introduction to light

WHAT IS LIGHT?

CHAPTER 1: INTRODUCTION OF LIGHT1Hanisah/EO301/JKE/PTSS/Dis'13

What is Light?What is Light?Hanisah/EO301/JKE/PTSS/Dis'132

Waves?Particles?LIGHT enables us to see Objects. The sun, a lighted candle and electric bulb give out the light. They are called luminous bodies.

or

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Hanisah/EO301/JKE/PTSS/Dis'133

Isaac Newton1643 - 1727Christian Huygens1629 - 1695In the17th century, two scientists had different views about the nature of light Light is particlesNo! Light is waves

In Late 1600, Sir Isaac Newton believed that light travels in the form of particles which named as corpuscles.

Then, in 1678 Christian Huygens argued that light might be some sort of a wave motion. Huygens came up with Huygens's Principle that explain the light is a wave traveling through a medium called aether.

Huygens deduced the Laws of Reflection and Refraction and could explain Double Refraction to prove the Theory of Wave.

However, if light behaves as waves, diffraction and interference should be seen using light.Hanisah/EO301/JKE/PTSS/Dis'134WAVE or PARTICLES?

Huygens failed provide any strong evidence to show that diffraction and interference of light occurred.

Furthermore, Huygens could not explain why light has different colours at all (He did not know that different colors of light have different wavelengths)

Therefore, Newtons Particle Theory is acceptable since his strong evidence about Particle nature of light.

However, In 1801, Thomas Youngs Double Slit Experiment showed that light diffracts and produces an interference pattern.

Therefore, Thomas Young successful provide evidence that light has WAVE properties.

He showed that light rays interfere with each other; such behavior could not be explained by particles.5WAVE or PARTICLES?Hanisah/EO301/JKE/PTSS/Dis'13

In the 1860s, Maxwell developed a mathematical model of electromagnetism.

He was able to show that these electromagnetic waves travel at the speed of light.

Therefore, he asserted that light was a form of high-frequency electromagnetic wave.

In 1900, Max Planck was able to explain the spectrum of a blackbody radiator by assuming that light energy is quantized. That quantum of light energy was later named a PHOTON.

A few years later, in 1905, Albert Einstein used Plancks idea to explain the photoelectric effect to support the particle behavior of light and came out with a QUANTUM THEORY.6WAVE or PARTICLES?Hanisah/EO301/JKE/PTSS/Dis'13

Isaac Newton (Particle theory of light)

Christian Huygens (Wave theory of light)

Thomas Young (Wave theory of light)

James Clerk Maxwell (Wave theory of light)

Max Planck (Particle theory of light)

Albert Einstein (Particle theory of light )

Louis de Broglie ( Wave-particle duality)


WAVE or PARTICLES?

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Wave-Particle Duality ExperimentExperiment to proof the Dual Nature of lightDouble slit exp. for particlesDouble slit exp. for wavesDouble slit exp. for electronsDouble slit exp. for particle-wave dualityHanisah/EO301/JKE/PTSS/Dis'138

Wave-Particle Duality ExperimentA gun sprays bullets towards a targetThe resulting pattern is a map of the likelihood of a bullet landing at each pointHanisah/EO301/JKE/PTSS/Dis'139

Double Slit Experiment for Particles

With waves, however, the result is very different, because of interference.If a slit is opened one at a time, the pattern would resemble that for bullets: two distinct peaks.But when both slits are open, the waves pass through both slits at once and interfere with each other

Hanisah/EO301/JKE/PTSS/Dis'1310Double Slit Experiment for Waves

Wave-Particle Duality Experiment

Now the quantum paradox: Electrons like bullets, strike the target one at a time.Yet, like waves, they create an interference patternHanisah/EO301/JKE/PTSS/Dis'1311Double Slit Experiment for Electrons/photons


If each electron passes individually through one slit, with what does it "interfere?" Although each electron arrives at the target at a single place and time, it seems that each has passed through.Thus, the electron is understood in terms of a wave-particle duality (Quantum-Mechanic Theory)Hanisah/EO301/JKE/PTSS/Dis'1312Double Slit Experiment for Light Wave-Particle Duality


So light is a wave or a particle ?Hanisah/EO301/JKE/PTSS/Dis'1313

On macroscopic scales, we can treat a large number of photonsas a wave.

When dealing with subatomic phenomenon, we are often dealingwith a single photon, or a few.

