Term
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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 |
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Term
INCIDENT LIGHT RAY ALONG THE NORMAL |
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Definition
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Term
AT A BOUNDRY BETWEEN TWO TRANSPARENT SUBSTANCES |
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Definition
| Light rays bend towards the normal into a more refractive substance and away from the normal into a less refractive substance |
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Term
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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
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Term
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Definition
Ratio of sini and sinr - same for each light ray
the ratio gives the refractive index , n,
n = sin i / sin r |
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Term
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Definition
| some light passes through (refraction) and some light bounces back (reflection) |
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Term
HOW DOES REFRACTION OCCUR? |
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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 |
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Term
SPEED AND REFRACTIVE INDEX |
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Definition
| The smaller the speed of light in the substance, the greater the refractive index |
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Term
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Definition
| Frequency of waves don't change when refraction occurs |
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Term
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Definition
| Gives the angle between the incident ray and the normal |
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Term
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Definition
| The angle between the refracted ray and the normal |
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Term
WHEN A LIGHT RAY PASSES FROM A VACUUM TO A TRANSPARANT SUBSTANCE |
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Definition
| n = the speed of light in a vacuum / the speed of light in the transparant substance |
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Term
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Definition
sin i / sin r = c / cs
s = Speed in the substance |
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Term
n IN SUBSTANCE WITH SPEEDS OF LIGHT |
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Definition
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Term
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Definition
| Speed and wavelength both change, frequency remains constant |
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Term
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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 |
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Term
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Definition
| The largest angle at which the refraction out of a denser medium is just possible |
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Term
CALCULATING THE CRITICAL ANGLE |
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Definition
| Use snells law. n = sin i / sin r |
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Term
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Definition
| Almost as optically active as a vacuum so the absoloute refractive index is used, in form of n = 1 / sin c |
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Term
TOTAL INTERNAL REFLECTION |
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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 |
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Term
TOTAL INTERNAL REFLECTION CAN ONLY
TAKE PLACE IF: |
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Definition
The incidence substance has a larger refractive index than the other substance
If the angle of incidence exceeds the critical angle |
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Term
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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 |
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Term
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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.
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Term
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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
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Term
COMMUNICATIONS OPTICAL FIBRES |
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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 |
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Term
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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 |
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Term
WHY IS THE CORE CLADDING NARROW? |
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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 |
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Term
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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 |
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Term
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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) |
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Term
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Definition
To observe interference of light, light up two closely spaced parallel slits using a suitable light source
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Term
COHERENT SOURCES OF WAVES |
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Definition
| Two slits act as this so they emit light waves with a constant phase difference and the same frequency |
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Term
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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 |
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Term
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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 |
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Term
WHAT IS USED TO LIGHT UP YOUNGS DOUBLE SLIT EXPERIMENT? |
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Definition
He used a candle
We use a light bulb
A low power laser beam can also be used instead of bulb and slit |
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Term
WHY ARE THE FRINGES FORMED? |
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Definition
| Due to the interference of light from the two slits |
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Term
WHERE A BRIGHT FRINGE IS FORMED |
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Definition
| Light from one slit reinforces light from the other slit so the light waves arrive in phase with each other |
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Term
WHERE A DARK FRINGE IS FORMED |
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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 |
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Term
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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 |
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Term
HOW DO THE FRINGES BECOME MORE WIDELY SPACED? |
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Definition
If distance from slit to screen is increased
Wavelength is increased
Slit spacing is reduced |
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Term
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Definition
| The difference in distance between two coherent sources to an interference fringe |
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Term
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Definition
| E.g double slits - emit light waves with constant phase difference |
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Term
IF NON LASER LIGHT IS USED |
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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 |
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Term
WHAT HAPPENS TO STRAIGHT WAVES FROM THE BEAM? |
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Definition
| The diffract after passing throught the slits. |
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Term
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Definition
| Where the diffracted waves overlap |
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Term
WHAT HAPPENS IF ONE GAP IS CLOSER TO THE BEAM THAN THE OTHER |
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Definition
| Each wavefront from the beam passes through the nearer gap first. Time intervals are the same so there is constant phase difference |
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Term
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Definition
| Can't form interference patterns - emit random wave lengths (white light) so cancellation and reinforcement would change randomly therefore no interference is possible |
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Term
DO TWO SOURCES HAVE TO BE IN PHASE TO BE COHERENT? |
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Definition
| No, they can also be in constant phase difference |
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Term
IN THE DOUBLE SLIT EXPERIMENT |
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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 |
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Term
VAPOUR LAMPS AND DISCHARGE TUBES |
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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 |
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Term
Light from filament lamp or sun |
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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 |
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Term
HOW DOES LIGHT FROM A LASER DIFFER WITH VAPOUR LAMPS AND FILAMENT BULBS
1 |
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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. |
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Term
HOW DOES LIGHT FROM A LASER DIFFER WITH VAPOUR LAMPS AND FILAMENT BULBS
2 |
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Definition
A laser is a convenient source of coherent light
Can illuminate double slits directly
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Term
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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. |
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Term
HOW DOES A LIGHT SOURCE EMIT LIGHT? |
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Definition
| Electrons inside it's atoms, moving to lower energy levels inside the atom. Each electron emits a photon. |
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Term
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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 |
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Term
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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. |
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Term
DIFFRACTION OF WATER WAVES |
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Definition
| Waves spread out more when the gap is made narrower or wavelength is larger |
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Term
WHY DO THE WAVE FRONTS HAVE BREAKS EITHER SIDE OF THE CENTRE? |
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Definition
| Because, the waves are diffreacted by adjacent sections on the gap being out of phase and cancelling each other out |
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Term
Diffraction of light by a single slit |
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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. |
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Term
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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 |
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Term
IF THE SINGLE SLIT PATTERN IS OBSERVED USING DIFFERENT SOURCES OF MONOCHROMATIC LIGHT |
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Definition
| Observations show, greater the wavelength, wider the fringe |
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Term
IF SINGLE SLIT PATTERN IS OBSERVED USING AN ADJUSTABLE SLIT |
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Definition
| Observations show that making the slit narrower makes the fringes wider |
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Term
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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
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Term
MICROSCOPES AND DIFFRACTION |
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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 |
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Term
CONDITIONS FOR INTERFERENCE |
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Definition
Each slit must be narrow enough to diffract light passing through
Two slits must be close enough so diffracted waves overlap on screen |
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Term
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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 |
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Term
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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 |
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Term
DIFFRACTION GRATING EQUATION |
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Definition
Distance of two points x sinθ = nth order beam x wavelength
dsinθ = nλ |
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Term
NUMBER OF SLITS ON GRATING |
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Definition
| Number of slits per metre on the grating N = 1/d (d is the grating spacing) |
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Term
FOR A GIVEN ORDER AND WAVELENGTH |
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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 |
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Term
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Definition
Expressed in decimals or minutes
1° = 60' |
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Term
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Definition
| Given by the value of d/λ rounded down to the nearest whole number |
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Term
WHERE ARE DIFFRACTION GRATINGS USED? |
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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 |
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Term
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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. |
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Term
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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 |
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Term
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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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