# Shared Flashcard Set

## Details

Optics
N/A
78
Physics
Not Applicable
10/07/2012

Term
 REFRACTION OF LIGHT
Definition
 Refraction is the change of direction that occurs when light passes across a boundry between two transparent substances. e.g air and a glass block. Light rays bend towards the normal from a less dense subastance to a more dense substance and away from the normal from a more dense substance to a less dense substance
Term
 INCIDENT LIGHT RAY ALONG THE NORMAL
Definition
 Causes no refraction
Term
 AT A BOUNDRY BETWEEN TWO TRANSPARENT SUBSTANCES
Definition
 Light rays bend towards the normal into a more refractive substance and away from the normal into a less refractive substance
Term
 ANGLE OF INCIDENCE
Definition
 The angle between the incident light ray and the normal at the point of incidence   The angle or refraction at a certain point is always less than the angle of incidence at that same point
Term
 SNELL'S LAW
Definition
 Ratio of sini and sinr - same for each light ray  the ratio gives the refractive index , n,      n = sin i / sin r
Term
 PARTIAL REFELCTION
Definition
 some light passes through (refraction) and some light bounces back (reflection)
Term
 HOW DOES REFRACTION OCCUR?
Definition
 The speed of the light waves is different in each substance.   Wave fronts are closer together in denser substances, because light travels slower in denser substances.   Slowing down, causes the rays to change direction
Term
 SPEED AND REFRACTIVE INDEX
Definition
 The smaller the speed of light in the substance, the greater the refractive index
Term
 FREQUENCY
Definition
 Frequency of waves don't change when refraction occurs
Term
 i
Definition
 Gives the angle between the incident ray and the normal
Term
 r
Definition
 The angle between the refracted ray and the normal
Term
 WHEN A LIGHT RAY PASSES FROM A VACUUM TO A TRANSPARANT SUBSTANCE
Definition
 n = the speed of light in a vacuum / the speed of light in the transparant substance
Term
 SINE AND SPEED
Definition
 sin i / sin r = c / cs s = Speed in the substance
Term
 n IN SUBSTANCE WITH SPEEDS OF LIGHT
Definition
 ns = c / cs = λ / λs
Term
 DURING REFRACTION
Definition
 Speed and wavelength both change, frequency remains constant
Term
 THE WHITE LIGHT SPECTRUM
Definition
 Prism splits white light into colours of spectrum. White light is made up of light from lots of wavelengths. Prism refreacts light by different amounts - shorter wavelength in air means greater refraction. Each colour in white light spectrum is refracted by different amounts. Speed of light depends on wavelength. Violet, shorter wavelength so lower speed in prism, greater refractive index
Term
 CRITICAL ANGLE
Definition
 The largest angle at which the refraction out of a denser medium is just possible
Term
 CALCULATING THE CRITICAL ANGLE
Definition
 Use snells law. n = sin i / sin r
Term
 AIR
Definition
 Almost as optically active as a vacuum so the absoloute refractive index is used, in form of n = 1 / sin c
Term
 TOTAL INTERNAL REFLECTION
Definition
 If angle of incidence exceeds the critical angle, total internal reflection occurs.    It is a light ray that is internally reflected at a boundry with a substance of lower refractive index if it's angle of incidence exceeds the critical angle
Term
 TOTAL INTERNAL REFLECTION CAN ONLY TAKE PLACE IF:
Definition
 The incidence substance has a larger refractive index than the other substance   If the angle of incidence exceeds the critical angle
Term
 AT THE CRITICAL ANGLE
Definition
 The angle of refraction is 90° to the normal because it occurs at the boundry.   sin90 = 1 so sin i / 1 = n2 / n1  =   sin i = n2/n1
Term
 WHY DO DIAMONDS SPARKLE?
Definition
 Diamonds have a very high refractive index. Low critical angle Light ray in a diamond may be totally internally reflected many times before emerging.
Term
 OPTICAL FIBRES
Definition
 The angle at which the light rays meet the normal at the surface is much greater than the critical angle so the ray continues down the fibre with total internal reflection. Light ray is totally internally reflected each time it reaches a fibre boundry, even if the fibre bends unless bend is too small.  At each point the light ray reaches the boundry, the angle of incidence exceeds the critical angle
Term
 COMMUNICATIONS OPTICAL FIBRES
Definition
 Light that enters from one end of the transmitter to reach the other end which is the receiver. Highly transparent so minimise absorption of light - surrounded in cladding of a lower refractive index
Term
Definition
 Total internal reflection takes place at the core cladding boundry - at any point where the two fibres are in contact, light would cross from on fibre to the other if there isn't cladding so signals wouldn't be secure and would reach the wrong destination
Term
 WHY IS THE CORE CLADDING NARROW?
