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35 Cards in this Set
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- 3rd side (hint)
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Light rays |
The ray approximation: a wave travels through a uniform medium in straight lines in the direction of the rays. |
If the waves encounter an object of dimensions d, we use the assumption that d >> λ so we can use the ray approximation |
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Refraction |
● no refraction takes place if incident light ray is along the normal At the boundary between 2 transparent substances: ● light ray slows down and bends towards normal if it passes into more optically dense material (higher refractive index) ● light ray speeds up and bends away from normal if it passes into less optically dense material (lower refractive index) |
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Change in wave speed in a medium |
• As photons move through a medium, they're absorbed and re emitted by electrons. Optically denser media take a longer time to re emit photons, so the speed of light is slower • for longitudinal waves, wave speed increases as density increases since more particles are able to transfer the energy |
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Refractive index |
Ratio of speed of light in vacuum to speed of light in medium |
n = c/v |
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Snell's law |
n1sinθ1 = n2sinθ2 Also sinθ1/sinθ2 = c1/c2 |
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Critical angle |
Angle of incidence at which angle of refraction is 90° and refracted light ray emerges along the boundary |
sinθc = n2/n1 |
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Total internal reflection |
When a light ray is completely reflected at a boundary back into the incident medium Only occurs if: ▪light ray enters a less optically dense medium with a lower refractive index ▪angle of incidence exceeds critical angle |
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Partial reflections |
When a light ray reflects off the boundary to a more optically dense material with a higher refractive index |
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Partial internal reflection |
When a light ray reflects off the boundary to a less optically dense material with a lower refractive index, and when angle of incidence <= to critical angle |
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Optical fibres |
Each time the light ray reaches the fibre boundary, the angle of incidence exceeds the critical angle so the light ray is totally internal reflected |
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Communications optical fibres |
Pulses of light enter from a transmitter at one end and reach a receiver at other end. Each fibre consists of a core surrounded by a layer of cladding. ■ cladding has lower refractive index than core so total internal reflection occurs at core-cladding boundary, reducing light loss ■ narrow core ensures that the light ray always hits the core-cladding boundary at an angle greater than the critical angle, so all the light is totally internally reflected |
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Medical endoscopes |
Consist of 2 bundles of fibres: one to send light to illuminate the body cavity; the other to send an image of the body cavity back to an observer. This second fibre bundle needs to be a coherent bundle, so that the fibre ends at each end are in the same relative positions. |
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Absorption |
■ optical fibres needs to be highly transparent to minimise absorption of light, which would result in energy loss and reduced amplitude of pulses as they travel down the fibre |
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Dispersion (optical fibres) |
● Modal dispersion - light rays enter the fibre at different angles, so they take different paths of different lengths, so take different amounts of time to reach the other end of the fibre ● Material dispersion - different wavelengths of light travel through the fibre at different speeds, so take different amounts of time to reach the other end Both lead to pulse broadening |
Use of monochromatic light (light of a single wavelength) prevents material dispersion |
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Pulse broadening |
Pulses sent down the fibre are broader at the other end, so can overlap and merge resulting in signal degradation and loss of information |
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Light through a prism |
The speed of light in a material depends on the wavelength, so different wavelengths of light are refracted by different amounts when they enter a glass prism, causing light to disperse Shorter wavelength in air means light travels slower in the material so is refracted by a greater amount (towards the normal) = greater refractive index for that wavelength of light |
Red = 650nm Violet = 350nm |
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Nature of electromagnetic radiation |
■ Wave theory of light was first suggested by Huygens but was rejected in favour of Newton's corpuscular theory ■ Newton thought light was composed of tiny particles ■ Young's double slit experiment demonstrated interference and the wave nature of light |
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Double slit experiment |
The use of two coherent sources of waves or the use of a single source passed though a single slit to illuminate double slits, produces an interference pattern - alternating bright and dark fringes, evenly spaced out |
2 loudspeakers connected to the same signal generator are coherent sources of sound waves |
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The interference pattern |
● formation of bright fringes: waves arrive in phase with each other and reinforcement occurs = constructive interference ● formation of dark fringes: waves arrive 180° out of phase and cancellation occurs = destructive inference w = λD/s The fringes become more widely spaced as D and λ increase, and as s decreases Only works if w is much smaller than D |
