Phy105 Waves & Optics Notes PDF

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I ÉFÉ Uo speed of observer Us speed source of fobserver source Observer stationary sauce is moving toward observer 0 a F f East at Observer stationary source is moving away from observe Uo f Fast observe movingtowardssome Exe SoEeyf T F Observe moving away f from S f clef  OPTICS Optics is the study of Light. By light we mean the visible part of the electromagnetic spectrum. Early scientists were curious to study light because without it human sense of vision cannot be stimulated. OPTICS  Vocabulary Image: That which is formed when light is reflected or refracted. Real Image: – It is made from “real” light rays that converge at a point. Can be projected unto a screen because light actually passes through the point where the image appears. Always inverted.  Vocabulary Virtual Image– “Not Real” because it cannot be projected on a screen It is formed from virtual rays. Normal: An imaginary line that is perpendicular to the reflecting surface. Reflection of light from a smooth plane surface Laws of Reflection The incident and I reflected rays lie in the same plane with the normal. The angle of incidence l equals the angle of r reflection. θi = θr  Image formation. When scattered light from an object falls on a plane smooth surface such as a mirror, the light is reflected, resulting in image 0 formation k The image is located behind the mirror. Image properties: Ear i) virtual ii) Erect iii) Same size as the object iv) Same distance from the reflecting surface as the object. v) Laterally inverted Virtual Images in Plane Mirrors 1 of Since light energy doesn't flow from the image, the image is "virtual". LATERAL INVERSION I Diffuse vs Specular Reflection Diffuse Reflection Light incident upon a rough surface kf Laws of reflection still hold; Normals are not parallel. Specular Reflection __ Mirror like reflection All Normals are parallel t.it ft Deviation of Light by a Plane-Mirror 1 1 3 The total angle between the straight-line path of 0 90 i i p the incident ray and the reflected ray is twice the glancing angle This is called the deviation of the light. It is a it adjacent measure of the angle through which the light has Grgtfe.to strayed from its initial straight-line path. joadjacentB D = 2θ 1 20 2 θ = 90 -i Grazing D = 180 – 2i 2C  Effects of Reflection From Plane surfaces To view your full image in a mirror, the mirror length needs to be only half your height. “real” you mirror only your image needs to be half as high as you are tall. Your image will be twice as far from you as the mirror. Effect of mirror rotation When a mirror is rotated through angle θ about the point of incidence, the reflected ray rotates through angle 2θ Plane of the mirror before M1 rotation Incident ray before and after mirror rotation θ θθ Plane of the mirror N2 after rotation We can see that the reflected ray has rotated through angle 2θ. Because the reflected ray ends virtually on the image, the image moves on an arc which also substends angle 2θ at the point of incidence. Worked Example A point object is placed 5 cm away from a plane mirror. Calculate the length of the arc that is traced by the image of the object as the mirror is rotated through angle 150 5 cm I1 5 cm Length of arc I1 I2 = (15/360)(2πx30) ≈ 7.9 cm  Applications of Reflection From Smooth Plane Surfaces Plane mirrors are used as looking glasses (dressing mirrors) In periscopes In solar cookers In kaleidoscopes a Mirror Periscope 71  Refraction Refraction: Sudden change in the direction of a wave as it goes from one medium to another. For refraction to occur, light ray must enter the medium obliquely It is always accompanied by change in speed A 1 Trirefraction A B ta tB ra v ff A XB  1 2 Refraction at Plane Surfaces Laws of Refraction insr 1. The incident ray, the normal and the refracted ray all lie in the same plane. 2. For two particular media, the ratio of the sine of the o self f angle of incidence to the sine of the angle of refraction is a constant, i.e. TETE sin i 11800 = constant = n2/n1 sin r n1 = Refractive index in the first medium n2 = Refractive index in the second medium The second law of refraction is also known as Snell’s Law.  ifi 1.5 9 11 o Refraction from plane surfaces In both cases the speed of the wave has decreased. 114 Except This is indicated by the decrease in wavelength! N Na i O r 0 sister 8 e g 1 57 2 11 Refraction In which medium does light travel faster? (glass rod appears bent) NOT Light ray f e Observe Absolute Refractive Index of a medium v is the speed of light in the new medium. n is the absolute index of c = 3.0 x 108 m/s refraction. This is a measure of optical density. n is defined as As the index increases the the ratio of the speed of speed decreases. light in air (and vacuum) Note: n(air) = 1.0 to the speed of light in a new medium. Relative Refractive Index When light travels from medium 1 of ◆ If nrel < 1 ; absolute refractive index n1 to speeds up medium 2 of absolute refractive index n2, then relative refractive index from ◆ If nrel > 1 ; medium 1 to medium2 is given by: slows down n2 c c n rel = =  n1 v2 v1 n2 v1 n rel = = n1 v2  Snell’s Law of Refraction (nksinθk = constant) When a wave slows down it bends closer to the normal.{n2>n1} When a wave speeds up it bends away from the normal. {n2