Wave-Particle DualityAs a conclusion, the scientists have observed that light energy can behave;Either like a wave as it moves through space, ORIt can behave like a discrete particles with a discrete amount of energy(quantum) that can be absorbed and emitted.

When light traveling through space, they act like waves.

When light interacts with atoms and molecules, they act like a stream of energy called photons or quanta.Hanisah/EO301/JKE/PTSS/Dis'1314

Theory of LightParticle TheoryWave TheoryTHEORY OF LIGHTTherefore, there are two THEORY OF LIGHT which explain the nature of light: Wave Theory Light as a wave Particle Theory Light as a particle (photon)Hanisah/EO301/JKE/PTSS/Dis'1315

Quantum Mechanic Theory ( Wave-Particle Duality)

WAVE TheoryWave as a Nature of LightSupported by;Hanisah/EO301/JKE/PTSS/Dis'1316

Christian HuygensThomas YoungJames Clerk MaxwellReflection and RefractionDiffraction and Interference (Youngs Double Slit Experiment)Electromagnetism

Wave Theory explain that the WAVE as a nature of light.

In Wave Theory, light is considered as an Electromagnetic (EM) Wave.

This EM wave consists two components which are Electric field (E) and Magnetic field (H) which oscillate and perpendicular to each other as well as to the direction of wave propagation as shown in Figure 1.Hanisah/EO301/JKE/PTSS/Dis'1317WAVE TheoryWave as a Nature of Light

According to the Wave Theory proposed by Christian Huygens, light is considered to be emitted as a series of waves (wave front) in all directions.Hanisah/EO301/JKE/PTSS/Dis'1318

WAVE TheoryWave as a Nature of Light

A wave has a wavelength () , a frequency (f ) and a velocity ().19WAVE TheoryWave as a Nature of Light

1 cycle1 wavelength1 periodAFigure 1.2 : WaveformTherefore, following properties can be defined for light by considering the wave nature in Figure 1.2

PROPERTIES OF LIGHT WAVES

Wavelength () - is the length that one cycle OR Distance between 2 crests. (Unit: meter, m)

Frequency (f) - How often cycle of wave repeats in one second OR number of cycles per sec. (Unit: Hertz, Hz)

Velocity (v) the distance covered by the wave in one second. (Unit: m/s)Hanisah/EO301/JKE/PTSS/Dis'1320WAVE TheoryWave as a Nature of LightAccording to Figure 1.2

PROPERTIES OF LIGHT WAVES

Period (T) - the duration of one cycle . It is reciprocal of frequency. (Unit: second, s)

Wave Number ( ) - the number of waves spread in a length of one meter . It is reciprocal of wavelength. (unit: m-1)

Amplitude (A) = the distance from the midline to the peak of wave. Amplitude is a measure of the intensity or brightness of light radiation.

Hanisah/EO301/JKE/PTSS/Dis'1321WAVE TheoryWave as a Nature of Light

CHARACTERISTICS OF LIGHT WAVES

The velocity of light wave is not constant. It depends on type of medium the wave travels through.

Velocity/speed of light wave (v) in vacuum is denoted by c. c = 3 x 108 m/s

The relationship among frequency(f), light velocity (c), and wavelength () is expressed mathematically as:Hanisah/EO301/JKE/PTSS/Dis'1322

WAVE TheoryWave as a Nature of Light.. equation 1.1


From equation 1.1, it can be seen that wavelength () is inversely proportional to the frequency (f).high frequency = short wavelengthlow frequency = long wavelength

Light wave have different colors of dispersion depends on the frequency (f) or wavelength ().

Different frequency, or wavelength of wave will give different color of light as shown in Table1.1.Hanisah/EO301/JKE/PTSS/Dis'1323WAVE TheoryWave as a Nature of Light


Light wave could diffract and interfere as shown in Thomas Youngs Double-Slit Experiment.24WAVE TheoryWave as a Nature of Light

DiffractionInterference


The propagation of light through space can be described in term of a traveling wave motion.

The light wave moves energy , without moving mass, from one place to another at a speed independent of its intensity or wavelength.

The light wave could moves in three different polarization;Linear PolarizationCircular PolarizationElliptical PolarizationHanisah/EO301/JKE/PTSS/Dis'1325WAVE TheoryWave as a Nature of Light


LinearCircularEllipticalWAVE TheoryWave as a Nature of Light

Supported by;Hanisah/EO301/JKE/PTSS/Dis'1327Isaac NewtonMax PlankAlbert EinsteinReflection and Refraction, phenomena of colorsBlack Body Radiator(Quantum Theory)Photoelectric Effect(Quantum Theory)

PARTICLE TheoryPhoton as a Nature of Light

Isaac Newton (1704) proposed that light consists of a stream of small particles, because ittravels in straight lines at great speedsis reflected from mirrors in a predictable wayHanisah/EO301/JKE/PTSS/Dis'1328Newton observed that the reflection of light from a mirror resembles the rebound of a steel ball from a steel plate


Particle Theory explain that the particle PHOTON as a nature of light.