Fringe spacing is distance between centres of adjacent bright fringes |
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Path difference |
Difference in the distance travelled by 2 waves meeting at a point A path difference of nλ: ▪constructive interference ▪reinforcement ▪waves arrive in phase A path difference of (n+0.5)λ: ▪destructive interference ▪cancellation ▪waves arrive 180° out of phase |
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Diffraction pattern |
Diffraction pattern consists of a central bright fringe with further fringes either side. Central bright fringe is twice as wide as other fringes and highest intensity. Peak intensity of each fringe decreases with distance from the central fringe. Each outer fringe is the same width. |
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Single slit diffraction |
The width of the central diffraction maximum increases as wavelength increases and slit width decreases |
A wider gap results in less diffraction, so less diffraction occurs through a telescope than through the pupil of the eye |
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Single slit diffraction and Young's double slits |
Interference can only occur where the waves of the 2 slits overlap. • each slit must be narrow enough to cause sufficient diffraction • the slits must be close enough so the diffracted waves overlap |
- Fringe spacing of inference pattern: w=λD/s - Width of central diffraction maximum: W=2λD/a |
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Diffraction grating |
Diffraction grating consists of many closely spaced parallel slits. When monochromatic light is directed at normal incidence to a grating, it diffracts through each slit and interference occurs between light from adjacent slits, creating points of reinforcement and cancellation. Central fringe (0 order) is along the incident light direction |
dsinθ = nλ no. of slits per metre = 1/d Angle of diffraction increases as λ increases and d decreases (no. of slits per metre increases) Points of minimum intensity in the single slit diffraction pattern will suppress certain orders in the diffraction grating pattern |
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Applications of diffraction gratings |
Used in spectrometers to study light spectra and measure wavelengths of light from a source by accurately measuring the angle of diffraction of a diffracted light beam. |
Slit width must be smaller than grating spacing to increase the amount of diffraction so diffracted waves overlap |
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Continuous spectrum |
Continuous spectrum of light emitted from a filament lamp. The hotter the light source, the shorter the wavelength of the most intense part of the spectrum. |
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Line emission spectra |
A glowing gas in a light source emits light of specific wavelengths, so it's spectrum consists of narrow lines of different colours. Each element has a unique characteristic spectrum from which it can be identified. |
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Line absorption spectra |
A continuous spectrum with narrow dark lines at specific wavelengths. When light is passed through a glowing gas, the elements of the gas absorb light of the same wavelength as the light they emit |
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Reflection |
Angle of incidence = angle of reflection. Specular reflection = reflection off a smooth surface (if surface imperfections < λ) Diffuse reflection = reflection off a rough surface scatters rays in random directions |
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Huygens's principle |
All points on a given wave front are taken as point sources for the production of spherical secondary waves, called wavelets, that propagate outward through a medium with speeds characteristic of waves in that medium. After some time interval has passed, the new position of the wave front is the surface tangent to the wavelets. |
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Dispersion |
For a given material, the refractive index varies with the wavelength of the light passing through it, and thus the light rays will refract through different angles upon entering. |
n is inversely proportional to λ |
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Image formation |
▪Object distance, p = distance to object ▪Image distance, q = distance to image ▪Real image: light rays pass through and diverge from the image point; can be displayed on a screen ▪Virtual image: light rays do not pass through the image point but only appear to diverge from that point; can't be displayed on a screen ▪Lateral magnification, M = image height / object height ▪1/p + 1/q = 1/f ▪Focal point: point at which rays of light parallel to the principal axis converge or appear to diverge from ▪Focal length, f = distance to focal point |
Images can always be located by extending diverging rays back to a point at which they intersect. |
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Flat mirrors |
■ Always produce a virtual image ■ Image distance = object distance ■ M = +1 (Image is the same size as object and upright) ■ Image formed appears to be reversed |
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Concave mirrors |
▪focuses incoming parallel rays to a focal point, f = R/2 ▪light originating from a point source along the principal axis, further than R from the mirror, reflect off the mirror and pass through the image point ▪In the paraxial ray approximation, light rays make small angles with the principal axis. ▪produce a real image |
Rays that make large angles with the principal axis converge to other points on the principal axis, producing a blurred image - spherical aberration |
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Convex mirror |
▪The image formed by the object is virtual, upright, diminished and behind the mirror. ▪rays from any point on an object diverge after reflection as though they were coming from some point behind the mirror. ▪f < 0 |
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