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Astronomy 1 – Fall 2014. Lecture 5; October 16, 2014. Previously on Astro-1. The Nature of Light Electromagnetic radiation Relation of speed to wavelength and frequency Wavelength dependence of scattering Doppler effect and Doppler formula Blackbody Radiation
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Astronomy 1 – Fall 2014 Lecture 5; October 16, 2014
Previously on Astro-1 • The Nature of Light • Electromagnetic radiation • Relation of speed to wavelength and frequency • Wavelength dependence of scattering • Doppler effect and Doppler formula • Blackbody Radiation • Describes the spectrum of light emitted by opaque sources • The temperature of the blackbody determines • The spectrum (Wien’s Law) • The energy flux (Stefan-Boltzman Law) • Kirchoff’s Laws • A hot body produces a continuous spectrum • A hot transparent gas produces emission lines • Cool transparent gas in front of a hot body produces absorption lines
Today on Astro-1 • Light can have particle-light properties. • The particles of light are called photons. • Planck’s Law: E = h= hc/ • Atoms absorb & emit photons at discrete energies • Spectroscopy and the composition of objects • Geometrical Optics • Reflection/Mirrors • Refraction/Lenses • Telescopes • Optical • Other wavelengths
Kirchoff’s Laws 1. A hot, dense object such as a blackbody emits a continuous spectrum covering all wavelengths. 2. A hot, transparent gas produces a spectrum that contains bright (emission) lines. 3. A cool, transparent gas in front of a light source that itself has a continuous spectrum produces dark (absorption) lines in the continuous spectrum. CLM - Fall 2014
What causes spectral lines? The structure of atoms CLM - Fall 2014
Rutherford’s Experiment CLM - Fall 2014
Rutherford’s model of the atom. Today we know this is not exactly correct – electrons do not orbit the nucleus, but the basic idea is right -- protons and neutrons exist in the nucleus, and electrons are outside of it. CLM - Fall 2014
Planck’s Law “Light is also a Particle” or E = Energy of a photon h = Planck’s constant = 6.625×10-34 J s c = speed of light λ = wavelength of light ν = frequency of light CLM - Fall 2014
What is the Energy of a Photon? Example: DNA molecules are easily broken when hit with ultraviolet light at 260 nm. How much energy does a single photon at this wavelength have? 7.6 x 10-19 J 7.6 x 10-17 J 7.6 J 5.7 x 10-49 J 7.6 x 1019 J CLM - Fall 2014
The Bohr model of the atom Niels Bohr 1885-1962 Was a postdoc with Rutherford. In 1912, to explain discrete nature of spectral lines, hypothesized that electron orbits are quantized (quantum mechanics!). Bohr and Einstein, 1925 CLM - Fall 2014
The quantum nature of light is related to the quantum nature of atoms! CLM - Fall 2014
In 1885 Swiss schoolteacher Johann Jakob Balmer, by trial and error, created a formula that can predict where lines of hydrogen fall in the spectrum of a star. We still call these Balmer lines. R = Rydberg constant = 1.097×107 m-1 n = any integer greater than 2 CLM - Fall 2014
The Balmer series and fomula. R = Rydberg constant = 1.097×107 m-1 Bohr figured out the physical explanation for Balmer’s formula – the spectra from stars depends on the structure of atoms! N = lower orbital n = higher orbital
Electron Transitions in the Hydrogen Atom The same wavelength occurs whether a photon is emitted or absorbed. CLM - Fall 2014
Every Element Has a Unique Set of Spectral Lines Atomic number is the number of protons in an atom. CLM - Fall 2014
The Ring Nebula is a shell of glowing gases surrounding a dying star. Nitrogen & Sulpher Hydrogen & Oxygen
Spectroscopy Reveals the Chemical Composition of Celestial Objects CLM - Fall 2014
Light is a wave… it is affected by motion too. Bottom line: you can tell how fast something is moving from its spectrum… sort of
Extra-solar planets: most have been discovered using Doppler shift measurements of their parent stars (since 1995)
Spectral Lines (iclicker Question) • Professor Martin used a spectrograph on the Keck telescope to observe a distant galaxy. She detected 2 absorption lines from sodium atoms. The wavelengths she measured were 0.22 nm bluer than the wavelengths of 589.0 and 589.6 nm where she expected to find the lines. What should she conclude? • There are cool clouds between the observer and the galaxy. • There gas between the galaxy and the observer is hotter than the galaxy. • The gas clouds are moving away from the galaxy towards the observer. • The gas clouds are falling into the galaxy. • Both A and C
Structure of Atoms(icliker Question) • Most of the mass of ordinary matter resides in the • A) electrons and nuclei, shared equally • B) nuclei of atoms • C) electrons around the nuclei of atoms • D) energy stored within the atom in electromagnetic forces • E) Atoms have no mass. CLM - Fall 2014
Light from Every Point on an Extended Object Passes through Every Point in the Lens …and creates extended images.
Importance of the telescope ‘Three great events stand at the threshold of the modern age and determine its character: 1) the discovery of America; 2) the Reformation; 3) the invention of the telescope and the development of a new science that considers the nature of the Earth from the viewpoint of the universe’ (Hannah Arendt, ‘The Human Condition’) Galileo’s Telescope
Telescope Demo(iclicker Question) The objective lens of our telescope demo has a focal length of 500 mm. Suppose we replace the 250 mm eyepiece with a 50 mm eyepiece. How will the magnification of the meter stick change? Increase 5x Increase to 10x Decrease to 50x Increase to 50x The magnification does not change.
Telescope Demo(iclicker Question) The objective lens of our telescope demo has a focal length of 500 mm. Suppose we replace the 250 mm eyepiece with a 50 mm eyepiece. How should we move the objective lens? Move the objective lens closer to the eyepiece. Move the objective lens further from the eyepiece. Move the objective lens away from the optical axis. There is no need to refocus the telescope. Back and forth by trial and error; it cannot be predicted.
Telescope Demo(iclicker Question) Suppose the professor covers the bottom half of the objective lens. What will the lass see through the telescope on the sceen? The top of the ruler, and this image will be inverted left to right. The bottom of the ruler because the image is inverted, and this image will be inverted left to right. The top of the ruler, and this image will NOT be inverted left to right. The bottom of the ruler because the image is inverted, and this image will NOT be inverted left to right. The same image we saw previously.
Light gathering power depends on size of objective lens or primary mirror
How much more light gathering power does a 10m telescope have than an 0.5 m telescope? Answer: The light gathering power is proportional to the square of the mirror’s diameter. (10m)2/(0.5m)2 = 100m / 0.25m = 400 So you can see objects about 400 times fainter with the 10m telescope in the same amount of time.
The Secondary Mirror Does Not Cause a Hole in the Image This illustration shows how even a small portion of the primary (objective) mirror of a reflecting telescope can make a complete image of the Moon. Thus, the secondary mirror does not cause a black spot or hole in the image. (It does, however, make the image a bit dimmer by reducing the total amount of light that reaches the primary mirror.)
Reflecting Telescopes This view of the Gemini North telescope shows its 8.1-meter objective mirror (1). Light incident on this mirror is reflected toward the 1.0-meter secondary mirror (2), then through the hole in the objective mirror (3) to the Cassegrain focus
