© 2004 Jones and Bartlett Publishers Chapter thru 4-4 Light and the Electromagnetic Spectrum Courtesy of Astrophysics Data Facility at the NASA Goddard Space Flight Center

© 2004 Jones and Bartlett Publishers The Temperature Scales 1. We started measuring temperatures long before we understood what temperature is. The two scales in common use are the Fahrenheit and Celsius scales. 2. The freezing point of water is defined as 32°F or 0  C, while the boiling point of water is defined as 212°F or 100  C. 3. The two scales are related by: TC = (5/9)  (TF − 32)

© 2004 Jones and Bartlett Publishers 4. Neither scale (nor any other scale that defines its zero mark arbitrarily) has anything to do with what temperature “is.” The Kelvin scale, however, has its zero mark defined in a physical meaningful way, as being the state of minimum atomic motion.

© 2004 Jones and Bartlett Publishers 5. On the Kelvin scale, the “absolute zero” corresponds to the lowest possible temperature. The scale is related to the Celsius scale by: T K = T C Notice that temperature differences on the Kelvin and Celsius scales are the same.

© 2004 Jones and Bartlett Publishers 6.Temperature is a fundamental quantity. It is a good approximation to say that given an object’s temperature we can calculate the average speed of each of its constituent particles.

© 2004 Jones and Bartlett Publishers 4-2 The Wave Nature of Light 1. Our understanding of light changed many times over the years. Two lines of thought came to us from the ancient Greeks. Aristotle pictured light as an “aethereal” motion while others considered light as a stream of very small, fast-moving particles. 2. Newton theorized that light consists of tiny, fast- moving particles. He showed that color is a fundamental property of light and that a prism does not add color to light. 3. A spectrum is the order of colors or wavelengths produced when light is dispersed, as by a prism.

© 2004 Jones and Bartlett Publishers Characteristics of Wave Motion 1. Like acts as a wave. The wavelength ( ) of a wave is the distance from a point on the wave to the next corresponding point (e.g., from one crest to the next crest). 2. Frequency (f) is the number of repetitions per unit time. The units of frequency is given in cycles/second or hertz (Hz). 3. The product of the frequency and wavelength of a wave equals the wave’s speed:  =  f.

© 2004 Jones and Bartlett Publishers Light as a Wave 1. White light is made up of light of many wavelengths, all traveling at the same speed. The speed of light in vacuum (c) is about 300 million meters/second (3.00  10 8 m/s). 2. The approximate vacuum wavelength range of visible light is 380 nm (for violet) to 720 nm (for red). A nanometer (nm) is a unit of length = 10 −9 m = 10 Angstrom (Å) (a non-SI unit). © Comstock Images/Jupiterimages

© 2004 Jones and Bartlett Publishers 3. The frequency of a 700 nm red light is f = 4.3  Hz, while that of a 400 nm violet light is f = 7.5  Hz. 4. The color we see is not a property of the light itself but a manifestation of the system that senses it, that is our eyes, nerves, and brain.

© 2004 Jones and Bartlett Publishers Advancing the Model: Measuring the Speed of Light 1. The speed of light in vacuum is now defined to be c = 299, km/s. Light has a smaller speed when going through other transparent media. 2. The first accurate measurement of c was made by the Danish astronomer Ole Roemer. Around 1675, he attributed the differences between the observed and anticipated times for eclipses of Jupiter’s moons to the time required for light to travel from Jupiter to Earth.

© 2004 Jones and Bartlett Publishers 4-1 thru 4-4 Lecture Question 1 The text states that color is not a property of light, what does it mean by that?

© 2004 Jones and Bartlett Publishers 4-3 The Electromagnetic Spectrum 1. The electromagnetic spectrum is the entire array of electromagnetic waves and extends from long wavelength, low frequency radio waves to short wavelength, high frequency gamma rays. 2. The frequency range from radio waves to gamma rays is from below 102 to above 1024 Hz. The corresponding wavelength range is from above 106 m to below 10−16 m.

© 2004 Jones and Bartlett Publishers 3. Based on frequency or wavelength, the EM spectrum is usually broken into these regions: radio (AM/FM/microwave), infrared, visible, ultraviolet, X-rays, gamma rays. Figure 4.03.

© 2004 Jones and Bartlett Publishers 4. These waves are called “electromagnetic” because they consist of combined, perpendicular, electric and magnetic fields that result when a charged particle accelerates. 5. Our atmosphere is transparent to visible light and to part of the radio spectrum, but most of the rest of the EM spectrum is blocked to some degree. Astronomers refer to windows in the Earth’s atmosphere (the visual and radio windows) that allow certain wavelengths to pass.

© 2004 Jones and Bartlett Publishers Figure 4.04: The curve height indicates the relative amount of radiation of a given wavelength blocked by the atmosphere. Visual window Radio window

© 2004 Jones and Bartlett Publishers 4-1 thru 4-4 Lecture Question 2 What is a window in Earth’s atmosphere?

© 2004 Jones and Bartlett Publishers 4-4 The Colors of Planets and Stars Color from Reflection—The Colors of Planets 1. Planets have their colors because the material on their surfaces or in their clouds absorbs some of the wavelengths of sunlight and reflects a combination of wavelengths that appear, for example, as the rusty red of Mars or the blue of Neptune.

© 2004 Jones and Bartlett Publishers Color as a Measure of Temperature 1. An intensity/wavelength graph (the thermal spectrum) of an object emitting electromagnetic radiation can be used to detect its temperature. 2. The peak of the intensity/wavelength curve ( max) for a light-emitting object falls at a wavelength that depends upon the object’s surface temperature (T). 3. According to Wien’s law: max (in nm) = 2,900,000 / T (in kelvin)

© 2004 Jones and Bartlett Publishers Figure 4.07: Color as a measure of temperature.

© 2004 Jones and Bartlett Publishers 4. Wien’s law was derived from theoretical calculations in the late 19th century, when scientists studied ideal objects called blackbodies. A blackbody is a theoretical object that is a perfect absorber and emitter of radiation. The intensity/wavelength curve of a blackbody is called the blackbody curve. Almost all objects of interest in astronomy can be approximated as blackbodies.

© 2004 Jones and Bartlett Publishers Figure 4.09: The Sun is almost an ideal blackbody at 5800 K.

© 2004 Jones and Bartlett Publishers 5. Stefan (in 1879) and Boltzmann (in 1884) showed that the energy emitted per second and per square meter of the surface by an object of temperature T (in kelvin) is proportional to the fourth power of the temperature: energy flux  T The Stefan-Boltzmann law allows us to find the radius of a star, since from observations we can find the temperature and total energy emitted by the star each second.

© 2004 Jones and Bartlett Publishers 4-1 thru 4-4 Lecture Question 3 Describe a blackbody and how it is used to determine the temperatures of stars.