Definition
 To prevent multipath dispersion. Occurs in wide core - light along axis of core travels shorter distance per metre of fibre thatn light which undergoes total internal reflection.   Pluse of light in wide core, would become longer and could merge with next pulse
Term
 PULSE DISPERSION
Definition
 In a wide core and also if white light is used instead of monochromatic light (light of a single wavelength or small range) This is spectral dispersion because speed of light in glass depends on it's wavelength. Violet more slowly than red so the difference in speed would cause the the white ligth waves to become longer so the pulses would merge
Term
 MEDICAL ENDOSCOPE
Definition
 Two bundles of fibres - insterted into body. Illuminated using light sent throught one of the fibre bundles  - lens on the other ened of the fibre bundle is used to form an image of the body part. Light that forms this image travels along the fibres to the other fibre bundle where the image can be observed. Bundle must be coherent (fibres on both ends must be relatively in the same position)
Term
 DOUBLE SLIT INTERFERENCE
Definition
 To observe interference of light, light up two closely spaced parallel slits using a suitable light source
Term
 COHERENT SOURCES OF WAVES
Definition
 Two slits act as this so they emit light waves with a constant phase difference and the same frequency
Term
 YOUNGS FRINGES
Definition
 Alternate bright and dark fringes, can be seen on a white screen places where the diffracted light from the double slits overlaps. Fringes evenly spaced and parallel to the double slits
Term
 IF SLIT IS TOO WIDE
Definition
 Each part that produces a fringe pattern - displaced slightly from pattern because of the adjacent parts of a single slit. So dark fringes become narrower than bright so contrast is lost
Term
 WHAT IS USED TO LIGHT UP YOUNGS DOUBLE SLIT EXPERIMENT?
Definition
 He used a candle We use a light bulb  A low power laser beam can also be used instead of bulb and slit
Term
 WHY ARE THE FRINGES FORMED?
Definition
 Due to the interference of light from the two slits
Term
 WHERE A BRIGHT FRINGE IS FORMED
Definition
 Light from one slit reinforces light from the other slit so the light waves arrive in phase with each other
Term
 WHERE A DARK FRINGE IS FORMED
Definition
 The light from one slit cancels out the light from the other slit so the light waves from the two slits are 180° out of phase
Term
 FRINGE SEPERATION
Definition
 The distance from the centre of a bright frigne to the centre of the next bright fringe is fringe seperation. This depends on slit spacing and distance from slits to screen.   Fringe spacing = Wave length of light x distance from slit to screen / slit spacing   w = λD/s
Term
 HOW DO THE FRINGES BECOME MORE WIDELY SPACED?
Definition
 If distance from slit to screen is increased Wavelength is increased Slit spacing is reduced
Term
 PATH DIFFERENCE
Definition
 The difference in distance between two coherent sources to an interference fringe
Term
 COHERENCE
Definition
 E.g double slits - emit light waves with constant phase difference
Term
 IF NON LASER LIGHT IS USED
Definition
 Each wave crest or trough from a single slit always passes through one double slit, before the other so the double slits have constant phase difference
Term
 WHAT HAPPENS TO STRAIGHT WAVES FROM THE BEAM?
Definition
 The diffract after passing throught the slits.
Term
 INTERFERENCE
Definition
 Where the diffracted waves overlap
Term
 WHAT HAPPENS IF ONE GAP IS CLOSER TO THE BEAM THAN THE OTHER
Definition
 Each wavefront from the beam passes through the nearer gap first. Time intervals are the same so there is constant phase difference
Term
 LIGHT FROM LIGHT BULBS
Definition
 Can't form interference patterns - emit random wave lengths (white light) so cancellation and reinforcement would change randomly therefore no interference is possible
Term
 DO TWO SOURCES HAVE TO BE IN PHASE TO BE COHERENT?
Definition
 No, they can also be in constant phase difference
Term
 IN THE DOUBLE SLIT EXPERIMENT
Definition
 Fringe seperation depends on colour of light Each colour has different wavelength.    Fringe seperation is greater for longer wavelength (red) than shorter wavelength (blue)    The longer the wavelength used, the greater the fringe seperation
Term
 VAPOUR LAMPS AND DISCHARGE TUBES
Definition
 Light with dominant colour (but still produce some other wavelengths too)    Dominant colour is more intense   In effect monochromatic light source because spectrum is dominated bt certain light
Term
 Light from filament lamp or sun
Definition
 Different colours of spectrum   Continuous range of wavelengths   If this light is directed to a fliter of a certain colour, the light from the fliter is a certain colour because the range of wavelengths is much narrower
Term
 HOW DOES LIGHT FROM A LASER DIFFER WITH VAPOUR LAMPS AND FILAMENT BULBS   1
Definition
 Highly monochromatic - specify wavelength to nm. Perfectly parallel and monochromatic so convex lens can focus it to a fine spot. beam power can be concentrated in a very small area.    Eye lens can focus laser on a tiny spot in retina. Intense concentration would destroy retina.
Term
 HOW DOES LIGHT FROM A LASER DIFFER WITH VAPOUR LAMPS AND FILAMENT BULBS   2
Definition
 A laser is a convenient source of coherent light   Can illuminate double slits directly
Term
 HOW DOES A LASER WORK?
