Chapter 1. The Nature of Light

Overview

Astronomers are passionately interested in the nature of light because it is the only information we get from stars. The stars, and most of the planets for that matter, are so far away that we will probably never visit them. So, for now, astronomers must learn to decode information hidden in starlight if we are to learn anything at all about the nature of these distant objects. Fortunately, the properties of light are well understood. After starlight is split into a rainbow by passing it through a prism or similar device, careful inspection of the light provides detailed information about the temperature, composition, and motion of stars.

Drawing of Newton's experiment showing that colors do not come from the glass of a prism. The second prism undoes the effects of the first prism, rather than adding more color, which it should if the prism creates the color. (Click image above to enlarge)

Observation

Tutorials

(In the original, there is no action taken when "Tutorials" heading is clicked, so no text would appear immediately below the "Tutorials" heading here either.)

Light and Temperature (first tutorial/L2 section)

Considering Star Colors and Temperatures

Nature of Light and Electromagnetic Radiation

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Although we use light every day, many of us have not stopped to think about the nature of light. What are the main characteristics of light, anyway? Light is composed of tiny packets of energy called photons. Light moves extremely fast—nearly 300,000 kilometers every second (186,000 miles per second). Light is often described as having wave-like characteristics called wavelengths. A more appropriate name for light and light energy is electromagnetic radiation or EM radiation. The EM radiation our eyes are capable of seeing is usually called visible light and ranges in wavelengths from about 400 to 700 nanometers (A nanometer is one-billionth of a meter). Stars and most other objects, as it turns out, emit a wide range of different wavelengths of EM radiation in addition to visible light. Most of the names for EM radiation are familiar to you. Listed from short wavelengths to long wavelengths, there are gamma rays, X rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves. In fact, visible light constitutes only a minuscule portion of the types of EM radiation emitted by stars. Because each type of EM radiation has a different wavelength, each also has a different energy. Energy and wavelength are inversely proportional. Blue light, for instance, has a much higher energy and a much shorter wavelength than red light, and red light has much lower energy but much longer wavelength than blue light.

Self Test: Electromagnetic Spectrum

Drag the circles adjacent to the terms below to their appropriate location on the diagram.

Click here (http://bcs.whfreeman.com/aol/content/cat_030/ch05/m05t001p3ST.html) to view the exercise in an external window.

Emission of Light by Electrons in Atoms

When atoms are heated or bathed in EM radiation, electrons absorb the extra energy and move to higher states when orbiting the nucleus. Because electrons are negatively charged and the particles in the nucleus are positively charged, the electrons quickly return to their lower orbits. However, to move to a lower orbit, the electron must release some energy. Electrons release this energy in the form of light.

(Click image to view animation in an external window.)

Temperature and Color

When atoms in objects absorb and reemit higher and higher levels of energy, such as when they are heated to thousands of degrees, they emit shorter and shorter wavelengths of light. This simple, inversely proportional relationship between the temperature, measured in kelvins, of an emitting object and the maximum wavelength of light emitted, , measured in meters, is known as Wien's law. Wien's law is calculated as

_{max meters}= 0.0029 / T _kelvin

This sequence of drawings shows how the appearance of a heated bar of iron changes with temperature. As the temperature increases, the amount of energy radiated by the bar increases. The color of the bar also changes as the temperature goes up because the dominant wavelength of light emitted by the bar decreases.

What this means is that the hotter an object is, the shorter the wavelength, or bluer, the light being emitted is. Objects that are at tens of thousands of degrees emit mostly ultraviolet light. Gases, like those in the outer atmosphere of the Sun, are at millions of degrees and emit primarily X rays.

Self Test: Wien's Law

According to Wien's law, the wavelength of maximum emission of a blackbody is inversely proportional to its temperature in Kelvins. Given the maximum wavelength of light emitted by a star in nanometers, use the on-line Wien's law calculator below to determine the approximate surface temperature of the following stars to rank order them from hottest to coolest. Note: 206 nm is equal to 206 X 10^-9 m.

Rigel, _max = 206 nm

Deneb, _max = 302 nm

Arcturus, _max = 644 nm

Vega, max = 251 nm

Betelgeuse, _max = 935 nm

To use this calculator enter the observed peak wavelength (lambda_max). Press the OK button to determine the object's temperature in kelvin (K). Note: "e" stands for a power-of-ten exponent. For example, 5.8e3 = 5.8 x 10³, 5e-7 = 5.0 x 10^-7.

Infrared Radiation

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Because our eyes are sensitive to only a very small range of EM radiation, we often do not realize that all objects emit EM radiation. In fact, all objects that have a temperature above absolute zero (-273°C) emit some EM radiation. Objects like your computer mouse are at room temperature, or about 295 K. The computer mouse is emitting some EM radiation right this instant, most likely in the infrared, which our eyes do not detect. The reason you can see your computer mouse is that it is reflecting visible light in the room into your eyes. Infrared radiation is an important part of astronomy because infrared radiation can come through the interstellar clouds, which visible light does not penetrate. In the Hubble Space Telescope photograph, the left side shows only a few visible stars whereas the camera that is sensitive to infrared light can easily see the infrared radiation from the cool stars within the clouds.

(Click image to enlarge.)

Note: The Jet Propulsion Laboratory has created a short video about infrared radiation that is available online at http://coolcosmos.ipac.caltech.edu/

Blackbody Radiation Curves

All objects emit a wide range of EM radiations. We can make a graph of the emitted radiations of any object by plotting the wavelength emitted versus the brightness. The resulting curve is called a blackbody curve. A blackbody curve illustrates the intensity of light at every wavelength that is emitted by a blackbody (an idealized case of energy emission from a hot and dense object) at a particular temperature. The higher the temperature, the shorter the wavelength of maximum emission (at which the curve peaks) and the greater the amount of light emitted at every wavelength.

INSTRUCTIONS: To graph a blackbody curve, enter the temperature (in kelvins) and click on "New Curve". Up to five curves can be displayed at one time. The "Clear" button will remove all curves. The "Redraw" button will refresh the graph. This is useful when the function domain (i.e., wavelength) has been changed. To see the temperature for each curve, check the "Temperature" box. Check the "Wien's law" box to see the wavelength of maximum emission. Check the "Visible light", "Ultraviolet", or "Infrared" boxes to delineate that portion of the spectrum.

Spectroscopy (second tutorial/L2 section)

......subsections of content go here.

Activities

Introductory text for activities.

...Light and Temperature (L2 section)

......Java Wavelength Calculator app and questions go here.

...Spectroscopy (L2 section)

......Java Doppler app and questions go here.

Examination

Introductory text for 20 questions that follow.