Key Ideas and Terms

2-1 Light Travels through Empty Space at a Speed of Nearly 300,000 km/s

• The speed of light, abbreviated as c, was historically measured over large, interplanetary distances.

2-2 Glowing objects, like stars, emit an entire spectrum of light

• Sunlight is a mixture of all colors that can be spread out into a spectrum.
• Light travels in waves and is characterized by wavelength and its frequency.
• The electromagnetic spectrum of light includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays.
• Light has properties of both waves and particles where individual packets are called photons.

2-3 An object’s temperature is revealed by the most intense wavelength of its spectrum of light

• Wien’s law specifies the dominant wavelength of light emitted by objects and reveals their temperatures.
• Astronomers use the Stefan-Boltzmann law to determine the temperature of an object by carefully measuring the energy flux of light emitted.

2-4 An object’s chemical composition is revealed by the unique pattern of its spectrum of light

• A hot, dense object emits a continuous spectrum covering all wavelengths.
• A hot, transparent gas produces an emission line spectrum containing bright lines.
• A cool, transparent gas in front of a light source that itself has a continuous spectrum produces an absorption line spectrum.
• Spectroscopy is the study of the unique pattern of spectral lines emitted by every chemical substance.
• Electrons can “jump” only to specific orbits within atoms, creating the particular sets of spectral lines.

2-5 An object’s motion through space is revealed by the precise wavelength positions of its spectrum of light

• The Doppler effect describes how an observed wavelength can be shifted due to the relative motion of an energy-emitting object.
• An observed spectrum is redshifted when the distance between an object and an observer is increasing.
• An observed spectrum is blueshifted when the distance between an object and an observer is decreasing.

2-6 Telescopes use lenses, mirrors, and electronics to concentrate and capture incoming light for study

• The ability of a telescope to capture light is a telescope’s light-gathering power and is closely related to the telescope’s diameter.
• A refracting telescope uses an objective lens to concentrate incoming light to a point that is a focal length from the lens.
• A telescope using a primary mirror to concentrate incoming light is called a reflecting telescope.
• The magnification achieved by a telescope is accomplished by the eyepiece lens.
• Angular resolution is the ability of a telescope to see two glowing objects as distinct and separate light sources.
• The new telescope technology of charged-coupled devices (CCDs), adaptive optics, and interferometry dramatically improves modern telescope performance.
• Telescopes in orbit around Earth detect and resolve light that does not penetrate the atmosphere.