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Ch 27 Light Notes
Mrs. Peck
Light is the only thing we see when light hits mat. it can be absorbed, reflected, reemitted (transmitted), refracted What we see: made visible by light reflected from sources Sources of Light: primary (1°) is the sun secondary (2°) is brightness of sky other sources: flames, white-hot filaments of lamps, glowing gases in tubes Dual Nature of Light- particle-wave theory: light can act as either particles or waves particle theory- supported by fact that lt moves in straight line instead of spreading out like waves light consists of massless bundles of concentrated electromagnetic E photon- a particle that travels only at speed of light and whose E is related to the frequency of the radiation in the wave model Production of light: an EM (electromagnetic) wave prod. by vibration of electrically charged particle wh/ releases E in form of radiation 1. Electrons in atoms gain E through vibration 2. electron gains E and jumps to higher energy level (shell) 3. this position is unstable and electron will return to its original position (shell)...releasing E as it goes back to lower level 4. energy released in a tiny packet called a photon 5. movement of electrons back and forth between E levels creates a “stream of photons” which is an EM wave that carries the E released by electrons (radiation) wave theory- light is type of E that travels as EM transverse wave does not need a medium to travel energy in EM wave is partly electric & partly magnetic EM wave prod. by vibration of electrically charged particle 1. particle with electric charge set into vibration 2. the electric field around particle vibrate too 3. when electric field vibrates---this creates a magnetic field 4. vibration of an electric and magnetic field creates EM waves that hold energy release by original vibration of the particle
EM Wave
Speed of Light (c) in a vacuum is a constant 300,000 km/ s or 187,500 mi/ s light-year- distance it takes for light to travel in one year
Electromagnetic Wave- (EM) transverse wave: partly electric & magnetic and carries energy emitted by vibrating electrons Electromagnetic Spectrum- range of entire EM waves from radio waves to gamma rays arranged from longest wavelength (lowest freq-E) to shortest wavelength (highest freq-E)
Radio waves: longest wavelengths, lowest frequency, can convert radio wave into electric energy wh/ is converted into sound waves TV and radio stations broadcast radio waves Microwaves: device in microwave called magnetron produces microwaves by accelerating charged particles. Microwaves reflected off of metal fan into cooking area. Energy of microwave vibrates water molecules in food .... increasing temp of food. Radar: “radio detection and ranging” used to detect speed and location of obj device emits pulses of microwaves wh/ are reflected by object Infrared Waves: EM waves lower than the red of visible light IR waves absorbed by obj. causing particles in obj to vibrate faster, therefore, increasing temp of obj sun not only source of IR waves: stars, planets, buildings, organisms Visible Light: very narrow range in EM spectrum that humans can see visible spectrum- range of colors of light ROYGBIV Ultraviolet Ray- EM waves higher than violet light pro: used to kill bacteria on food & surgical instruments, produces vit. D in skin when exposed to UV rays con: overexposure to UV causes sunburn, skin cancer, eye damage,wrinkles X-rays- easily penetrate a variety of materials, have high frequency therefore, high energy. Used in medical field, too much exposure kills cells therefore, use lead aprons to protect body Gamma Rays- highest freq, carry large amts of energy, can easily penetrate mat. Created by radioactive atoms or nuclear explosions. Used to treat some forms of cancer
if light is incident on material it can behave in 4 ways: 3 1. reflected- incident wave turns back when encounters edge of a medium 2. absorbed- by material because incident wave frequency matches natural freq. of medium 3. refracted- transmitted thru mat. - but at a slower speed inside mat. so...lt. gets bent 4. transmitted- transmitted thru mat. at original speed (no refracting-bending)
light absorbed & reflected
light absorbed & transmitted
Light is an incident wave that hits mat.--causes electrons in mat to vib. Frequency visible light = 1014 Hz
Transparent- medium that allows light to pass thru in straight lines
frequency of reemitted (transmitted) light is
=
to frequency of incident light
air, glass, & water are transparent to visible light
atmosphere- transparent: visible light and some infrared (high frequency) glass: Visible Lt.- EM waves have lower freq. than UV (nat freq. of glass electrons). Set electrons to vib. with lower amplitudes. Atoms hold E for less time, decrease chance of collisions, decrease loss of E as heat. Therefore, E is reemitted as visible light in straight lines (not blurry) glass is transparent to all freq. of visible light. Glass atoms transmit vibrations from atom to atom till vibration reaches other side of glass. Electrons transmit an EM wave with the same frequency as that of the incident light wave.