From quantum perspective, light consist of particles called photon.

So, What is PHOTON?

Photon is a very tiny little particle that has energy and movement (momentum) but it has no mass or electrical charge.

According to Einstein, Photon is considered as discrete Packet of Energy (Quantum).

Hanisah/EO301/JKE/PTSS/Dis'1329PARTICLE TheoryPhoton as a Nature of Light

Hanisah/EO301/JKE/PTSS/Dis'1330Photon is a very tiny small particle that couldnt see by eyes

Photon = Packet of Energy = Wave PacketPARTICLE TheoryPhoton as a Nature of Light

Hanisah/EO301/JKE/PTSS/Dis'1331CHARACTERISTICS OF LIGHT PHOTON

Photon has no mass and electrical charge.

Photon carries electromagnetic energy, E and momentum, p as well as intrinsic angular momentum (or spin) associated with its polarization properties.

Photon travels at the speed of light in vacuum;c = 3 x 108 m/sPARTICLE TheoryPhoton as a Nature of Light


Photon has a wavelike character that determines its localization properties in space and time, and the rules by which it interferes and diffracts.

Photon are always in motion.

Photons can produces :-Infrared LightVisible Light (e.g: sunlight)Ultraviolet (UV) Light - UVa & UVb rays that give you sunburnsX raysPARTICLE TheoryPhoton as a Nature of Light


According to quantum theory, a photon has an energy, E given by;

E = hf = hc/ (unit: Joule,J)

Where, h = Plancks Constant = 6.625 x 10-34 J/s c = velocity of light = 3 x 108 m/s = wavelength of light (in meter)

A photon also carry momentum, p. The momentum is related to the energy by;

p = E/c = h/l (unit: Js/m)


Hanisah/EO301/JKE/PTSS/Dis'1334Question 1 Photon in a pale blue light have a wavelength of 500nm. What is the energy of this photon? Then, calculate the momentum of photon.(answ: E = 3.97 x 10-19 J , p = 1.32 x 10-27 Js/m)

Question 2Find the energy of a photon travelling with 200 THz frequency and its momentum. (answ: E = 1.325 x 10-19 J , p = 4.42 x 10-28 Js/m)

Question 3Given the momentum of photon is 6.84 x 10-28. Find the frequency of photon. (answ: f = 309.7 THz)PARTICLE TheoryPhoton as a Nature of Light


The Energy (E) of the light photon is proportional to the frequency (f) and inversely proportional to the wavelength ().

The higher the frequency (OR lower the wavelength) the higher the energy of the photon. - Higher frequency photon gains more energy - Lower frequency photon gains less energy

For example, BLUE ray has more energy than RED ray because BLUE ray has higher frequency and shorter wavelength. (please refer to Table 1.1)PARTICLE TheoryPhoton as a Nature of Light


Photon can interacts with other particles such as electrons, protons, neutrons etc.

When photons bump into another atoms, some of their energy can get the electrons in those atoms moving faster than they were before - that's what we call heat. That's why you get hot sitting in the sun.

A Photoelectric Experiment by Einstein shows that a very energetic photons of BLUE light (has very high frequency) could knocked the electrons out from metal surface to produce a current as shown in below Figure 1.3.PARTICLE TheoryPhoton as a Nature of Light

Hanisah/EO301/JKE/PTSS/Dis'1337The increasing of frequency will increase the energy of photon. Therefore, the photons of BLUE light could eject the electrons compare to RED light because the BLUE light has higher frequency.

Figure 1.3: Photoelectric EffectPARTICLE TheoryPhoton as a Nature of Light

ELECTROMAGNETIC SPECTRUMThe range of frequencies of electromagnetic radiation is called the Electromagnetic Spectrum.Hanisah/EO301/JKE/PTSS/Dis'1338

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Visible light is a small part of the energy range of Electromagnetic waves.

The whole range is called the Electromagnetic spectrum and visible light is in the middle of it.


