Radiasi Elektromagnetik

AS3100 Lab. Astronomi Dasar I Prodi Astronomi 2007/2008 B. Dermawan 1

Spektrum Elektromagnetik

Nick Strobel’s Astronomy 2

Informasi Astrofisika (1)

• Tujuan astrofisika: Menggambarkan, memahami dan memprediksi fenomena fisis yang terjadi di alam semesta – Materi alam semesta: rapat/renggang, panas/dingin, stabil/tidak stabil – Informasi yg diterima pengamat ditransformasikan menjadi sinyal sbg basis klasifikasi ini 3

Informasi Astrofisika (2)

• Tujuan observasi: Strategi dalam rangka mengumpulkan informasi astrofisika – Menyusun variabel/parameter fisis yang diukur; menganalisis informasi agar tidak

over-interpreted

atau terbuang; menyimpan informasi guna telaah di masa datang – Tiap teknik observasi 

filter informasi

yg menghasilkan citra, spektrum, kurva cahaya, dll. pada suatu daerah panjang gelombang 4

Kurir Informasi Astrofisika (1)

Radiasi elektromagnetik berkaitan dengan kondisi fisis sumber Keadaan dan gerak partikel, atom, molekul atau bulir debu:

temperatur

,

tekanan

,

medan magnet

Nick Strobel’s Astronomy Perambatan radiasi e.m. dipengaruhi oleh kondisi sepanjang lintasan: kurvatur lokal alam semesta, distribusi lokal materi (lensa gravitasi), serapan dan hamburan selektif (ekstingsi) materi antar bintang dan atmosfer bumi 6

Kurir Informasi Astrofisika (2)

Materi

• Berkas kosmis (

cosmic-rays

) – Terdiri atas elektron, inti atom dari proton hingga inti berat – Berasal dari proses energi tinggi di galaksi (ledakan supernova).

– Partikel bermuatan ini berinteraksi dgn medan magnet galaksi  distribusi spasial sangat isotropik Léna et al. 1996

Kelimpahan elemen di tata surya

Kelimpahan elemen relatif thd Silikon (Si=100) berkas kosmik energi rendah (70-280 MeV per inti) 7

Malasan, priv. com 8

• Meteorit (

meteorites

)

– Ukuran : mikroskopik  berat beberapa ton – Saat dihasilkan: • Kini : oleh angin matahari • Masa lalu: – pembentukan tata surya – reaksi energi tinggi di permukaan bintang (ledakan nukleosintesis) – Awal alam semesta (kelimpahan helium dlm berkas kosmik) 9

Kurir Informasi Astrofisika (3)

Neutrino –

Interaksi lemah

n  p 

e

 p  n 

e

  

e

,  

e

–

Interaksi kuat

pp  pn     pn       , nn  np    pp  pp   0 ,  np       , np  np   0 e  :elektron, e + :positron n : neutron, p: proton  e :neutrino elektron  e : anti neutrino elektron  + ,   ,  0 : pions/pi-mesons dg muatan +1,-1,0  +,  : muons/mu-mesons dg muatan +1,-1,   : neutrino muon  : anti-neutrino muon Léna et al. 1996 10

Pengaruh pada Pencitraan Astronomis

11

Kurir Informasi Astrofisika (4)

Gravitational Waves – As the black holes, stars, or galaxies orbit each other, they send out waves of “gravitational radiation" that reach the Earth – A more massive moving object will produce more powerful waves, and objects that move very quickly will produce more waves over a certain time period NASA 12 NASA

Kurir Informasi Astrofisika (5)

Observation in situ • Allows local measurements • To experiment in the same way as a physicist, a chemist, or a biologist 13

Apakah Cahaya Itu?

