The Nature Of Light PDF

The Nature Of Light Visible light is Electromagnetic Radiation that is detectable by the human eye. EMR is energy propagation by periodic variation of the electric E, and magnetic field B, strengths caused by the acceleration of charged particles. Fig 4.1 These are not mechanical waves, but the...

The Nature Of Light Visible light is Electromagnetic Radiation that is detectable by the human eye. EMR is energy propagation by periodic variation of the electric E, and magnetic field B, strengths caused by the acceleration of charged particles. Fig 4.1 These are not mechanical waves, but they display similar behaviour, and are able to travel through vacuum. The speed of EMR propagation c, commonly called the speed of light is 3 x 108 m/s in a vacuum. (300,000 km/s or 186,000 mph.) Visible light, (often called white light) actually comprises of all of the EMR between 400 and 700 nm. (nanometres.) That is, between blue and red in the spectrum. Fig 4.2 The sense of sight is due to the fact our EM detectors (eyes) pick up the energy reflected from objects and transmit it to the brain. Page 88 of 114 The colour we see depends on the source of light and the atomic structure of the material reflecting it. Electrons either absorb the energy or re- radiate it, (scatter or reflect.) As an example: we see grass as being green because it uses Chlorophyll to change light into energy. Chlorophyll absorbs the blue and red colors of the spectrum and reflects the green. The green is reflected back out to the viewer. The sky is blue because atmospheric dust absorbs all the energies except blue. Anything that is black, absorbs all colours of light, so reflect no colours at all. White objects do not absorb any light so just reflect back the incoming light. So if you were to shine a blue light on a white object, the white object would appear blue. You may have experienced this when sitting in the sun. If you are wearing a white shirt, the shirt will remain relatively cool to the touch. However, if you are wearing a black shirt, you will find that the shirt feels relatively hot to the touch. Black absorbs all the light energies and frequencies of the spectrum whereas white does not. Wave or Particle? Light is assumed to be wave-based, but there is also evidence that light is composed of particles with mass. (photons). Some of the evidence that light is composed of particles is:  Light is affected by gravity -- it is bent around large planets, so must have mass.  Light exerts a force, \- light from the sun causes the deflection of comet tails;  Light can generate a photoelectric effect. Some of the evidence that light is composed of waves is:  Light can be reflected and refracted;  Light can be dispersed, broken down into spectral components meaning that each colour has a different wavelength;  Polarisation and Polaroid lenses blocking out one plane of light waves;  Light experiences a Doppler effect (red shift). The analysis of light uses a combination of the two behaviours with great success. Reflection of Light Page 89 of 114 Reflection of light and other electromagnetic radiation occurs when waves encounter a boundary that does not absorb the radiation's energy and bounces the waves off the surface. The incoming wave is known as the incident wave and the wave that is bounced from the surface is called the reflected wave. The law of reflection states that the angle of incidence equals the angle of reflection. The angles are measured against a line perpendicular to the surface of the reflective material, called the 'normal'. Fig 4.3 REFRACTION When light waves pass from one transparent medium to another, they change velocity and direction. The angle of refraction is dependent on the density of the material through which the light passes. For example, when light travels from air to water, it slows down and bends towards the normal.. Most substances have a refraction index n which gives an indication of their density, how much the light slows down and, therefore, how much the light bends through the substance. The higher the refractive index number, the denser the material and the more the light will slow down and refract or bend as it passes through the substance. Fig 4.4 n = speed of light in a vacuum speed of light in material also n = sin I Snell's Law sin r A person may see a fish in the water but, in reality, the fish is in a different position because the light from the image is refracted as it leaves the water Fig 4.5 Example of n Air is 1.00029; Page 90 of 114 Diamond is 2.42; Glass is 1.5 to 1.7; Ice is 1.31; Water is 1.33. Total Internal Reflection As the incidence angle i, increases, less and less energy is