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4: Light and Spectra

Last updated

Jul 23, 2025
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- 3.6: Eclipses
- 4.1: Looking at a rainbow, what do you see?

Lumen Learning
Lumen Learning

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Red, orange, yellow, green, blue, indigo-violet mystic vision arcs
Red, orange yellow green blue indigo follow graceful violet glows
Lively prism light enchanting rainbow delights brightly dance then hides
Misty promise made sunny days ahead storm’s ending I see ROY G BIV

Lorraine Margueritte Gasrel Black
Rainbow in Motion Haiku

Learning Objectives

Upon completion of this module, the student will be able to:

Describe light and the electromagnetic spectrum
Recognize Wien’s Law and Stefan-Boltzmann Law
Differentiate among Continuous, Emission Line, and Absorption Line spectrums
Describe Doppler Effect, both in sound and light
Identify the astronomical implications of spectral observations

This module covers visible light as well as other forms of the electromagnetic spectrum, which provide us information about the Universe. An object’s spectrum can also provide an incredible amount of information, from the object’s velocity to its distance.

4.1: Looking at a rainbow, what do you see?
This page explores light as radiative energy, detailing its division into a spectrum that showcases its colors via prisms. Light is composed of photons moving at 186,000 mi/s and exhibits characteristics of electromagnetic waves, defined by wavelength and frequency, which indicate wave peak distance and the rate of peaks passing a point, respectively.
4.2: Thermal Radiation
This page explains temperature as a measure of particle movement, using the Kelvin scale where 0K is Absolute Zero. It describes Wien's Law, which states that the brightest color of an incandescent object is inversely proportional to its temperature. The page also highlights the Stefan-Boltzmann Law, indicating that the power emitted per unit area is proportional to the fourth power of an object's temperature.
4.3: The Electromagnetic Spectrum
This page explains the Electromagnetic Spectrum (EMS), highlighting the range of frequencies of electromagnetic radiation that acts as both waves and particles. It details the visible light segment (400 nm to 700 nm) and the various types of radiation, including Radio, Microwave, Infrared, Ultraviolet, X-rays, and Gamma Rays. It notes that only specific wavelengths penetrate Earth’s atmosphere and provides a mnemonic for remembering the order of the EMS.
4.4: Spectra
This page explains the importance of spectra in astronomy, detailing three main types: continuous spectra produced by hot, dense solids; emission line spectra that show bright lines from heated atoms; and absorption line spectra marked by dark lines from absorbed photons. These spectra are essential for understanding the composition and behavior of stars and celestial bodies.
4.5: Moving Objects, Spectra, and the Doppler Effect
This page explains the Doppler Effect, which describes changes in an object's spectrum due to its movement relative to an observer, notably in sound. Its applications in astronomy include analyzing stellar composition, gauging the motion of celestial bodies through red or blue shifts, and measuring their speed. The historical context highlights key contributions by astronomers Herschel and Ritter in understanding infrared and ultraviolet radiation.

Learning Objectives

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