July 11 , Lecture 4, AST201
Light is an electromagnetic wave. Particles of light are called photons. Light is both
waves and particles. Electromagnetism, 1/137, infinite, make atoms cling to each
other. Electromagnetic radiation = light
The observed speed of light is constant. It is not what the source or velocity of the
observer, reference frame of the observer, velocity of the source of the light. No type
of matter that can be transmitted faster than the speed of light.
Wavelength is the distance between adjacent peaks of the electric (and magnetic)
field. While frequency is the number of times each second that the electric (and
magnetic) field vibrates up and down (or side to side) at any point. All light travels
with speed c=5,300,000 km/s
Speed of light = wavelength x frequency
Increasing frequency and energy
Red Orange Yellow Green Blue Indigo Violet
Light can have any wavelength
Electromagnetic radiation is more than just visible light. (infrared) ROYGIBIV
X-Rays, Infrared, Microwave, Radio
(Insert slide for The Electromagnetic Spectrum)
(Insert slide of major space observatories)
Atmosphere actually absorbs and reacts to other types of radiation, lose energy
before reaching the ground. Gamma and X-ray, launch telescopes into space. Use
balloons. For visible, telescopes on the ground and high altitude and stable
atmosphere. Infrared, telescopes on a plane. Radio on the ground.
Before about 1950, all astronomy was done using visible light.
Since 1950, astronomy has used all forms of radiation. Flood of new information
coming in from using all forms of radiation.
Visible light from human view. UV light from the bee’s view.
Gamma Rays, X-rays, Ultraviolet light, Visible light, infrared light, microwaves, radio
Increasing wavelength, decreasing frequency, decreasing energy, “redder”
The higher the photon energy, the shorter its wavelength. Different wavelengths show different physical conditions.
X-rays. Differing in photos. Epicenters are very bright.
Ultraviolet compared to visible light. Bright stars, blue, massive, hot, young.
Infrared, wider radiation. Cool low mass stars, emit more infrared. Hot dust.
(insert slide of Milky Way)
How do Light and Matter interact?
1) Emission- the matter has energy that emits as radiation (light bulb)
2) Absorption- the matter takes in radiation, gaining its energy (hot day at the
3) Transmission- allowing light to pass through, transparent objects transmit
light, opaque objects block (absorb) light. (windows, glasses)
4) Reflection or scattering- radiation “bounces” off and goes in another
direction (mirrors, sea)
Why is a rose red? The rose reflects red light.
Why is the sky blue? Air preferentially scatters blue light.
Light of all colors from the sun.
The atmosphere scatters blue light much more than red light, so that the blue light
reaches you from all directions in the sky. This makes the sky appear blue. At sunset
(or sunrise), sunlight must pass through more atmosphere to reach you. Most blue
light is scattered away, leaving red light that gives you