Sifat-Sifat Cahaya

Dr. Ahmad Marzuki

Physics Department

Sebelas Maret University

Diambil dari berbagai sumber

Apa itu cahaya?

Semenjak abad ke 17 orang telah

mengamati cahaya bahawasannya

cahaya dapat

1. merambat melalui garis lurus

2. memantul

3. membias

4. transmit energy dari satu titik

ke titik yang lain

WAVE THEORY (tokoh:

Christian Huygens dan Robert Hooke,

Cahaya merupakan sebuah

gelombang

PARTICLE (corpuscular) THEORY (Tokoh: Isaac Newton dan Pierre Laplace)

Cahaya pada dasarnya adalah deretan/barisan partikel-pertikel kecil yang

biasa disebut corpuskel

Ada dua teori yang biasa digunakan

untuk menerangkan fenomena di atas

Teori Partikel Newton dapat dengan mudah menerangkan

Perambatan cahaya secara lurus, pemantulan, transmisi

energi namun gagal menerangkan fenomena pembiasan.

Newton’s explanation of refraction required that

light must travel faster in water than in air.

Teori gelombang Huygen dapatdengan mudah menerangkan

hal pemantulan, transmisi energi dan pemantulan

namun gagal menerangkan mengapa cahaya merambat

menurut garis lurus

The wave theory’s explanation of refraction

required that light must travel

slower in water than in air.

Perdebatan tentang apa sebenarnya cahaya itu

berlanjut

hingga pertengahan 1800’s.1801- interference of light was discovered

1816 - diffraction of light (actually observed

in the 1600’s but not given much significance)

was explained using interference principles

Teori partikel tidak dapat menerangkan kedua fenomena di atas

The final blow to the particle

theory came in 1850 when

Jean Foucault discovered that

light traveled faster in air

than in water.

Pada masa berikutnya secara umum kemudian

dipercaya bahwa cahaya merupakan sebuah

gelombang.

Gelombang apa dia?

Pada tahun 1865 James Maxwell (diinisiasi antara lain oleh Michail Faraday)

mengembangkan teori gelombang elektromagnetik yang menyatakan bahwasannya

cahaya merupakan merupakan gelombang e/m : a periodic disturbance involving electric

and magnetic forces.

In 1885, Heinrich Hertz

experimentally confirmed

the e/m theory.

Implikasi dari persamaan Maxwell

Light travels at constant speed

00

1

c

Light consists of an oscillation of electric and magnetic field

• Young’s Double-Slit Experiment indicated light behaved as

a wave (1801)

• The alternating black and bright bands appearing on the

screen is analogous to the water waves that pass through a

barrier with two openings

Light has wavelike property

Light: Wavelength and Frequency

• Example

– FM radio, e.g., 103.5 MHz (WTOP station) => λ = 2.90 m

– Visible light, e.g., red 700 nm => ν = 4.29 X 1014 Hz

• Visible light falls in the 400 to

700 nm range

• In the order of decreasing

wavelength

– Radio waves: 1 m

– Microwave: 1 mm

– Infrared radiation: 1 μm

– Visible light: 500 nm

– Ultraviolet radiation: 100 nm

– X-rays: 1 nm

– Gamma rays: 10-3 nm

Electromagnetic Spectrum

Light: spectrum and color

• Newton found that the white light from the Sun is composed of light of different color, or spectrum (1670).

Visible light is that portion of the

electromagnetic spectrum which stimulates the

retina of the

human eye.

Visible spectrum

wavelengths range

from about 400 nm (violet) to 760 nm (red).

Light travels at about 3 x 108 m/s through

empty space and slightly slower through air.

Remember that for all waves, v = f.

At the end of the century, many physicists felt that

all the significant laws of physics had been discovered.

Hertz even stated, “The wave theory of light is, from

the point of view of human beings, a certainty.”

That view was soon to change.

Around 1900,

the photoelectric effect was observed.

“the emission of electrons by a substance

when illuminated by e/m radiation”

Careful study of the photoelectric effect

was performed by many scientists.

The wave theory could not totally explain the

photoelectric effect, but a variation of the

old particle theory could!

Max Planck and

Albert Einsteinsubsequently proposed the

QUANTUM THEORY.

The Quantum TheoryThe transfer of energy between

light radiation and matter occurs in

discrete units called quanta, the magnitude

of which depends on the frequency of radiation.