refracted and more is reflected. At the Critical Angle, 100% of the light is reflected and Total Internal Reflection is occurring. This phenomenon is important in the design of fibre optic cable Fig 4.6 Activity: Add i and r to the above diagram Dispersion The index of refraction also varies with the wavelength of the radiation. If white light enters a prism, the different wavelengths of the component colours are refracted by different amounts. This is termed dispersion. Fig 4.7 A rainbow is the cumulative effect of sunlight being dispersed through a large number of raindrops. Polarisation Page 91 of 114 The rope will be vibrating up-and-down, side-to-side, and all the directions in- between, giving it a really complex overall motion. Now, suppose you passed the rope through a vertical slit. The rope is a really snug fit in the slit. The only vibrations still happening on the other side of the slit will be vertical ones. All the others will have been prevented by the slit. Fig 4.8 What emerges from the slit could be described as \"plane polarised radiation\", because the vibrations are only in a single (vertical) plane. Now look at the possibility of But if the second slit is at 90° to the first one, the string will stop vibrating entirely to the right of the second slit. Fig 4.9 The second slit will only let through horizontal vibrations - and there aren\'t any. The energy is completely polarized. Fig 4.10 Polarising sunglasses etc. do the optical equivalent of this using certain materials in place of the slits, to reduce the energy of the radiation, and cut glare. Page 92 of 114 Mirrors and Lenses Plane Mirrors Reflection off a plane surface: Note direction of energy propagation gets reversed Fig 4.11 If we have an extended object, this will create an image. To find out where the image appears to be, extend the line of sight To get the sensation of depth, we need binocular vision Fig 4.12 This is based on angle of incidence = angle of reflection θ₁ = θ₂ Fig 4.13 Page 93 of 114 Spherical Mirrors This is true even if the surface is curved: for example cut from a sphere. Fig 4.14 E.g. concave mirrors: different areas of the mirror reflect the wave according to the local angle of incidence This is reversible: if we have a source at the centre of a curved mirror, a plane wave is reflected. Focal length of a mirror Fig 4.15 f = R/2 (Approx) This means that light from infinity is focussed a distance f away Convex mirrors cause waves to diverge Note that these behave as if there is a focus behind the mirror Page 94 of 114 Driving and security mirrors are convex to increase coverage. For accurate focusing, a parabolic mirror is required. Fig 4.16 Activity Light from the centre of a sph erical mirror is reflected back there (why?) Fig 4.16 Lenses The use of lenses is an application of refraction Light is bent as it passes through transparent material of different densities. The velocity of light changes and we get a refractive index. Velocity in medium 1 = n and n = sin i Velocity in medium 2 sin r We have already seen how a single surface refracts. All optical instruments have at least 2 surfaces. A prism deflects light via two successive refractions according to Snell\'s law sinθ1 = n.sinθ2 entering the prism and, sinθ3 = n.sinθ4 exiting the prism. Fig 4.17 We can build up a lens from a series of prisms Fig 4.18. Fig 4.19 Page 95 of 114 Fig 4.19 We could add a 2nd prism, to deviate light more, so that two rays go to the same place Fig 4.20 Fig 4.21 More prisms are added and ground smooth. There are a variety of lenses, but essentially they are converging or positive (convex) diverging or negative (concave) Fig 4.22 The most important quantity for a lens is the focal length f: i.e. how far from the lens do parallel rays get focused. Fig 4.23 Page 96 of 114 Concave lenses cause light to diverge, but the rays can be traced back to an (imaginary) focus Fig 4.24 Images, real and virtual Real images are those where light actually converges, whereas virtual images are locations from where light appears to have converged. Real images occur when objects are placed outside the focal length of a converging lens or outside the focal length of a converging mirror. A real image is illustrated below. Note that it is magnified, but inverted. Fig 4.25 A real image has to be where the light is, which means in front of a mirror, or behind a lens.) A virtual image are formed by diverging lenses. Image is upright but diminished. Fig 4.26 If a convex lens does not focus the light passing through it at a single focal point, the image will not be sharp.This is termed spherical aberration and is common in less expensive lenses. Page 97 of 114 Sometimes