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
Ada dua teori yang biasa digunakan untuk menerangkan fenomena di atas
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
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 consists of an oscillation of electric and magnetic field
Light travels at constant speed
c
1
0 0
Light has wavelike property • 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
Electromagnetic Spectrum • 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
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 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 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
Dual properties of Light: Planck’s Law • 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
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.
Structure of Atom • 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 • 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
Periodic Table
• 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
Bohr’s Model of Atom • 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 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 electrontransition 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
Doppler Effect • 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 • Questions: what if the object’s motion perpendicular to our line of sight?
