8 Light Nature_Waves and Photons

8/9/2019 8 Light Nature_Waves and Photons

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Light

Content of Lecture1. The double nature of light (particulates photons - and electromagnetic waves)

2. Velocity of Light (speed of propagation, wavelength and frequency)3. The Electromagnetic Spectrum

4. The Paradox of the Dual Nature of Light, the Max Planck photons

5. The mathematical solution of the paradox and Maxwell relationship

6. Photon interaction with matter

7. SPECTROSCOPY (light absorption)

8. Photometry (colorimetery and spectroscopy, spectrophotometery)

9. Absorption, Emission, Scattering

10. Light Absorption and chemical analysis

11. Physics of Photosynthesis12. Beer and Lambert Law

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8.1 The nature of light

Importance of light can hardly be overestimated in everyday life,photosynthesis, scientific studies and analytical measurement.

Astronomers generally cannot conduct experiments. They areonly allowed to collect and analyze light from distant objects.

Light carries information about luminosity, temperature,

composition and velocity of celestial objects.

Light NatureLight may be thought of as particles of energy that move through

space with wavelike properties; this is known as the doublenature (or duality) of light.1. Light particulates

Interaction of light with matter, which is the basis ofabsorptionand emission spectroscopy, may be best understood in terms ofthe particulate (corpuscular or discrete) nature of light.

2. Light wavesLight propagation, interference, diffraction, and refraction, aremost easily explained using the wave theory.The wave properties are described in terms offrequency,wavelength, and amplitude.

Light as electromagnetic wave Concept of electromagnetic radiation being waves began in 1862.

Light is energy carried in form of traveling wave composed of

electric and magnetic fields. Electric and magnetic fields vary in intensity and are at rightangles to each other and to direction wave is propagating. These fields propagate until energy of wave is converted intosome other form of energy.

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Electromagnetic radiation in natural world occurs overa widerange of wavelengths or frequencies.

Velocity of Light - Historical

Galilee suggested that velocity of light is finite rather than infinite,but large compared with sound velocities.

First definite evidence that light moves at a finite velocity found

by Danish astronomer Ole Roemer (1644-1710)

Scottish physicist James ClerkMaxwell (1831-1879) showed that:product ofwavelength (it is a distance) and frequency (it is a

reciprocal of time) gives velocity at which the electromagnetic

wave travels For any wave * = Cwavelength*frequency = speed of propagation

cm * 1/s = cm/sor

= C / frequency = speed of propagation / wavelength

1/s = cm/s / cm Hertz is the unit used for measuring frequency for all

electromagnetic radiation (1 Hz = 1 oscillation per second) The amount of energy a wave transports is proportional to

square of the wave's amplitude The speed of light in vacuum, c, is:

3 x 105 km/s = 3 x 108 m/s = 3 x 1010 cm/s

(exactly 2.9979 x 105 km s-1) in vacuumOr 186282 mi/s

It takes about 2.5 seconds for a radio communication traveling at the

speed of light to get to the moon and back. Sunrays take 8 minutesto reach the Earth.

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A light year is the distance about (5.8 trillion miles) light travels inone year at the speed of 186,282 miles per second.

This is upper limit for velocity at which energy can be transportedin universe. Speed of light is a fundamental constant of nature

to obtain light speed in any media, divide light speed invacuum by the refractive index of the media.

Radio, Infrared, Visible, Ultraviolet, X-rays, and -rays are alllight. The distinction between them is artificial (since it is onlybased on their wavelength).

1 Angstrom, A= 0.10 nm = 10-10 m = 10-8 cm= 10-7 mm

1 m = 1000 mm1 mm = 1000 m1 cm = 104 m1 m = 10-4 cm = 103 nm1 m = 1000 millimicron (i.e 1000 nm)2 m (d clay) = 2000 millimicron (i.e 2000 nm)1 millimicron = 10 A (i.e. 10 nm)1 m = 10-3 mm1 m = 10-4 cm1 nm = 10-9 m

1 m = 109 millimicron (i.e nm)

1 millimicron = 10 A1 m = 1010A1 nm = 10 A1 m = 1000 millimicron (i.e. 1000 nm)

= 10-6 m

= 104A = 10-4 cm = 10-3 mm

Electromagnetic Spectrum: (from longer to shorter) Radio very long (1 m to 104 m = 10 km)

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Microwaves long (10-2 to 102 cm) i.e. 0.1 to 1000 mm Infrared (10-4 to 10-2 cm) i.e. 1 to 100 microns Visible light (about 10-5 cm) i.e. 0.1 micron

average 0.5 m (= 5*10-7 m) = 500 nm (= 5*10-5 cm)from 3500 A (350 nm) (violet) to 7000 A (700 nm) (red)

Ultraviolet (10-6 to 10-5 cm) i.e. 10 to 100 nm X-rays (10-8 to 10-6 cm) i.e. 0.1 to 10 nm

(i.e. 1 to 10 )

Gamma rays (10-10 to 10-8 cm) i.e. 0.001 to 0.1 nm

Visible light Spectrum400Violet 500 Blue 600 Green 700Red (values in nm)

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Paradox of Light Dual Nature

Max Planck The ultimate origin of the difficulty lies in the fact (or

philosophical principle) that we are compelled to use the wordsof common language when we wish to describe a phenomenon,not by logical or mathematical analysis, but by a pictureappealing to the imagination. Common language has grown byeveryday experience and can never surpass these limits.