the human eye does not focus images well enough on the back of the eye, the retina. In these cases spectacles, contact lenses or corrective surgery can be used. Fig 4.27 Page 98 of 114 OPTICAL FIBRE CABLE An optical fibre is a thin strand of high quality glass. Very little light is absorbed in the glass. Light getting in at one end is totally internally reflected, even when the fibre is bent. Optical fibres are used in telecommunication because they can carry enormous amounts of information in light pulses transmitted through them. This information is carried at very high speed -- about 2/3rds the speed of light. Many optical fibres are combined to form an optical fibre cable. Fibre optics are also used in medicine in flexible inspection probes which can carry a low heat light source and transmit images back to an eyepiece or video screen. Real optical fibre -- glass cable so pure that light visible through it, even when many kilometers long -- thickness comparable to that of single human hair Laser at end of cable switches on & off to send digital bits -- billions of bits per second Multiple lasers -- different colors (frequencies) -- multiple signals on same fibre Capable of carrying a signal quite a distance ≈ 100 km Fig 4.28 Optical fibre consists of three parts: Outer coating or buffer -- protects from physical damage -- typically plastic Core -- generally made of glass -- light propagates along core without layer of cladding, but cladding performs necessary functions Cladding -- generally made of glass or plastic -- index of refraction less than that of core material: Fig 4.29 Reduces loss of light into surrounding air Reduces scattering loss at surface of core Page 99 of 114 Fig 4.30 Protects fibre from absorbing contaminants Adds mechanical strength Optical signals typically extend partially into cladding material Optical fibre cable -- light travels down core by constant refraction off side walls Phenomenon of refraction used to transfer light from source to receiver -- light rays bend as they traverse through another medium of different density Typically infra red signal -- just below frequency of visible light Losses in optical fibre cables : Attenuation -- caused by absorption & scattering -- loss of optical power as light travels along fibre -- induced by imperfections in structure of optical fibre: impurities & contamination -- limits distance optical signal (pulse) can travel Fig 4.31 Bending Losses -- bending the fibre causes attenuation -- bending classified according to bend radius or curvature: microbend or macrobend Microbends -- small microscopic bends -- likened to dents in cladding & core Fig 4.32 Macrobends -- Page 100 of 114 Bends having a large radius of curvature -- if fibres bent too sharply, macrobend losses occur Fig 4.33 Advantages of Optical Fibre Cable  Lighter weight & smaller size -- substantially saves weight & space in aircraft  Reduction of cross-talk -- no light interferes with light on adjacent cables  Immunity to electromagnetic interference -- EMI does not affect light frequencies  Lower signal attenuation -- attenuation approx 1% that of coax or waveguide  Wide bandwidth: 100 MHz to 1GHz -- greater signal throughput in comms system  Lower cost -- optical fibre materials cost much less than copper  Safety -- hazards or short circuits & sparks eliminated  Corrosion -- fibre material is inert -- corrosion effects minimised Disadvantages:  Coupling -- from light source to fibre & splicing is critical -- exacting joints necessary  Special techniques & equipment -- because of size & nature of fibre cable material  Ultra-clean environment -- necessary when terminating to avoid particle pollution Applications Page 101 of 114 Optical fibre data busses increasingly incorporated in modern aircraft: Boeing 777's, Airbus A380, FA-18 E & F model Hornets & Apache helicopters, to name a few Typical functions performed by optical fibre networks: Avionics databus (ARINC & MIL-STD) Flight data sensor interface Engine sensor interface Feedback links in Automatic Flight Control System (AFCS) These instruments are now also being used in industry for internally inspecting machinery. Aircraft engines are very commonly inspected internally with an instrument called a borescope. Factors Affecting Transmission  Purity of glass fibre;  Length of fibre -- if over 1 km long, optical regenerators are used to modify the signal;  Quality of cladding around the fibre (cladding assists total internal reflection);  Type of wavelength of light used -- infra red is less susceptible to attenuation (loss of quality);  Quality of connections and minimising bends in the cable. Page 102 of 114 This page intentionally le