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Light Rays Dr. Entesar Ganash 1. The Nature of Light “Is light a wave or a particle?” The particle model of light (Newton’s Particle Model): - Light was considered to be a stream of particles. - Newton held that particles were emitted from a light source &they stimulated the sight sense upon entering the eye. - Reflection and refraction can be explained. However, light Interference phenomenon could not be explained. The wave model of light: This model was proposed that light might be some type of wave motion. To Huygens, light was a type of vibratory motion, spreading out & generating a light sensation when impinging on the eye. in 1678, Huygens explained reflection & refraction on the basis of this theory. In 1801, Young showed light Interference phenomenon In 1873, Maxwell stated that light was a type of high-frequency electromagnetic wave. In 1887, Hertz confirmed Maxwell’s theory. 1. The Nature of Light “Is light a wave or a particle?” In 1905, Einstein explained the photoelectric effect using quantization model, which assumes the energy of a light wave is existed in particles called photons. According to Einstein’s theory, the photon energy (E) is proportional to the frequency (f) of the electromagnetic wave (E= h f ), where h is Planck’s constant. Hence, “Is light a wave or a particle?” is an inappropriate question Light must be regarded as having a dual nature. Sometimes light acts like a wave, and other times it acts like a particle. 2. The Ray Approximation in Ray Optics The field of ray optics (geometric optics) involves the study of the light propagation. Geometric optics is an approximate treatment of light in which light are represented as straight- line rays. The direction of light ray propagation is changed by refraction & reflection. knowledge of geometrical optics is useful for the important design & manufacture of magnifiers, microscopes & telescopes. We can understand the formation of an image with the fundamentals of ray optics. 2. The Ray Approximation in Ray Optics A plane wave of wavelength λ is incident on a barrier in which there is an opening of diameter d. A barrier Diffraction The ray approximation is very useful for the study of mirrors, lenses, prisms, & optical instruments 3. Reflection When a beam of light strikes such an interface, some light is scattered backward. This phenomenon is known as reflection. smooth surface Specular reflection, Reflection of light from a smooth surface . The reflected rays are all parallel to one another. any rough surface Diffuse reflection, Reflection from any rough surface. The reflected rays travel in random directions. Lows of Reflection At a planar mirror Angle of incidence = Angle of reflection (the first part of Law of reflection) Be carful the angles are always measured from the normal to the surface. The incident and reflected beams lie within a plane together with the surface normal. (the second part of Law of reflection) 4. Refraction At a planar dielectric surface such as glass plate, reflection & transmission occur at the same time. Refraction is a phenomenon that describes the propagation of light through interface, which separates 2 materials. The refraction changes the light’s direction of travel. The transmitted part of the incident beam is refracted. Its change of direction can be explained by the refraction index (n) that is defined by n= c v 8 where c is speed of the light in a vacuum ( c = 2.998 ×10 m / s ) & v is the velocity of light in a medium. 4. Refraction Optically Dense medium has a higher n than thinner medium. E.g. Water ( n ≅ 1.333 ) is optically more dense than air ( n ≅ 1.00029 ). Low of Refraction A refracted ray lies in the plane of incidence and has a refracted angle (θ t ) which is related to incident angle( θ i ) by Snell’s law or Law of refraction that is given as ni sin θ i = nt sin θ t Low of Refraction we can have three basic results: a) nt = ni ⇒ θ t = θ i i.e. the refraction does not bend the light beam, which continues moves in the same direction. b) nt > ni ⇒ θ t < θ i i.e. the refraction bends the light beam toward the normal c) nt < ni ⇒ θ t > θ i i.e. the refraction bends the light beam a way the normal. Here the subscript 1 ≡i &2 ≡t 5. Total Internal Reflection Rays of light from point source (S) in glass incident on glass-air interface ray a: part of ray light reflects at interface & the rest travels without change in direction . Rays b, c and d: there are both reflection and refraction at interface. 5. Total Internal Reflection Rays of light from point source (S) in glass incident on glass-air interface Ray e : as the incident angle increases the refracted angle increases. When the refracted ray reaches the interface that means the refracted angle is equal to 90 (relative to a normal), then the incident angle is called the critical angle ( θ C ). This Snell's law. Rays f and g: here the incident angles larger than θ C & there is no refracted ray & all the light is reflected. This situation is known as total internal reflection. The total internal reflection can occur when the incident light in medium has a high (n). that means the light travels from a dense medium to a thinner one ( here from glass to air). Total Internal Reflection Optical Fibers Losing of light intensity is very little A cladding is a material that has a lower n than the core. Physicians use it to 1. inspect internal organs of the body 2. do surgery without making large incisions. 6. 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. The laws of reflection & refraction can be derived by using Huygens’s principle. A plane wave A spherical wave Radius of each circular arc is c∆t Wavelets 8. Dispersion The refraction index depends on the wavelength of the light i.e. n( λ). A light beam consists of rays of different wavelengths which will refracted at different angles. Dispersion is the spreading of light according to its wavelengths λ References •“ Physics for Scientists & Engineers with Modern Physics ” by Serway & Jewett 2014,ch 35. •“ Fundamental of Physics ” by Halliday, Resnick & Walker 2008 John Wiley & Sons, 8 ed. •http://sst2011-s208sci.blogspot.com/2011/08/applications-of-total-internal.html •http://www.physicsclassroom.com/class/refrn/Lesson-1/The-Direction-of-Bending •http://cnx.org/content/m12895/latest/ •http://thewaythetruthandthelife.net/index/2_background/2-1_cosmological/2-1-12_childoverview/sir_isaac_newton.htm •http://turningmirrors.com/22-halo/halo-plot-dispersion_ice