What is light?
description
Transcript of What is light?
![Page 1: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/1.jpg)
What is light?
![Page 2: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/2.jpg)
Why light is important
I2
![Page 3: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/3.jpg)
Seeing the messengers?
Abu Ali Hasan Ibn Al-Haitham, ~1000 AD
![Page 4: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/4.jpg)
Particle theory of light
‘Is light not a body?’Sir Isaac Newton (1643-1727)
“The light and heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the interspace of air in the direction imparted by the shove.”Lucretius, ~50 BC
![Page 5: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/5.jpg)
Corpuscular theory of light
Reflection Dispersion
![Page 6: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/6.jpg)
Corpuscular theory of light
“Light does not bend into the shadow”
Light moves in straight lines
![Page 7: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/7.jpg)
Wave theories of light
Christian Huygens (1629-1695)
Each point on the existing wave-front can be considered to act as a source of waves (sometimes referred to as "secondary wavelets").
![Page 8: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/8.jpg)
Wave theories of light
![Page 9: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/9.jpg)
Wave theories of light
refraction
diffraction
![Page 10: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/10.jpg)
New observations
Thomas Young (1773-1829)
Light is a wave!
![Page 11: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/11.jpg)
What kind of wave?Light is an electromagnetic wave!
“This velocity is so nearly that of light, that it seems we have strong reason to conclude that light itself (including radiant heat, and other radiations if any) is an electromagnetic disturbance in the form of waves propagated through the electromagnetic field according to electromagnetic laws.”
![Page 12: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/12.jpg)
Crazy for Maxwell
![Page 13: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/13.jpg)
Experimental support
"I do not think that the wireless waves I have discovered will have any practical application."
Heinrich Hertz (1857 - 1894)
![Page 14: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/14.jpg)
Light as waves
=c
![Page 15: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/15.jpg)
Light as waves
100 m
167 times as small as the diameter of a hair!
0.6 m
600 nm = 0.6 m
![Page 16: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/16.jpg)
Light as wavesSodium vapor lamps are sometimes used for public lighting. They give off a yellowish light with wavelength of 589 nm. What is the frequency of this radiation?
€
υ =cλ
€
=3.00 ⋅108m /s589 ⋅10−9m
€
=5.09 ⋅1014 s−1 = 5.09 ⋅1014Hz
![Page 17: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/17.jpg)
Spectrum of EM radiation
![Page 18: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/18.jpg)
Spectrum of EM radiation
![Page 19: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/19.jpg)
Are we done?diffraction refraction
dispersion reflection
![Page 20: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/20.jpg)
Particle nature of lightPhotoelectric effect
electrons are emitted:
- only when is higher than certain value
- below that , intensity doesn’t matter
- instantaneously when light is applied
One-to-one interaction of a light quantum (h) and the electron.
![Page 21: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/21.jpg)
Atoms of light“On a Heuristic Viewpoint Concerning the Production and Transformation of Light”, Einstein, 1905
Light is composed of light quanta
€
E = hυ
Light is electromagnetic radiation “associated with singular points just like the occurrence of electrostatic fields according to the electron theory”
“I therefore take the liberty of proposing for this hypothetical new atom, which is not light but plays an essential part in every process of radiation, the name photon”, Gilbert N. Lewis, 1926
![Page 22: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/22.jpg)
Photons
Electromagnetic radiation has a frequency and a wavelength:
=c
Frequency is related to the energy of a photon:
E = h =hc
Photons are bosons (s=1), many photons can have the same ‘state’:
Etot = nh
![Page 23: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/23.jpg)
Quantization of energy
€
Ephoton =195,000J /mol
6.022 ×1023 photons /mol
A mole of photons in the yellow/red visible region of the spectrum has an energy of 195 kJ/mol. Calculate the wavelength of the light.
1. Calculate the energy for 1 photon.
2. Find the wavelength.
€
=3.24 ×10−19 J
€
=hcE
€
=6.626 ×10−34 Js( ) 3.00 ×108m /s( )
3.24 ×10−19J
€
=6.14 ×10−7m
![Page 24: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/24.jpg)
Do photons have mass?
Since photons have particle-like properties, they should have mass.
€
E = hν = hcλ
The (relativistic) mass of photons can be calculated from Einstein’s equation for special relativity.
€
E =mc2
€
m = Ec2
€
=hc /λc2
€
=h /λc
![Page 25: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/25.jpg)
Where’s the photon?
![Page 26: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/26.jpg)
Where’s the photon?Single photon interference
![Page 27: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/27.jpg)
What is the photon’s duration?
If the frequency of the photon is well known, the duration of the photon is infinite!
![Page 28: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/28.jpg)
Strange particles
Photon ‘state’ is defined by:
- momentum- Energy- polarization
Position and time (duration) of the photon are not well-defined!
- spin (s=1)
“Nobody knows, and it’s best if you try not to think about it.” Richard Feynman
“These days, every Tom, Dick and Harry thinks he knows what a photon is, but he is wrong”, Albert Einstein
![Page 29: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/29.jpg)
Semi-classical theory
Classical EM
€
hυ
€
hυ
The propagation of light through vacuum can also be treated quantum mechanically: quantum electrodynamics
![Page 30: What is light?](https://reader035.fdocuments.us/reader035/viewer/2022070419/56815c36550346895dca1e70/html5/thumbnails/30.jpg)
More light on the horizon….