Light

Properties Light is key to understanding the universe by analyzing light; we learn what planets & stars are made of and their temperature. Light is radiant energy - it can travel through empty space without a physical link. Ex: explosion -light will reach us but not sound (it needs a medium)

Light Travels at 2.99 x 10 5 km / sec Known as “c” in empty space Slows in transparent materials & slows differently with blue slower than red

Light is a wave or particle: Electromagnetic wave model - alternation of electric & magnetic energy like a vibration

Light is a wave or particle: Photon model - light is a stream of photons which are packets of energy. It enters eye and produces light. # of photons indicates brightness

Light and color Color of light is determined by the lights wavelength. (distance between crests) Measured in nanometers (billionths of a meter) m Denoted as lambda.

Frequency Number of wave crests that pass a given point in one second. Red - long wavelength, low freq. Violet - short wavelength, high freq.

Characteristics Speed of light = wavelength x frequency = constant in space White light is a mixture of all colors, sun Newton separated with a prism & recombined with a lens We ignore background (ambient) color and notice changes = white light

Wavelength Color Wavelength violet 380–450 nm blue 450–495 nm green 495–570 nm yellow 570–590 nm orange 590–620 nm red 620–750 nm

Arranged according to wavelength

Radio waves Increased wavelength, decreased freq., lowest amount of energy Galaxies and pulsars Built radio telescope - saw forming & exploding stars, pulsars, planets, active galaxies, extraterrestrial

Infrared waves Heat radiation just off red wavelength of visible light. Given off by sun, can feel the heat but cannot see it. The wavelength is longer than red so our eyes cannot detect it. Also comes from planets and interstellar dust

Visible light The only part of the electromagnetic spectrum that we can see. Our eyes can only detect wavelengths between 400 nm & 700 nm.

Ultraviolet waves - UV Shorter wavelengths than visible light. Just outside the color violet. Comes from stars.

X - rays 1940 discovered x rays coming from sun. Also comes from hot gases surrounding black holes, collapsed stars and distant galaxies.

Gamma rays Shortest wavelength, greatest frequency, and most energy Absorbed by the upper atmosphere so not found at Earth’s surface. Source from active galaxies and gamma - ray bursts

All waves: Are electromagnetic waves (vibration of electric and magnetic energy) that travel at the speed of light. Difference is the wavelength or frequency.

Wien’s Law A wavelength - temperature relation If increase wavelength, the temperature decreases. T = 3 x 10 6 / wavelength