White light is composed of all the

colors of the rainbow• Rainbow of colors of the visible

spectrum can be split by a prism

RedOrangeYellowGreenBlueIndigoViolet

The visible spectrum

A single color can’t be split again

Spectrum can be recombined into white light

Light is electromagnetic

radiation

High

er fr

eque

ncy

What is the speed of light?

How do we know?

Measuring the speed of light

Light takes over 16 minutes to travel the diameter of earth’s orbit (2 AU)

Speed of light is the universal speed limit

• No physical object can travel at the speed of light

• No information can travel faster than light

• We can’t find out about the universe faster than c

• Distance is a time machine!

Light behaves as particles and as waves

• Particles of light are called photons• Energy of a photon increases with

shorter wavelengths

E = h c / λ

Most electromagnetic radiation is blocked by

atmosphere• Good for life on earth, but bad for

observing the universe• Much of the observation of the

universe must take place from space

• Visible light and radio waves are biggest “windows” in atmosphere

Types of telescopes• Refracting telescopes focus light

with an objective lens

• Reflecting telescopes focus light with an objective mirror

Refracting telescopes• Bend light to collect & magnify

Refracting telescopes are made with two lenses

Image is inverted in eyepiece!

Light gathering power increases with aperture

Aperture also increases resolution

Magnification does no good if resolution is poor!

Limitations of refracting telescopes

• Spherical aberration• Chromatic aberration• Distortion of lens by gravity• Unwanted refractions

(imperfections in glass)• Glass blocks some light (especially

outside visible, e.g. UV)

Spherical aberration

Chromatic aberration

(same effect that makes prism separate colors)

Lick Observatory,Mt. Hamilton

• 36” telescope is 2nd largest refractor in the world

• Severely impacted by light pollution from Silicon Valley

Reflecting telescopes• Avoid many of the

problems of refracting telescopes

• Can be made much larger• up to 200” single mirror• much larger with new

technology– up to 10 meter Keck

telescopes

Reflecting telescopes use a mirror to focus light

• No chromatic aberration

• Easier to make large mirrors that don’t distort

• Still have to correct for spherical aberration (or use parabolic mirror)

a) Newtonian c) Cassegrainb) prime focus d) coudé

Type of refactors

Astronomical observations from surface of the Earth

are limited• Turbulent air causes twinkling and

poor “seeing” (lower resolution)• Most non-visible wavelengths are

blocked by atmosphere• Light pollution is increasing problem• Space-based observatories don’t

have these limitations (but cost $$$)

Light pollution

Space-based observatories• Hubble Space Telescope

• 2.4 meter orbiting telescope• Observes in visible, near-IR, UV

• Many others for IR, UV, X-ray, gamma ray

New technology is revolutionizing ground-

based astronomy• Active optics• Adaptive optics• Segmented mirrors• Optical interferometry

Observing and recording the sky

• First observations were made directly through telescopes, recorded by hand sketches

• Photographic plates & film allowed long time exposures to capture greater amounts of light

• CCDs now record images digitally

M27 (Dumbell Nebula) seen through an amateur telescope

Amateur long exposure photograph

Radio Astronomy• First radio signals from

space detected in 1932• Can be done from the

ground• Collecting dishes need

to be large for long-wavelength radio waves

Infrared (IR) telescopes• Infrared Telescope Facility (IRTF),

Mauna Kea (NASA)• Infrared Astronomical Satellite

(IRAS)• Space Infrared Telescope Facility

(SIRTF) to be launched this spring

Ultraviolet (UV) astronomy• International Ultraviolet Explorer

(IUE) 45 cm• Far Ultraviolet Spectroscopic

Explorer (FUSE)

X-rays and gamma rays• High-energy photons

cannot be focused like light

• Very specialized orbiting instruments

• Chandra X-ray observatory

• Compton gamma ray observatory