water- transparent to visible light (except red....red gets absorbed)
4 Opaque- medium that reflect and/ or absorb light w/ o reemission, and consequently do not allow light thru them waveʼs frequency matches the natural frequency of medium 1. 2. 3. 4. 5.
wave absorbed has same frequency as Natural Frequency of electrons of the medium resonance occurs-- increasing amplitude of vibrating electrons electrons hold onto this energy for a longer time and begin to vibrate adjacent atoms turned into random KE (translational KE) --increase Int. E--increase Temp of medium no energy is transmitted from medium
glass: electrons in glass have nat. freq. in UV range., when hit by UV electrons vib at increase amplitude: resonance. Increase in vib energy of electrons given off to neighboring atoms in glass in form of heat. The energy does not get reemitted from glass as UV but is transformed into heat. Therefore, glass is not transparent to UV rays. req. lower than visible lt, vib. both electrons as well as all particle of transparent mat. increasing the int. E---increasing the temp.
metals- opaque, reemitted light seen as a reflection outer electrons not bound to any particular atom; therefore, light hits them-sets them in vib--their E does not spring from atom to atom but is reemitted as visible light....seen as a reflection.....thatʼs why metals are shiny
atmosphere- almost opaque: UV rays....small amount gets thru opaque to longer IR waves clouds- semitransparent to UV....can get sunburned on cloudy day
water- reflects UV rays absorbs IR rays........IR wave freq. is the natural frequency of water molecules absorbs red visible light too....(after ~ 15m red is absorbed)....if have a red object below 15-30m it will appear to be black for their is no red light available to shine on it and be reflected to your eye
Translucent- mat. that transmits light or reemits light but it is scattered (not straight line) as it passes thru mat. therefore you can only see blurry images eg. frosted glass, wax paper
5
MEDIA
TRANSPARENT
OPAQUE
atmosphere
glass
water
ray- a thin beam of light Shadows- a shaded region that results when light falls on obj. & thus canʼt reach into region on the far side of the obj. two types of shadows: 1. umbra 2. penumbra
Shadows
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umbra- the darker part of a shadow where all the light is blocked sharp shadow penumbra- a partial shadow that appears where light from part of the source is blocked & light from another part of the source is not blocked blurry edged shadow 1. light from broad source partially blocked 2. light from different direction fills in edges of shadow solar eclipse- moonʼs shadow on small area of Earth moon b/w sun and Earth........shadow on Earth: center is umbra with edges-penumbra shadowed
lunar eclipse- Earthʼs shadow is on the moon Earth b/w sun and moon.....Earthʼs shadow is on the moon
polarization-aligning of vib. in a transverse wave, usually by filtering out waves of other directions most light sources electrons vibrate in random directions. Net resultant light is horizontal & vertical.
polaroid filter: consists of long molecules that line up like parallel slits, only light waves that are vibrating in same direction as slits will pass thru. polarized light consists of light waves that vibrate in only one direction polarized sunglasses- slits aligned vertically
(yellow ? p 417) filter out horizontal light (reduces glare from water, cars ect) light that passes thru is vertically polarized (only 50% of incident light passes thru)
*glare from water and cars is polarized horizontally so use vertical polarized lenses to filter glare out 3D Movies: - movie filmed by 2 cameras short distance apart (avg. eye spacing) - use 2 film projectors with polarized filters in front of them at rt. angles (vert. & horiz.) -wear 3D glasses w/ each lens polarized at rt angles just like projector (vert. & horiz.) -left eye can only see left projected image & right eye can only see left projected image -each eye sees a separate image from slightly diff. angle....the brain interprets the 2 images as a single image w/ a feeling of depth (binocular vision)
fig 27.21
