 2 summary
• Conservation of energy
Wext - Wfriction = DPE + DKE
- Work done by a force = F × d//
- Looked at work done by external forces and by friction
- GPE = mgh, KE = ½ mv2, PPE = PV, SPE= ½ kx2, Thermal energy = constant × T
- Ramps, roller coasters, balls…….
- Power = energy/s
• Bernoulli’s equation
Etpv = P + ½ rv2 + rgh
- Conservation of energy for an incompressible fluid
• Sound
- Wave basics: f, A, T, l, v (and relationships)
- How to get different notes from a violin
- Harmonics on a violin string
Review Topics – midterm 3
Blackbody spectrum
- Introduction to EM waves and the EM spectrum
- Kelvin temperature scale
- Stefan-Boltzman law
- Shape of BB spectrum at different temperatures –
 why the sun produces visible light efficiently and incandescent lightbulbs don’t.
- Green house effect
Static electricity
- Coulomb’s law for force between point charges
- Voltage and electric potential energy (EPE)
Electric circuits
- Ohm’s Law
- Power dissipation law
- Batteries in series
- Bulbs in series and Parallel
Last couple of lectures review
Class Review Topics
Radio waves
• F = qE
• Field around a point charge
• EM waves created by accelerating charges
• Optimizing transmission/reception of a radio wave
- Power, polarisation, antenna length
• Tuning your radio: Tank circuits
• Carrying information with a radio wave
- Modulation schemes (AM and FM)
- Bandwidth
• Dangers of radio waves?
• Net forces and acceleration
• Conservation of energy (work, EPE, GPE, KE)
• Bernoulli’s equation
• Waves on a string
• Series and parallel circuits
Fluorescent lights
• Atomic discharge lamps
• Role of the phosphor coating
• How the eye perceives color
3