CHAPTER 4: Visible Light and Other Electromagnetic Radiation

CHAPTER 4: Visible Light and Other Electromagnetic Radiation

WHAT DO YOU THINK? • How hot is a “red hot” object? • What color is the Sun? • How can we determine the age of space debris found on Earth?

You will discover… • the origins of electromagnetic radiation • the structure of atoms • that stars with different surface temperatures emit different intensities of electromagnetic radiation • that astronomers can determine the chemical compositions of stars and interstellar clouds by studying the wavelengths of electromagnetic radiation that they absorb or emit • how to tell whether an object in space is moving toward or away from Earth

BLACKBODY RADIATION • A heated iron poker will begin to glow emitting photons. This is different from a burning process because no chemical change is involved. The amount and wavelength of the radiation changes with temperature. • As the object heats up, it gets brighter, emitting more photons of all colors (wavelengths). • The brightest color (most intense wavelength) of the radiation changes with temperature. AS THE TEMPERATURE RISES, THE POKER BECOMES BRIGHTER AND GLOWS ORANGE WHEN FIRST HEATED THE POKER GLOWS DIMMLY AND IS RED AT HIGHER TEMPERATURES THE POKER BECOMES EVEN BRIGHTER AND GLOWS YELLOW

Shown is a plot of intensity versus wavelength for blackbodies at different temperatures. At higher temperatures the most intense wavelengths are shorter. Since the observed color depends on these emitted wavelengths, blackbodies at different temperatures will appear different colors.

Stellar surfaces emit light that is close to an ideal blackbody. We can estimate the surface temperature of a star by examining the intensity of emitted light across a wide range of wavelengths.

A spectroscope is used to examine the wavelengths of light emitted from a source When a chemical is burned, the light produced is made of only specific wavelengths. Different chemical elements have their own series of wavelengths.

Elements are arranged in order of increasing number of protons (atomic number) and their properties. The elements in each column have similar chemical properties.

The combination of lines from a stellar spectrum allow us to determine which chemicals are present and in what quantities. For example, by matching the spectrum of iron to the absorption lines from the Sun, we see that there is iron present in the Sun’s atmosphere.

A grating spectrograph separates light from a telescope into different colors by passing it through a grating of tiny parallel grooves. Peacock feathers are natural gratings.

THE SPECTRUM OF HYDROGEN GAS ABSORPTION SPECTRUM Signature wavelengths appear as dark lines on an otherwise continuous rainbow. Lines appear as dips in the intensity versus wavelength graph. EMISSION SPECTRUM Signature wavelengths appear as bright lines on an otherwise black background. Lines appear as peaks in the intensity versus wavelength graph.

Different types of spectra are produced depending on how a how blackbody and a cloud of gases are observed

ATOMIC STRUCTURE At the center of an atom is a dense nucleus which contains positively-charged particles, called protons, and particles with no charge, called neutrons. This nucleus is surrounded by a cloud of negatively-charged particles called electrons. Most of the mass of an atom is contained within its nucleus. The neutron and proton in the nucleus are each about 1800 times more massive than the electrons in the surrounding cloud. However, most of the space of an atom is occupied by the electron cloud. The nucleus of an atom is 100,000 times smaller than the atom itself. The remaining space is filled by the electron cloud. THE ELECTRON CLOUD EXTENDS FAR FROM THE NUCLEUS NUCLEUS TINY AND MASSIVE CENTER CONTAINING PROTONS AND NEUTRONS

The number of protons contained in the nucleus, called the atomic number, determines the element of the atom. Elements containing the same number of protons in their nucleus (and are thus the same chemical element) but have different numbers of neutrons are called isotopes. Because two protons of like charge repel each other, there must be another force which holds the nucleus of an atom together. We call this force the strong nuclear force,and it is the strongest of the four fundamental forces. However, it only has a range of effect inside the atomic nuclei.

The electrons in an atom can only exist in certain allowed orbits with specific energies. The lines seen from the chemicals are made when an electron moves from one energy level to another. When an electron moves from a lower energy level to a higher one, a photon is absorbed. When an electron moves from a higher energy level to a lower energy one, a photon is emitted. The energy of the photon, and thus its wavelength, are determined by the energy difference between the two energy levels.

The visible light portion of the spectrum of a hydrogen atom shows the photons representing transitions to and from the n = 2 energy level to a higher energy level, forming a series of lines, the Balmer Series. This spectrograph shows the hydrogen Balmer absorption lines from 13-40.

Emitted photons sent out in all directions will cause the gas surrounding a star to glow different colors, depending on which gases are abundant. HYDROGEN RICH CLOUDS GLOW RED. OXYGEN RICH CLOUDS GLOW GREEN.

Radial Velocity The proper motion of a star is its motion perpendicular to our line of sight across the celestial sphere. This is so small that it can only be measured for the closest stars. The radial velocity of a star is its motion along our line of sight either toward or away from us. Using the spectrum, we can measure this for nearly every object in space.

Recall that the wavelength of light, and therefore the wavelength of the photons that light contains, is slightly shifted when the source is traveling toward or away from the observer—the Doppler Effect. Stars moving toward us show spectral lines that are shifted to blue. Stars moving away from us show spectral lines that are shifted to red. The amount of the shift increases with the radial speed. The Balmer series lines from the spectrum of the star Vega are all shifted toward the blue side by the same amount. From this we determine that Vega is moving toward us (blueshift) with a speed of 14km/s (determined from the amount of the shift).

We can examine the proper motion of nearby stars over long periods of time. This picture is made from three overlapping photographs taken over a four-year period. The three dots in a row are Barnard’s Star seen moving over the four-year period.

WHAT DID YOU THINK? • How hot is a “red hot” object? • 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 wavelengths of electromagnetic radiation, with blue-green most intense. • How can we determine the age of space debris found on Earth? • We measure how much long-lived radioactive elements have decayed in the object.

Key Terms absorption line absorption line spectrum atomic number blackbody blackbody curve continuous spectrum diffraction grating element emission line emission line spectrum energy flux excited state ground state ion ionization isotope Kirchhoff’s laws luminosity quantum mechanics radial velocity radioactive spectral analysis spectrograph Stefan-Boltzmann law strong nuclear force transition (of an electron) virtual particles Wien’s law molecule nucleus (of an atom) periodic table Planck’s law proper motion

CHAPTER 4: Visible Light and Other Electromagnetic Radiation