Ch.3 Reflection (反射) Reflection of Light (光的反射) normal angle of angle of incidence reflection incident ray reflected ray plane mirror Incident Rays The light ray moving towards the mirror (入射線) Reflected Rays The light ray moving out of the mirror (反射線) Normal The line perpendicular (垂直) to the mirror (法線) Angle of incidence The angle between the incident ray and the normal. (入射角) Angle of reflection The angle between the reflected rays and the normal. (反射角) 3 Laws of reflection (反射定律) Laws of reflection: 1. The angle of reflection r is equal to the angle of incidence i. i = r normal incident ray i r vertical plane reflected ray plane mirror Laws of reflection: normal 2. The incident ray, the reflected ray and the normal all lie in the same plane. incident ray vertical plane reflected ray plane mirror When the angle of incidence equals to zero (i = 0) , the angle of reflection is also zero (r = 0). i=0 r=0 air mirror 6 7 40° 60° 40° 60° 30° 45° 30° 45° 8 5.3 Experiment (實驗) 平面鏡 電源 光箱 量角器 9 i / r / 0 30 45 60 75 Conclusion (結論) The angle of incidence is ____________ the angle of reflection. 10 i / r / 0 0 30 30 45 45 60 60 75 75 Conclusion (結論) The angle of incidence is _equal_ the angle of reflection. 11 Regular reflection (單向反射) Will the reflected rays remain parallel (平行) after reflection? For flat (平) and smooth (滑) surface, parallel reflected rays parallel incident rays i r i r i r plane mirror Each light ray obeys the laws of reflection. same i same r The reflected rays are therefore parallel. Regular reflection (單向反射) occurs at: plane mirror The following reflecting surfaces can form clear images due to regular reflection. metal surface a calm water surface Diffuse reflection (漫反射) For rough (粗糙) surface, parallel incident rays i1 r1 i2 i3 r2 reflected rays are not parallel r3 rough surface Each light ray obeys the laws of reflection. different i different r Diffuse reflection occurs at a rough surface (e.g. a piece of paper). e.g. a piece of paper Lateral inversion (橫向倒置) Mirror images are laterally inverted. the object image formed by the plane mirror The laterally inverted words on an ambulance (救護車) An image formed by a plane mirror is 1. same size as the object, 2. laterally 3. inverted. (橫向倒置) same distance 21 22 Uses of plane mirrors (平面鏡的應用) rear-view mirrors (倒後鏡) 23 Explain why the Chinese characters 救護車 are printed in the special way as shown. 24 So that the driver of the car in front of the ambulance can see the Chinese characters 救護車 without lateral inversion. 25 Periscope (潛望鏡) A periscope consists of two mirrors The mirrors make an angle of 45 with the line joining them. Periscopes are used in a double-deckers (雙層巴士) Periscope (潛 望鏡) used in submarine (潛水艇). 28 Plane mirrors used in interior design (室內設計). 29 Plane mirror used by dentist (牙醫). 30 In Figure 5.4, draw ONE light ray from the head of the object to the eye after reflection from the two mirrors. 31 32