## The Nature of Light

• As opposed to other scientists, Astronomers generally cannot conduct "hands-on" experiments. They primarily collect and analyze light from distant inaccessible objects.

• A source of light produces photons - small packages of energy which leave the source and travel through space (or any transparent material) at the speed of light - 300,000 km/s.

• Photons have a wave nature. Each one is a small packet of waves.

• Different types of light (e.g. Radio, Infrared, Visible...) are distinguished by their wavelength, as are different colors of light in the visible part of the electromagnetic spectrum.

• Within the visible portion of the spectrum our eyes sense different wavelengths of light as different colors.

• "White" is our brain's response to our eyes receiving all colors at the same time.

• The principal regions of the electromagnetic spectrum (from long wavelengths to short) are Radio, Infrared, Visible, Ultraviolet, X-ray, and Gamma-ray

• The different methods of detection of these sorts of light define these different regions, even though there is no fundamental difference between the different kinds of photons except for wavelength..

• The only thing that distinguishes radio waves from gamma rays is their wavelength.
• Radio light: = meters (m)
• Infrared: = microns (1 m = 10 m)
• Visible: = 0.5 m = 500 nm (1 nm = 10 m)

• Each photon carries a discrete amount of energy which depends upon its wavelength.

• Energy, wavelength, and thus color, are all equivalent.

• h is just a constant of nature -- Planck's constant. The value is not important here.
• c is a constant as well -- the speed of light. Its numerical value (3x108 meters/sec) is not important to understanding this equation (but it is a value you need to know).

• What IS important is that this equation says that short wavelength photons (e.g. X-rays and -rays) carry much more energy per photon than photons that have a long wavelength (e.g. radio or infrared light).

• Light sorted according to wavelength produces a spectrum.

Revised October 2, 2003