ELECTROMAGNETIC SPECTRUM

E = hf = hc/lHanisah/EO301/JKE/PTSS/Dis'1340ELECTROMAGNETIC SPECTRUM

The higher the frequency of the light, the higher the energy of the wave.Since color is related to frequency and wavelength, there is also a direct relation between color and energy.Hanisah/EO301/JKE/PTSS/Dis'1341

Table 1.1: Spectrum of Visible Light

Shorter Wavelength

Higher Frequency

Increasing EnergyELECTROMAGNETIC SPECTRUM

Dispersion of light by Glass PrismHanisah/EO301/JKE/PTSS/Dis'1342

Decreasing Wavelength

Increasing Frequency

Increasing EnergyELECTROMAGNETIC SPECTRUM

WHAT IS LIGHT?Definition: LIGHT is a special kind of electromagnetic energy with a wavelength range from 380nm to 740nm (visible light).

This electromagnetic energy consists two components which are electric field, E and the magnetic field, H which oscillate and perpendicular each other.

This electromagnetic radiation are produced by the vibrations of a charged particles called photons.


CHARACTERISTICS of Light

Light travels in a straight line. For example, the light from candle through pin hole lies in a straight line.

This straight line is called a ray of light.

Light travels at a high speed. The speed of light in vacuum is expressed as, c = 3x108 m/sHanisah/EO301/JKE/PTSS/Dis'1344

A bundle of rays is called a beam of light. A beam of light may be parallel, converging or diverging as shown in below Figure 1.4Hanisah/EO301/JKE/PTSS/Dis'1345(a) Parallel(b) Converging(c) DivergingCHARACTERISTICS of LightLight travels in a vacuum at a constant speed.

However when light travels in non-vacuum media such as air, glass, water, the speed of light will decrease (air 0.03% slower, glass 30% slower)

Light consist of different types of colours.

These colours are differentiated on the basis of their wavelengths in the visible light spectrum (see Table 1.1)

Light has no mass but carries energy and momentum, p where the energy of light is proportional to the frequency but reciprocal to the wavelength.

When the frequency of light increase, the energy of light also increase and vice versa.

Different colors of light has different energy because it has different frequencies.

Light is emitted and absorbed in the form of Quanta(Photons) but propagated in the form of waves

Hanisah/EO301/JKE/PTSS/Dis'1346CHARACTERISTICS of Light

Pass Through - The rays of light can pass through the objectAbsorption - The rays of light can be absorbed by the object.Reflection The rays of light can be reflected off the object.Scattering - The rays of light can be scattered off the object.Refraction - The rays of light can be refracted through the object.

Light has different phenomena when it interact with other objects such as;


SOURCES OF LIGHTThe sources of light are many and varied.

Usually there are 2 categories of source;Natural Source (Sun, Star, radio star, lightning, or any Body that exists at a temperature over absolute zero).Man-made Source (Incandescent light, Fluorescent light, heater, lasers, antennas, radars, and X-ray tubes).

All materials with temperature above absolute zero emit electromagnetic radiation (light).

For example, atoms and molecules which has their own characteristics set of spectral lines.

In this case, we are study two THEORY of light source;Atom EquilibriumBlackbody RadiationHanisah/EO301/JKE/PTSS/Dis'1348

Atom absorb energy, unstable conditionAtom is stable or equilibrium by released energy as a lightAtom EquilibriumLight is emitted when the atoms is changed from one form to another form of energy.Extra energy is released as a light.

Hanisah/EO301/JKE/PTSS/Dis'1349Energy from outside

atomatomLight energy is released

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Atom EquilibriumWe model the energy of an atom with the electrons.

A nucleus of an atom is surrounded by electrons that in their orbits/shells.


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Atom EquilibriumWhen all the electrons are in unexcited or ground state, the atom is assumed at the lowest energy level (atom is stable).

When the atom absorbs energy (heat it up), electrons will excited and jumped to higher-energy shells (atom is unstable).

As electrons jump from one shell to another, an amount of energies (Quanta) are emitted.

This is how an atom can emit the LIGHT.Hanisah/EO301/JKE/PTSS/Dis'1351

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Atom EquilibriumEach electron that jumped emits one photon of light.

What color is this light?

Depends on how big the jump between orbits was.

The bigger the jump, the higher the energy.

The energy determines color; a blue photon has more energy than a red photon.

Shine all the colors together, you get white light!Hanisah/EO301/JKE/PTSS/Dis'1352

Blackbody RadiationThe term "black body" was introduced by Gustav Kirchhoffin 1860. Then, have been analyzed by Max Planck (1900) he came out his Plancks Quantum Theory.

