Radiation:

Visible Light is only one part of the electromagnetic spectrum, if we were to only use optical telescopes we would limit our view of the universe. So we use other types of electromagnetic radiation to gather information from the universe.

Electromagnetic radiation can be described in terms of a stream of photons, which are massless particles each traveling in a wave-like pattern and moving at the speed of light. Each photon contains a certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons. The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons. Electromagnetic Waves have different wavelengths.

Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom. Electromagnetic waves can be described by their wavelength, energy, and frequency

The electromagnetic spectrum includes, from longest wavelength to shortest: radio waves, microwaves, infrared, optical, ultraviolet, X-rays, and gamma-rays.

Parts of a wave T = 1/f f = 1/T l x f = v All electromagnetic waves move at a very specific speed – the speed of light which is 3 x 108 m/s In the time it takes to snap your fingers light will have traveled ¾ away around the earth. The light from the Andromeda galaxy is 2.5 million years old!

The components of visible light:

Absorption spectra If you look more closely at the Sun's spectrum, you will notice the presence of dark lines. These lines are caused by the Sun's atmosphere absorbing light at certain wavelengths, causing the intensity of the light at this wavelength to drop and appear dark. The atoms and molecules in a gas will absorb only certain wavelengths of light. The pattern of these lines is unique to each element and tells us what elements make up the atmosphere of the Sun. We usually see absorption spectra from regions in space where a cooler gas lies between us and a hotter source. We usually see absorption spectra from stars, planets with atmospheres, and galaxies.

Emission spectra An emission spectra occurs when the atoms and molecules in a hot gas emit extra light at certain wavelengths, causing bright lines to appear in a spectra. As with absorption spectra, the pattern of these lines are unique for each element. We can see emission spectra from comets, nebul and certain types of stars.

This image of the Pleiades star cluster shows haloes around the stars due to the wave nature of light.

If light is a particle… If light is a particle, then only the couple of rays of light that hit exactly where the slits are will be able to pass through.

If light is a wave… There are still only two light rays that actually go through the slits, but as soon as they pass through they start to diffract.

Notice that at some points the two sets of waves will meet crest to crest, at other spots crest meets trough. Where crest meets crest, there will be constructive interference and the waves will make it to the viewing screen as a bright spot. Where crest meets trough there will be destructive interference that cancel each other out… a black spot will appear on the screen. Occurs when one ray travels an extra distance of one-half wavelength and are out of phase Occurs when the path of the two rays differ by one wavelength

All macroscopic objects – fire, ice cubes, people, stars – emit radiation at all times, regardless of their size, shape or chemical composition. The temperature of an object is a direct measure of the amount of microscopic motion within it – the hotter the object – the faster its molecules move and the more energy they radiate.

The Blackbody Spectrum: Intensity is a term often used to specify the amount or strength of radiation at any point in space. No natural object emits all its radiation at just one frequency. Instead, the energy is spread out over a range of frequencies with some frequency demonstrating a more significant intensity. The blackbody curve, describes the distribution of the reemitted radiation.

As temperature increases the curve shifts towards the higher frequency Consider how the color of metal changes as it is heated

Wein’s Law: Relates the temperature of an object to the wavelength at which it emits the most radiation. = 0.29cm/T Stefan’s Law: with T measured in Kelvins, the total amount of energy emitted per square meter of its surface persecond can be calculated F = s T4 , where sigma is known as the Stefan-Boltzman constant – 5.67 x 10-8 W/m2K4

Astronomers often use blackbody curves as thermometers to determine the temperatures of distant objects, even the surface temperature of the sun.

The Doppler Effect: Stars that you are moving towards appear more blue and those you are moving away from appear to be more red. The motion induced change in the observed frequency of a wave is called the Doppler effect.