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

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.

Electromagnetic Waves

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/photoelectric

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

HOW DO WE MODEL LIGHT?TWO MODELS

Wave ModelParticle 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.

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.

Light and Transparent Materials

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!

100 Watts 40 Watts

Same frequencies of light, just different intensities!

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.

Electromagnetic radiation (visible light range)

Electrons emitted

Produces electron flow (electricity)

Photovolatic cell (solar cell)

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/

emspectrum.html