Definition
 Photons emitted, causes more photons to be emitted as it passes through a light emitting substance. The stimulated photons are in phase with the photons that caused them so the laser is coherent.
Term
 HOW DOES A LIGHT SOURCE EMIT LIGHT?
Definition
 Electrons inside it's atoms, moving to lower energy levels inside the atom. Each electron emits a photon.
Term
 WHITE LIGHT FRINGES
Definition
 Each component colour of white light produces it's own fringe pattern. Each pattern centered on the screen at the same position. Central fringe is white ecasue every colour contributes to the centre. Inner fringes are bule and red on outer side. Red are more spaced out than blue. Outer fringes merge into white light. They become more faint as the distance from the centre increases because where different colours overlap and reinforce
Term
 DIFFRACTION
Definition
 Spreading of waves when they pass through a gap or edge. Important in optical instruments and microscopes and cameras.  Less diffraction occurs through a wide gap than narrow gap. Our eye pupil is narrow, the telescope lens is wide so less diffraction.
Term
 DIFFRACTION OF WATER WAVES
Definition
 Waves spread out more when the gap is made narrower or wavelength is larger
Term
 WHY DO THE WAVE FRONTS HAVE BREAKS EITHER SIDE OF THE CENTRE?
Definition
 Because, the waves are diffreacted by adjacent sections on the gap being out of phase and cancelling each other out
Term
 Diffraction of light by a single slit
Definition
 Can be demonstrated by directing a single parallel light beam at the slit. Diffracted light forms pattern that can be observed on a white screen. Central fringe with other fringes on either side. Intensity greatest at centre of central fringe.
Term
 FRINGES
Definition
 Central fringe is twice as wide as each outer fringe Peak intensity of each fringe decreases with distance from centre each outer fringe is the same width outer fringes are less intense than central
Term
 IF THE SINGLE SLIT PATTERN IS OBSERVED USING DIFFERENT SOURCES OF MONOCHROMATIC LIGHT
Definition
 Observations show, greater the wavelength, wider the fringe
Term
 IF SINGLE SLIT PATTERN IS OBSERVED USING AN ADJUSTABLE SLIT
Definition
 Observations show that making the slit narrower makes the fringes wider
Term
 WIDTH OF CENTRAL FRINGE
Definition
 W = (wavelength of light(λ) / width of single slit(a)) x (2 X Distance from slit)   so width of fringe is proportional to λ/a
Term
 MICROSCOPES AND DIFFRACTION
Definition
 Fitted with blue filter as better resolution with blue light than white Electron microscope resolution increases with voltageas speed of electrons increases Greater speed of electrons, smaller de Broglie's wavelength so less diffraction of electrons as passed through magnetic field
Term
 CONDITIONS FOR INTERFERENCE
Definition
 Each slit must be narrow enough to diffract light passing through Two slits must be close enough so diffracted waves overlap on screen
Term
 DIFFRACTION GRATING
Definition
 Plate with many parallel slits close together. Beam of monochromatic light directed on it, light transmitted in certain directions because:  Light passing through each slit is diffracted, and diffracted light from adjacent slits reinforce and cancel out
Term
 ZERO ORDER BEAM
Definition
 Central beam - same direction as incident beam. Other transmitted beams are numbered outwards from zero order beam. Angle of diffraction between each beam and central beam increases if light of longer wavelength is used of if a grating with closer slits is used
Term
 DIFFRACTION GRATING EQUATION
Definition
 Distance of two points x sinθ = nth order beam x wavelength   dsinθ = nλ
Term
 NUMBER OF SLITS ON GRATING
Definition
 Number of slits per metre on the grating N = 1/d (d is the grating spacing)
Term
 FOR A GIVEN ORDER AND WAVELENGTH
Definition
 Smaller the value of d, the greater the angle of diffraction so the larger the number of slits per metre, the bigger the angle of diffraction
Term
 FRACTIONS OF A DEGREE
Definition
 Expressed in decimals or minutes   1° = 60'
Term
 MAXIMUM NUMBER OF ORDERS
Definition
 Given by the value of d/λ rounded down to the nearest whole number
Term
 WHERE ARE DIFFRACTION GRATINGS USED?
Definition
 Spectrometer - studies the spectrum of light Measures the light wavelengths very accurately  - measures angles to 1 arc minute (1/60 of a degree)  Spectrum analyser - electronic spectrometer linked to a computer gives display of variation of intensity of the wavelengths
Term
 CONTINUOUS SPECTRA
Definition
 Spectrum of light from a filament lamp has a continuous spectrum. Most intense part depends on temperature of light source, hotter the shorter the wave length of brightest part. By measuring wavelength of brightest part we can measure the temperature of the light source.
Term
 LINE EMISSION SPECTRA
Definition
 Wavelength of the lines are characteristics of the chemical elements that produces the light.  If a glowing gas contains more than one element, the elements can be identified using the line spectra
Term
 LINE ABSORPTION SPECTRA
Definition
 Continuous spectrum with narrow dark lines at certain wavelenghts. e.g filament light passed through glowing gas. Dark lines super imposed on cont. spectrum.
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