Chapter 3 Light and Telescopes

What do you think? What is the main purpose of a telescope? Why do stars twinkle?

If you pass white light through a prism, it separates into its component colors. R.O.Y. G. B.I.V spectrum long wavelengthsshort wavelengths

If white light contains all colors, how fast does it move? 186,000 miles per second 300,000 kilometers per second 3 x 10 8 m/s first successfully determined by Danish astronomer Ole Roemer in 1675

But, what is light? In the 17th Century, Isaac Newton argued that light was composed of little particles while Christian Huygens suggested that light travels in the form of waves. In the 19th Century, Thomas Young demonstrated that light bends slightly around corners and acts like interfering waves.

Thomas Young’s interference experiment

Scottish physicist James Clerk Maxwell showed mathematically in the 1860s that light must be a combination of electric and magnetic fields.

It wasn’t until 1905 that our current understanding of the nature of light emerged. Einstein showed that light sometimes behaves as particles and sometimes as waves. Photon energy = Plank’s constant x speed of light / wavelength

Visible light is only one type of electromagnetic radiation emitted by stars Each type of EM radiation travels at exactly the same speed - the speed of light!

Not all EM radiation can penetrate Earth’s atmosphere.

Different types of EM radiation require different types of telescopes A refracting telescope uses a lens to concentrate incoming light A reflecting telescope uses mirrors to concentrate incoming starlight

A refracting telescope uses a lens to concentrate incoming light Similar to a magnifying glass

A larger objective lens provides a brighter (not bigger) image

lenses reverse images

Three main functions of a telescope brighten (called light gathering power) see fine detail (called resolution) and least important, magnify magnification = (objective lens focal length / eyepiece lens focal length)

Refracting telescopes have drawbacks Spherical aberration Chromatic aberration

Special achromatic compound lenses and lens coatings can often fix this aberration

Refracting telescopes have drawbacks Spherical aberration Chromatic aberration Sagging due to gravity distorting the lens Unwanted refractions opaque to certain wavelengths of light

Reflecting telescopes use mirrors to concentrate incoming starlight

Newtonian Focus Prime Focus Cassegrain focus coude’ focus

Astronomer’s face two major obstacles in observing the stars Light Pollution from Cities Effects of Twinkling from Earth’s atmosphere

Tucson, Arizona in 1959 and 1980

Earth’s atmosphere hinders astronomical research Image of stars taken with a telescope on the Earth’s surface Same picture taken with Hubble Space Telescope high above Earth’s blurring atmosphere

Rapid changes in the density of Earth’s atmosphere cause passing starlight to quickly change direction, making stars appear to twinkle.

Advanced technology is spawning a new generation of equipment to view the universe CCDs (charge-coupled devices) Large telescopes on remote mountain tops –Mauna Kea in Hawaii –Cerro Pachon in Chile Adaptive Optics to counteract the blurring of Earth’s atmosphere Orbiting space observatories

A Charge-Coupled Device (CCD)

Ordinary Photographs vs. CCDs

Matching 10-m, multiple mirror Keck Telescopes in Hawaii with adaptive optics

High above Earth’s atmosphere, the Hubble Space Telescope provides stunning details about the universe

Observations at wavelengths other than visible light are revealing previously invisible sights Visible light imageradio wavelength image

Radio wavelength observations are possible from Earth’s surface

The Very Large Array (VLA) in New Mexico

Observations at other wavelengths are revealing previously invisible sights UV Ordinary visible infrared Map of Orion region

What did you think? What is the main purpose of a telescope? A telescope is designed to collect as much light as possible. It also improves resolution and magnifies images. Why do stars twinkle? Rapid changes in the density of Earth’s atmosphere cause passing starlight to change direction, making stars appear to twinkle.

Self-Check 1: List the major regions of the electromagnetic spectrum in order of wavelength and give common examples of each. 2: List the colors of the visible spectrum in order of wavelength. 3: Name the two main classes of telescopes and describe the physical laws that each uses to form images. 4: Describe how the focal length and diameter of a telescope influence its angular resolution, light-gathering power, and magnifying power. 5: Draw a refracting telescope and reflecting telescopes with Newtonian, Cassegrain, prime, and coude’ focus locations, showing the path of parallel light rays through each. 6: Compare the merits and deficiencies of the two major classes of telescopes. 7: Discuss the similarities and differences of radio telescopes and optical telescopes. 8: List the advantages of orbiting telescopes over Earth-bound telescopes in detecting electromagnetic radiation in each of the major spectral regions. 9: Identify examples of observations impossible from the ground that have been made by the Hubble Space Telescope.