Laboratory experiments designed to inquire about eitherlight's wave nature or its corpuscular nature

No experiment will simultaneously yield discrete and waveproperties of light.

"We can therefore say that the wave and corpusculardescriptions are only to be regarded as two complementaryways of viewing one and the same objective process, a processwhich only in definite limiting cases admits complete pictorialinterpretation...."

Particles of Light: Photons, The Discrete Nature of Light Max Planck(1858-1947) argued emission of radiant energy by

ideal radiators (blackbodies) is not continuous.Emission isdiscontinuous in discrete units, photons. Energy transported by

EM wave is not continuously distributed over wave front defined

by crests. Energy is actually located at discrete points, photons,along wave-front.

1905, Einstein used this idea of Max Planck (discrete nature foremission of light) to explain a phenomenon discovered in 1887

known as photoelectric effect.(for which he obtined Nobelprize, not for the relativity!).

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Light particles are called photons (discrete "packets" of energythat move with velocity of light in straight lines, they are masslessand electrically neutral)

Mathematical solution of the paradox Wavelength characterizes wavelike properties of light whereasenergy content refers to its discrete nature. Thefact thatwavelength and energy content can be linked in mathematicalequation (E photon = h c / ) is the strongest argument forduality--simultaneously wave and photon.In fact, each photon carries a discrete amount of energy E that

depends upon its wavelength (de Broglie equation):E h

h C= =

E(photon) = (constant) / (wavelength).h Planck's constant (6.626196 * 10-34 Joule-second). frequency (constant for a given monocromatic light).C speed of light. wavelength.

Remember that we have earlier shown Maxwell relationship:

= C / This mean that energy content in photon is inversely proportional

to its wavelength, i.e., short wavelength light (e.g. X-rays and -rays) carries much more energy (per photon) than light thathas a long wavelength (e.g. radio or infrared light).

This equation ties together the particle and wave nature of lightpermitting us to convert (back and forth) from wavelengths to

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Photons Interaction With Matter

Radiating body emits photons of differing discrete amounts of

energy in all directions. Photons are created inside atomsof radiating bodyfrom which

they receive their energy content. Photon energy content remains constant while traveling

through space. Photons may be absorbed (by atoms) when they encounter

matter; they lose their identity by transferring their energy toatoms.

Creation and destruction of photons by atoms is a classicexample of conservation of energy (i.e. the first law ofthermodynamics).

Microwave radiation - when absorbed - stimulatesrotational motion of molecules.

Infrared radiation - when absorbed - stimulates

vibrational motion of molecules. Visible and UV radiation - when absorbed - cause

electrons to be promoted to higher energy orbital. X-rays and short wavelength UV radiation - when

absorbed - break chemical bonds and cause ionizationdamage to living organisms.

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SPECTROSCOPY

8.2 light absorption

8.2.1 Beers law

8.2.2 Lambert law8.3 light measurements (photometry)

8.3.1 Colorimetry

8.3.2 SpectroscopySpectroscopy is the use of the absorption, emission, orscattering of

electromagnetic radiation by atoms or molecules (or atomic or

molecular ions) to qualitatively, or quantitatively study the atoms or

molecules, or to study physical processes.

Spectroscopy deals with theproduction, measurement, and

interpretation ofspectra arising from the interaction of

electromagnetic radiation with matter.

Methods differ in

1. Species to be analyzed (such as molecular or atomic spectroscopy)

2. Type ofradiation-matterinteraction to be monitored (such as

absorption, emission, or diffraction)

3. Region of the electromagnetic spectrum used in the analysis.

Spectroscopic methods are based on the absorption or emission of

radiation in the ultraviolet (UV), visible (VIS), infrared (IR), andradio (nuclear magnetic resonance, NMR).

Spectrophotometry is any procedure that uses light to measure

chemical concentrations.

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Spectrophotometery and the Absorption of Light

Analytical chemists frequently use the measurement of light

absorption to determine concentration of chemicals. The technique is

called spectrophotometry or spectroscopy.

Why is light absorbed?

Light is energy, and when energy is absorbed by a chemical, it

results in a change in energy levels of the chemical.

Molecules normally exist in discrete energy levels.

- Vibrational energy levels exist because molecular bonds

vibrate at specific frequencies.

- Electronic energy levels exist because electrons in

molecules can be excited to discrete, higher energy orbital.

A molecule will absorb energy (light) when the light energy (or

wavelength) exactly matches the energy difference between the two

energy states of the molecule.

For example, a carbon-hydrogen chemical bond (C-H) vibrates at

frequency 1 and a higher frequency 2.

Light energy is absorbed by the C-H chemical bond at a specific

wavelength of light when the bond energy difference between 1

and 2 equals the energy of the light.

E = h = h C / E = h ( 1 - 2)

where = frequency of light, = wavelength of light, h = Planck'sconstant, and C = speed of light.

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Infrared light contains enough energy to cause changes in vibrational

energy levels, and this forms the basis of Infrared Spectroscopy.

UV and visible light contains enough energy to excite electrons in

molecules, and forms the basis for UV-VIS Spectroscopy.

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8 Light Nature_Waves and Photons