Although we still commonly characterize

light as a wave, it is actually neither a

wave nor a particle. It seems to have

characteristics of both.

The modern view of the nature of

light recognizes the dual character:Light is radiant energy transportedin photons that are guided along

their path by a wave field.

Dual properties of Light:

(1) waves and (2) particles

• Light is an electromagnetic radiation wave, e.g, Young’s

double slit experiment

• Light is also a particle-like packet of energy - photon

– Light particle is called photon

– The energy of phone is related to the wavelength of light

• Light has a dual personality; it behaves as a stream of

particle like photons, but each photon has wavelike

properties

• Planck’s law relates the energy of a photon to its

wavelength or frequency

– E = energy of a photon

– h = Planck’s constant

= 6.625 x 10–34 J s

– c = speed of light

– λ= wavelength of light

• Energy of photon is inversely proportional to the

wavelength of light

• Example: 633-nm red-light photon

– E = 3.14 x 10–19 J

– or E = 1.96 eV

– eV: electron volt, a small energy unit = 1.602 x 10–19 J

Dual properties of Light: Planck’s Law

Tugas 1 :

Tanpa harus mengurangi waktu lebaran kalian, soal

no 1-1 hingga 1-11 harus kalian kerjakan.

Tugas dikumpulkan paling lambat tanggal: 13

September 2011

Things you should know

Spectral Lines• Bright spectrum lines can be seen when a chemical substance is

heated and valoprized (Kirchhoff, ~1850)

Each chemical element has its own

unique set of spectral lines.

Kirchhoff’s Laws on Spectrum• Three different spectrum: continuous spectrum, emission-line

spectrum, and absorption line spectrum

Kirchhoff’s Laws on Spectrum

• Law 1- Continuous spectrum: a hot opaque body, such as a perfect blackbody, produce a continuous spectrum – a complete rainbow of colors without any spectral line

• Law 2 – emission line spectrum: a hot, transparent gas produces an emission line spectrum – a series of bright spectral lines against a dark background

• Law 3 – absorption line spectrum: a relatively cool, transparent gas in front of a source of a continuous spectrum produces an absorption line spectrum – a series of dark spectral lines amongst the colors of the continuous spectrum. Further, the dark lines of a particular gas occur at exactly the same wavelength as the bright lines of that same gas.

• An atom consists of a small, dense nucleus at the center,

surrounded by electrons which orbit the nucleus.

• The nucleus contains more than 99% of the mass of an atom,

but concentrates in an extremely small volume

Structure of Atom

• A nucleus contains two types of particles: protons and neutrons

• A proton has a positive electric change, equal and opposite to that of an electron.

• A neutron, about the same mass of a proton, has no electric charge.

• An atom has no net electric charge

• The number of protons in an atom’s nucleus is the atomic number for that particular element

• The same element may have different numbers of neutrons in its nucleus, which are called isotopes

Periodic Table

• Electrons occupy only certain orbits or energy levels

• When an electron jumps from one orbit to another, it emits or absorbs a photon of appropriate energy.

• The energy of the photon equals the difference in energy between the two orbits.

Bohr’s Model of Atom

Bohr’s Model of Hydrogen

Bohr’s Model of Atom • Absorption is produced when electron absorbs incoming

photon and jumps from a lower orbit to a higher orbit

• Emission is produced when electron jumps from a higher

orbit to a lower orbit and emits a photon of the same energy

Bohr’s Atomic Model for Hydrogen

• The strongest hydrogen spectral line from the Sun, Hα line at 656 nm, is caused by electron-transition between n=3 orbit and n=1orbit

• Lyman series lines: between n=1 orbit and higher orbits (n=2, n=3, n=4,…)

• Balmer series lines: between n-2 orbit and higher orbits (n=3, 4, 5,…)

Doppler Effect• Doppler effect: the wavelength of light is affected by

motion between the light source and an observer

• Red Shift: The object is moving away from the observer, the line is shifted toward the longer wavelength

• Blue Shift: The object is moving towards the observer, the line is shifted toward the shorter wavelength

D/o = v/c

D = wavelength shift

o = wavelength if source is not moving

v = velocity of source

c = speed of light

Doppler Effect

• Questions: what if the object’s motion perpendicular to our line of sight?