Sifat gelombang & partikel

Sifat partikel dominan

Malasan, priv. com

Sifat gelombang dominan Interferensi Polarisasi

14

Cahaya Kasat Mata

Radiometri/Fotometri bertautan dg pengukuran radiasi kasat mata CIE 1931 Standard Observer: Acuan berdasar pd respons rata-rata mata di bawah iluminasi normal dan medan pandang 2  Tiga komponen model warna: • Lightness: Transformasi hitam  putih • Hue ; Transformasi putih  hitam • Saturation: jarak dari sumbu lightness Malasan, priv. com 15

Radiasi Ultraviolet

UV-A :

• Disebut juga ‘cahaya hitam’ • Paling tak berbahaya • Menyebabkan material fluoresensi berpendar kalau diradiasi • Aplikasi dalam fototerapi (medis)

UV-B :

• Bentuk radiasi yg paling destruktif • Penyebab kanker kulit • Penapis alamiah: Lapisan Ozon

UV-C :

• Diserap sempurna oleh atmosfer • Foton UV-C menumbuk Oksigen  Ozon • Aplikasi dalam purifikasi air dan udara (dg lampu UV-C

)

Malasan, priv. com 16

Radiasi Inframerah

• Radiasi dengan muatan energi foton ter-rendah • Umumnya dideteksi dengan detektor termal Malasan, priv. com 17

Daya Radiasi EM

Watt (W)  Satuan fundamental daya optik: laju energi 1 joule (J) per detik

Q



hc

 Malasan, priv. com 18

Zang 2006

Light: spectrum and color

Newton found that the white light from the Sun is composed of light of different color, or spectrum (1670) 19

Zang 2006

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 20

Zang 2006

Light is Electromagnetic Radiation

• •

The nature of light is electromagnetic radiation

In the 1860s, James Clerk

Maxwell

succeeded in describing all the basic properties of electricity and magnetism in four equations: the Maxwell equations of

electromagnetism.

21

Zang 2006

Radiation depending on Temperature

A general rule: The higher an object’s temperature, the more intensely the object emits electromagnetic radiation and the shorter the wavelength at which emits most strongly The example of heated iron bar. As the temperature increases – The bar glows more brightly – The color of the bar also changes 22

Zang 2006

Blackbody Radiation

• Hot and dense objects act like a blackbody • Stars, which are opaque gas ball, closely approximate the behavior of blackbodies • The Sun’s radiation is remarkably close to that from a blackbody at a temperature of 5800 K

The Sun as a Blackbody emits most strongly at infrared light

Zang 2006

Blackbody Radiation: Wien’s Law

Wien ’s law states that the dominant wavelength at which a blackbody emits electromagnetic radiation is inversely proportional to the Kelvin temperature of the object For example – The Sun, λ max = 500 nm  T = 5800 K – Human body at 37 degrees Celcius, or 310 Kelvin  λ max = 9.35 μm = 9350 nm 24

Zang 2006

Blackbody radiation: Stefan-Boltzmann Law

The Stefan-Boltzmann law states that a blackbody radiates electromagnetic waves with a total energy flux F directly proportional to the fourth power of the Kelvin temperature T of the object:

F

= 

T

4

F

= energy flux, in joules per square meter of surface per second  = Stefan-Boltzmann constant = 5.67 X 10 -8 W m 2 K -4

T

= object’s temperature, in kelvins 25

Zang 2006

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

26

Zang 2006

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 27

Zang 2006

Spectral Lines

• The Sun’s spectrum: in addition to the rainbow-colored continuous spectrum, it contains hundreds of fine dark lines, called

spectral lines

(

Fraunhofer

, 1814) • A perfect blackbody would produce a smooth, continuous spectrum with no dark lines 28

Zang 2006

Spectral Lines

• Bright spectrum lines can be seen when a chemical substance is heated and vaporized (Kirchhoff, ~1850) 29

Zang 2006

Each chemical element has its own unique set of spectral lines

.

30

Zang 2006

Kirchhoff’s Laws on Spectrum

• Three different spectrum: continuous spectrum, emission-line spectrum, and absorption line spectrum 31

Zang 2006 • •

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 32

Zang 2006

Bohr’s Atomic Model for Hydrogen

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

Lyman series lines

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

Balmer series lines

5,…) : between n-2 orbit and higher orbits (n = 3, 4, 33

Zang 2006 •

Doppler Effect

Doppler effect: the wavelength of light is affected by motion between the light source and an observer

34

Zang 2006

Doppler Effect

• •

Red Shift

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

Blue Shift

D

/



o

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

= v/c

v

D = wavelength shift,  o = velocity of source,

c

= wavelength if source is not moving, = speed of light  Nick Strobel’s Astronomy 35