Unformatted text preview: The Nature of Light A spectrum is produced when white light passes through a prism But, what is light? In the 17th Century, Isaac Newton argued that light was composed of little particles while Christian Huygens suggested that light travels in the form of waves. In the 19th Century, Thomas Young demonstrated that light bends slightly around corners and acts like interfering waves. Thomas Young's interference experiment Scottish physicist James Clerk Maxwell showed mathematically in the 1860s that light must be a combination of electric and magnetic fields. It wasn't until 1905 that our current understanding of the nature of light emerged. Einstein showed that light sometimes behaves as particles and sometimes as waves.
Photon energy = Plank's constant x speed of light / wavelength The study of light Light (electromagnetic radiation) can be described in two ways Wave model Wavelengths of radiation vary Radio waves measure up to several kilometers long Gamma ray waves are less than a billionth of a centimeter long White light consists of several wavelengths corresponding to the colors of the rainbow The study of light Light (electromagnetic radiation) can be described in two ways Particle model Particles called photons Exert a pressure, called radiation pressure, on matter Shorter wavelengths correspond to more energetic photons But, where does light actually come from?
Light comes from the movement of electrons in atoms An atom consists of a small, dense nucleus surrounded by electrons Atomic Vocabulary The nucleus contains protons and neutrons All atoms with the same number of protons have the same name (called an element) Atoms with varying numbers of neutrons are called isotopes. Atoms with varying numbers of electrons are called ions. Spectral lines occur when an electron jumps from one energy level to another If you pass white light through a prism, it separates into its component colors. long wavelengths R.O.Y. G. B.I.V short wavelengths spectrum If white light contains all colors, how fast does it move? 186,000 miles per second first successfully determined by Danish astronomer Ole Roemer in 1675 Electromagnetic radiation Visible light is only one small part of an array of energy Electromagnetic radiation includes Gamma rays X-rays Ultraviolet light Visible light Infrared light Radio waves Visible light is only one type of electromagnetic radiation emitted by stars Each type of EM radiation travels at exactly the same speed - the speed of light! What do you think? How hot is a "red hot" object compared to objects glowing with other colors? What color is the Sun? Peak color (wavelength) shifts to shorter wavelengths as an objects is heated increasing temperature What can we learn by analyzing starlight? A star's temperature Peak color (wavelength) emitted depends on an object's temperature Peak color (wavelength) shifts to shorter wavelengths as objects are heated The intensities of different emitted colors reveal a star's temperature Wien's law wavelength is inversely proportional to temperature max = (2.9 x 10-3) / TKelvin What color is our 5800K Sun?
The Sun emits all colors (wavelengths of electromagnetic radiation); however, the color it emits most intensely are in the blue-green part of the spectrum. What can we learn by analyzing starlight? A star's temperature A star's chemical composition The study of light Spectroscopy The study of the properties of light that depend on wavelength The light pattern produced by passing light through a prism, which spreads out the various wavelengths, is called a spectrum (plural: spectra) Spectral lines occur when an electron jumps from one energy level to another The study of light Spectroscopy Types of spectra Continuous spectrum Produced by an incandescent solid, liquid, or high pressure gas Uninterrupted band of color Dark-line (absorption) spectrum Produced when white light is passed through a comparatively cool, low pressure gas Appears as a continuous spectrum but with dark lines running through it The study of light Spectroscopy Types of spectra Bright-line (emission) spectrum Produced by a hot (incandescent) gas under low pressure Appears as a series of bright lines of particular wavelengths depending on the gas that produced them Most stars have a dark-line spectrum Instrument used to spread out the light is called a spectroscope Formation of the three types of spectra Conclusion Emission and absorption spectra allow astronomers to determine the elements present in a distant star. Each chemical element produces its own unique set of spectral lines when it burns The Sun's Spectrum Kirchhoff's Laws The brightness of spectral lines depend on conditions in the spectrum's source
Law 1 A hot object or a hot, dense gas produces a continuous spectrum -- a complete rainbow of colors without any specific spectral lines. (This is a black body spectrum.) The brightness of spectral lines depend on conditions in the spectrum's source
Law 2 A hot, rarefied gas produces an emission line spectrum - a series of bright spectral lines against a dark background. The brightness of spectral lines depend on conditions in the spectrum's source
Law 3 A cool gas in front of a continuous source of light produces an absorption line spectrum - a series of dark spectral lines among the colors of the rainbow. Absorption Spectrum of Hydrogen Gas What can we learn by analyzing starlight? A star's temperature A star's chemical composition A star's movement Barnard's Star The Doppler shift allows astronomers to measure radial velocity Spectral lines shift due to the relative motion between the source and the observer The study of light Doppler effect The apparent change in wavelength of radiation caused by the relative motions of the source and observer Used to determine Direction of motion Increasing distance wavelength is longer ("stretches") Decreasing distance makes wavelength shorter ("compresses") Velocity larger Doppler shifts indicate higher velocities The Doppler effect Doppler Shifts Red Shift: The distance between the observer and the source is increasing Blue Shift: The distance between the observer and the source is decreasing Stationary Slow Fast A Comparison of Three Stars
Our Sun Star "B" Star "C" Star "D" What can we learn by analyzing starlight? A star's temperature by peak wavelength by spectral analysis from Doppler shifts A star's chemical composition A star's radial velocity Nature of Light
The End What did you think? How hot is a "red hot" object compared to objects glowing with other colors? Of all objects that glow from heat stored or generated inside them, those that glow red are the coolest. What color is the Sun? The Sun emits all colors (wavelengths of electromagnetic radiation). The colors it emits most intensely are in the bluegreen part of the spectrum. Self-Check
1: State the Stefan-Boltzmann law and Wien's law and explain their meaning in the context of blackbody radiation and temperature determination. 2: Describe the evidence for the particle nature of light and indicate how the energy per photon is related to the wavelength and frequency in the wave model. 3: State Kirchhoff's three laws of spectral analysis and indicate what information is derived about the nature of the light source in each case. 4: Describe the Bohr model of the atom in terms of its constituents and their distribution. Explain how spectral lines can be produced by a low-density gas. 5: Describe how spectroscopic analysis provides information about the chemical composition of celestial objects and indicate for which part of the object the information is valid. 6: Indicate how the numbers of protons, neutrons, and electrons are used to define elements, ions, and isotopes. 7: Describe the origin of line series in the hydrogen atom and explain why the Balmer lines occur at visual wavelengths but the other line series do not. 8: Define excitation and ionization in the context of the Bohr model of atoms. Not all EM radiation can penetrate Earth's atmosphere. A refracting telescope uses a lens to Different types of EM radiation require different types of telescopes concentrate incoming light A reflecting telescope uses mirrors to concentrate incoming starlight wavelength is inversely proportional to temperature max = (2.9 x 10-3) / TKelvin The study of light Electromagnetic radiation All forms of radiation travel at 300,000 kilometers (186,000 miles) per second Peak color (wavelength) shifts to shorter wavelengths as an objects is heated hotter object cooler object ...
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- Summer '07
- Light, Thomas Young, spectral lines, Wien