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Reflection 1. Introduction When light travelling in one medium encounters a boundary with another medium, part of the incident light changes direction and returns into the medium from which it originated, which is called reflection. The following diagrams show some common examples of reflection in daily life. The reflection of Mount Hood in Trillium Lake (http://en.wikipedia.org/wiki/Reflectio n_(physics)) Reflection in a soap bubble (http://commons.wikimedia.org/wiki/ File:Reflection_in_a_soap_bubble_edi t.jpg) Window Reflection of Canal Houses on a Clear Winter Day http://goamsterdam.about.com/od/a msterdamphotos/ig/AmsterdamReflection-Pictures/Canal-HouseWindow-Reflection.htm 1 2. Ray diagram In optics, we use a ray to represent the propagation of light. a ray of light a beam of light Different types of light beam: parallel light divergent light convergent light 3. Reflection 3.1 Two types of reflection Diffuse Reflection Reflection from any rough surface is known as diffuse reflection. When the surface is rough, a beam of parallel light is reflected not as a parallel set but in various directions as shown in the Fig.1. Fig.1 (http://commons.wikimedia.org/wiki/File:Difracao.svg) 2 Specular (regular) Reflection Reflection of light from a smooth surface is called specular reflection. When a beam of parallel light strikes a smooth surface, the reflected rays are also parallel to each other as indicated in Fig.2 below. Fig. 2 (http://www.screentekinc.com/glossary/glossary-specular_reflection.gif) The difference between these two kinds of reflection explains why it is more difficult to see while driving on a rainy night. If the road is wet, specular reflection occurs at the smooth surface of the water and hence most of the light is reflected away from the driver. When the road is dry, the rough surface of the road reflects the light diffusely. Part of the light is reflected back towards the driver, allowing the driver to see the highway clearly. 3.2 Principle of reflection Consider a light ray travelling in the air and incident at an angle on a flat smooth surface, as shown in Fig. 3. Fig. 3 (http://electron9.phys.utk.edu/phys135d/modules/m10/images/Image1282.gif) 3 The incoming light is called the incident ray and the outgoing ray is called the reflected ray. The normal is a line drawn perpendicular to the surface at the point where the incident ray strikes. The incident and reflected rays make angles and , respectively, where the angles are measured from the normal to the rays as indicated in Fig. 3. and are called the angle of incidence and angle of reflection respectively. Experiment shows that the angle of reflection equals the angle of incidence: This relationship is called the principle of reflection. 4. Mirror 4.1 Characteristics of image formed by plane mirrors Mirror Image http://wpcontent.answers.com/wikipedia/commons/thumb/5/52/Mirror.jpg/200pxMirror.jpg What are the characteristics of image formed by plane mirrors? The image has the same size as the object. For example, when you look into the mirror, your image has the same height as you. The image is upright. For example, when you stand upright in front of the mirror, your image is also upright. (Your feet will not appear on top of your head!) The image is virtual. It cannot be captured in a screen. The image distance (distance between the image and the mirror) equals the object distance (distance between the object and the mirror). The image is laterally inverted. For example, when you raise your right hand in front of the mirror, the image in the mirror raises the left hand. 4 4.2 Ray diagram to illustrate reflection by plane mirrors Consider the point source of light placed at O in Fig. 4, as a distance p in front of a plane mirror. The distance p is called the object distance. p q I O p=q Fig. 4 mirror Light rays leave the light source and strike the mirror. Light rays are reflected and continue to diverge. However, if we extend the reflected light rays in the opposite direction (behind mirror), they appear to converge at a point (light appears to be given out from that point), I, behind the mirror. Point I is called the image of the object at O. Images are classified as real or virtual. A real image is formed when light rays pass through and converge at a point; a virtual image is formed when the light rays do not pass through the image point but appear to be given out from the point. Hence, the image formed by plane mirror in Fig. 4 is virtual. Real images can be displayed at a screen (as at a movie), but virtual images cannot be displayed on a screen. Fig. 5 shows another ray diagram for images formed by plane mirrors. In this case, instead of a point source, the object is represented by an arrow. To form the image of the object, we have to choose two end points of the object and find the images of these two points. (At least two rays are required to find one image point.) Object Image of Object Mirror Fig. 5 5 4.3 Image formed by curved mirrors Concave Mirrors A concave mirror has a reflecting surface that bulges inward. A concave mirror tends to focus light towards a focal point. The image form by concave mirrors depends on the object distance. Effect on image of object's position relative to mirror focal point Object's position (S), focal point (F) S<F (Object between focal point and mirror) Image Diagram Virtual Upright Magnified (larger) 6 S=F (Object at focal point) the image is formed at infinity. (Note that the reflected light rays are parallel and do not meet the others. In this way, no image is formed or more properly the image is formed at infinity.) F < S < 2F (Object at 2x focal point) Real Real Inverted (vertically) Same size S = 2F Inverted (vertically) Magnified (larger) 7 S > 2F Real Inverted (vertically) Diminished (smaller) (http://en.wikipedia.org/wiki/Concave_mirror) image form on the inside of a spoon (http://studio.nathanielguy.com/previews/a_spoon_full_of_sky_preview.jpg) An important property of concave mirrors is that light rays passing through the focal point are reflected as parallel lights (parallel to the principle axis). The reflected light of a ray parallel to the principal axis will pass through the focal point. Convex Mirrors A convex mirror is a curved mirror in which the reflecting surface bulges outwards. The image is always virtual, diminished and upright. Convex mirror lets motorists see around a corner. (http://en.wikipedia.org/wiki/Concave_mirror) 8 5. Application of reflection Plane Mirror We see plane mirror in our everyday life. The following are two examples. A Dance Studio http://gotrhythmgardiner.com/db1/00073/gotrhythmgardiner.com/_uimages/DanceStudio2.JPG A Dressing Room http://www.mirrotek.com/blog/wp-content/uploads/2008/11/3vu1448b.jpg Many optical instruments use plane mirrors. Submarines have periscopes which help the crew to see what is above the water when the submarines are under water. The working principle of periscope is shown in Fig.6. 9 Fig. 6 A kaleidoscope makes use of two or more plane mirrors placed at an angle to see another to form multiple images of small colourful objects. Concave Mirror & Convex Mirror Convex and concave mirrors and reflectors have many uses. Fig. 7 shows how a concave reflector produces a parallel beam of light in a car headlamp. 10 Fig. 7 concave mirror produces a parallel beam of light The parallel beam of light allows people far away from the car to see the car coming. It also allows the driver to see what is ahead in the highway. Reflecting telescopes make use of mirrors to reflect light from the stars or other celestial bodies. The working principle is shown on Fig. 8. Fig.8 http://upload.wikimedia.org/wikipedia/commons/thumb/6/64/Gregorian_Telescope_Lightpath.s vg/800px-Gregorian_Telescope_Lightpath.svg.png Concave mirrors are also used by dentists to produce a magnified image of one’s teeth when held close to the teeth. Dentist Mirror 11 Convex mirrors are usually used in shops for security and on roads or in car parks to allow drivers to see around blind spots such as sharp corner. Security Mirror 12