LIGHT

WHAT IS LIGHT? All frequencies or wavelengths of electromagnetic radiation Seven bands of electromagnetic waves, based upon differences in frequency and wavelength. A continuous spectrum A form of energy.

WHAT CAUSES Electromagnetic waves (or LIGHT)? Vibrating electrically charged particles A changing electric field sets up a changing magnetic field, which sets up a changing electric field, and so on. Electromagnetic wave (a combination of electric and magnetic fields

Electromagnetic Wave http://www.nrc-cnrc.gc.ca/obj/inms-ienm/images/research_images/optical_comb/COMBFIG1.gif

Electromagnetic Waves Light is a portion of the family of electromagnetic waves that includes radio waves, microwaves, and X-rays. The range of electromagnetic waves is called the electromagnetic spectrum.

The Electromagnetic Spectrum Non-mechanical waves are produced by vibrating electric charges (changing electric field produces a changing magnetic field and vice-versa) Spectrum means a continuous series of wavelengths and frequencies Divided into 7 characteristic portions-differ by wavelength As frequency increases, wavelength decreases Light shows a dual nature Modeled as Wave (reflects, refracts, diffracts, interferes) Modeled as Particles: a stream of photons As the frequency increases, so does the energy of the photon All objects emit electromagnetic radiation (higher the temperature, the higher the frequency of the electromagnetic spectrum)

http://phet.colorado.edu/en/simulation/photoelectr ic

Visible Light Smallest portion of electromagnetic spectrum Different frequencies represent different colors White light contains all frequencies of visible light (all colors) ROYGBV—from longest to shortest wavelength or lowest to highest frequency, red light has lower frequency and is less energetic than violet light White light is the presence of all color (reflection); black light is the absence of all color (absorption). The color we see represents only those frequencies of visible light that are reflected by the object Primary colors of light (Red + Blue + Green = White) Red + Blue (magenta) Blue + Green (cyan) Red + Green (yellow) Photoelectric effect: visible light photons cause electrons to move in solar cells, producing electricity. The cone cells in the retina are light-sensitive and respond to the different frequencies of color The pupil will change size to adjust for intensity (brightness) of light

Graphical Representation of Wavelength and Light

TWO MODELS HOW DO WE MODEL LIGHT? Wave Model Particle Model Physicists recognize the dual nature of light (light travels like a wave, but hits like a particle). Photon: a massless particle of light (carries energy—greater the frequency, the greater the photon energy)

LIGHT AS A WAVE AND A PARTICLE Light behaves like a wave because light reflects, refracts, diffracts and interferes. Light also behaves like a particle, traveling in a stream photons. Photons are discrete packets of light energy. Evidence for the particle model is the photoelectric effect.

ENERGY OF A PHOTON Photon energy is proportional to the frequency of light. The higher the frequency, the higher the photon energy. For example, radio wave photons carry less energy than do ultraviolet light photons.

Light and Transparent Materials A light wave incident upon a pane of glass sets up vibrations in the atoms. Because of the time delay between absorptions and reemissions, the average speed of light in glass is less than c.

INTENSITY OF LIGHT The amount of light illuminating a surface or passing through space is called the intensity (brightness). Intensity relates to the number of photons, not the energy of a photon.

INTENSITY OF LIGHT An increase in the number of waves or photons (or the amount of light) Brightness of light means intensity. Not related to frequency or the energy per photon! Same frequencies of light, just different intensities! 100 Watts 40 Watts

SPEED OF LIGHT Nothing in the Universe travels faster than the speed of light. Speed 3 x 108 m/s 300,000 km/s 186,000 mi/s

WHAT AFFECTS THE SPEED OF LIGHT? Speed of light is affected by the type of medium it travels through. In general, light travels fastest through a vacuum, then gases, liquids and solids. This is opposite of mechanical waves. Sound waves travel fastest through solids, not gases.

Photoelectric Effect: the ejection of electrons from certain metals when exposed to certain frequencies of light. Electrons emitted Electromagnetic radiation (visible light range) Photovolatic cell (solar cell) Produces electron flow (electricity)

Energy transformations Observed with photovoltaic cell, motor and propeller Visible light absorbed by photvoltaic cell (light energy to electrical energy) Electric motor causes propeller to spin (electrical energy to mechanical energy)

WHAT IS LIGHT? All forms of electromagnetic radiation http://www.colorado.edu/physics/2000/waves_particles/ http://didaktik.physik.uni- wuerzburg.de/~pkrahmer/ntnujava/emWave/emWave.html http://imagers.gsfc.nasa.gov/ems/waves3.html http://ihome.cuhk.edu.hk/~s016969/physproj/index.htm http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectr um.html