Sifat-Sifat Cahaya
Dr. Ahmad Marzuki Physics Department Sebelas Maret University
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
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
0 0 1
c• 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: 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 10
8m/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
andAlbert Einstein
subsequently proposed the
QUANTUM THEORY
.The Quantum Theory
The 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 transported
in photons that are guided along
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
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
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
• 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 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
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?
