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What is light?. I 2. Why light is important. Seeing the messengers?. Abu Ali Hasan Ibn Al-Haitham, ~1000 AD.

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Why light is important l.jpg

I2

Why light is important


Seeing the messengers l.jpg
Seeing the messengers?

Abu Ali Hasan Ibn Al-Haitham, ~1000 AD


Particle theory of light l.jpg

“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

Sir Isaac Newton (1643-1727)

Particle theory of light

‘Is light not a body?’


Corpuscular theory of light l.jpg

Reflection 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.”

Dispersion

Corpuscular theory of light


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Corpuscular theory of light 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.”

Light moves in straight lines

“Light does not bend into the shadow”


Wave theories of light l.jpg
Wave theories of light 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.”

Each point on the existing wave-front can be considered to act as a source of waves (sometimes referred to as "secondary wavelets").

Christian Huygens (1629-1695)


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Wave theories of light 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.”


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diffraction 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.”

refraction

Wave theories of light


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Thomas Young (1773-1829) 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.”

New observations

Light is a wave!


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What kind of wave? 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.”

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.”


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Crazy for Maxwell 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.”


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Heinrich Hertz (1857 - 1894) 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.”

Experimental support

"I do not think that the wireless waves I have discovered will have any practical application."


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c 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.”

 =

Light as waves


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100 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.”m

0.6 m

Light as waves

600 nm

= 0.6 m

167 times as small as the diameter of a hair!


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Light as waves 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.”

Sodium 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?


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Spectrum of EM radiation 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.”


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Spectrum of EM radiation 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.”


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diffraction 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.”

refraction

dispersion

reflection

Are we done?


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Particle nature of light 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.”

Photoelectric effect

electrons are emitted:

- instantaneously when light is applied

- only when  is higher than certain value

- below that , intensity doesn’t matter

One-to-one interaction of a light quantum (h) and the electron.


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Light is composed of light quanta 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.”

Light is electromagnetic radiation “associated with singular points just like the occurrence of electrostatic fields according to the electron theory”

Atoms of light

“On a Heuristic Viewpoint Concerning the Production and Transformation of Light”, Einstein, 1905

“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


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c 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.”

Photons are bosons (s=1), many photons can have the same ‘state’:

 =

Etot = nh

hc

E = h =

Photons

Electromagnetic radiation has a frequency and a wavelength:

Frequency is related to the energy of a photon:


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Quantization of energy 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.”

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.


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Do photons have mass? 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.”

Since photons have particle-like properties, they should have mass.

The (relativistic) mass of photons can be calculated from Einstein’s equation for special relativity.


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Where’s the photon? 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.”


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Where’s the photon? 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.”

Single photon interference


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What is the photon’s duration? 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.”

If the frequency of the photon is well known, the duration of the photon is infinite!


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Strange particles 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.”

Photon ‘state’ is defined by:

- momentum

- Energy

- polarization

- spin (s=1)

Position and time (duration) of the photon are not well-defined!

“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


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Semi-classical theory 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.”

Classical EM

The propagation of light through vacuum can also be treated quantum mechanically: quantum electrodynamics


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More light on the horizon…. 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.”


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