Definition: Blackbody is an idealized physical body which absorbs and emits the radiation of energy completely.

No electromagnetic radiation passes through it and none isreflected.Hanisah/EO301/JKE/PTSS/Dis'1353

A hollow metallic sphere coated inside with platinum black with a small aperture in its wall can act as a near Blackbody.

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Blackbody RadiationAs we know, any heated body (blackbody included) could radiates light over the whole spectrum of frequencies.

When the black body is heated to different high temperatures, it emits radiations of light at different wavelengths/frequencies/energies/colors.

A black body emits a temperature-dependent spectrum of light.

Thisthermal radiationfrom a black body is calledblackbody radiation.

The hotter the emitter (the bodies), the more energy emitted and the shorter the wavelength.


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Blackbody RadiationTotal power radiated by Blackbody; WB

WB = AT4 (unit: watts)

= emissivityA = surface area, m2T = temperature in unit Kelvin (K) = Stefan-Boltzmanns constant = 5.67 x 10-8 Wm-2 K-4

Noted:Graybody, WG is one that does not emit as a perfect blackbody but emit at fraction of the theoretical maximum of a blackbody.

Emissivity for Blackbody usually is 1( = 1.0) but for Graybody the is always less than 1 ( < 1.0).

Unit conversion of Temperature;Hanisah/EO301/JKE/PTSS/Dis'1355

Fahrenheit(F) to Celsius(C) C = 5/9(F 32)Where,Celsius(C) to Kelvin (K) K = C + 273.15The area, A for sphere is;A = 4r2

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Question 1 Calculate the power radiated by a blackbody at room temperature of 82F and surface area is 1m2. (answer: WB = 464.991 Watts)

Question 2Calculate the power radiated by a blackbody at room temperature of 82F, emissivity = 0.7 and surface area is 1m2.(answer: WG = 325.494 Watts)Black Body RadiationHanisah/EO301/JKE/PTSS/Dis'1356

Unit of LightThere are two types of System International (SI) unit of light;RadiometryPhotometry

RadiometryRadiometry is a field of detection and measurement of light energy.It uses a standardized system for characterizing the radiant energy.

PhotometryPhotometry is the astronomical measurement of brightness or intensity and colour.It measured as the amount of light energy that strikes a certain surface area on the earth over a certain period of time.

Below Table 1.2 shows the differences between unit of Radiometry and Photometry.


Unit of Light58Symbol (SI units)Radiometric term and unitsPhotometric term and unitsDefinitionQRadiant Energy (J)Luminous Energy (talbot)Quantity of energyRadiant Power (W)Luminous Power (lm)Total power/flux emitted in all direction.I(E)Irradiance (W/m2)Illuminance (lm/m2) or (lux, lx)Total power falling on unit area (Flux density)I(I)Radiant Intensity (W/sr)Luminous Intensity (lm/sr) or (candela. cd)Total power emitted by a point source into unit solid angleLRadiance (W/sr.m2)Luminance/Brightness (lm/sr.m2) or (cd/m2)Intensity per unit area in a given direction.WRadiant emittance(W/m2)Total power radiated in all direction from unit area

Table 1.2: Difference between Radiometry and Photometry unit

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Unit of LightIn this chapter, we only discuss Photometric standard unit.

Luminous Energy (Q) Luminous energy is the measure of the perceived energy of lightalso called the quantity of lightUnit SI : Lumen second or talbot

Luminous Power/ Flux ()Luminous flux/power is the measure of the perceived power of light. Unit SI : Lumen (lm)1 lumen = the luminous power of light produced by a light source that emits 1 candela of luminous intensity.


Unit of LightIlluminance I(E) Illuminance is the total luminous power incident on a surface, per unit area It is a measure of the intensity of the incident light.Unit SI : Lux (lx) OR lm/m2

Luminous Intensity I(I)Luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle.Unit SI : Candela (cd) OR lm/sr


Unit of LightLuminance (L)Luminance is a measure of the luminous intensity per unit area of light travelling in a given direction.It describes the amount of light is emitted from a particular area, and falls within a given solid angle. Unit SI : cd/m2 or lm/sr.m2Luminance is often used to characterize reflection of light from flat or diffuse surfaces.The luminance indicates how much luminous power will be perceived by an eye looking at the surface from a particular angle of view.Luminance is an indicator of how bright the surface will appear (the measurement of Brightness).Hanisah/EO301/JKE/PTSS/Dis'1361


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introduction to light

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