S M K Pahi Bidang sains dan matematik Lesson 5.1 - Understanding light reflection LESSON 5.1 – Common terminology of reflection of light on a plane mirror Understanding reflection of light Normal : A line at right angles to the Introduction : mirror’s surface. Light is a form energy that enables us to see. Incident ray : A ray of light that is directed Light is emitted or given by hot objects, like a candle onto the mirror’s surface. flame, or the glowing filament of light bulb,or the sun. An object which produces its own light is said to be Reflected ray : A ray that is reflected by the luminous. mirror’s surface. Most of the objects we can see do not produce their own light – they are non-luminous. What we can see Angle of : The angle between the incident is the light is reflected off them. incidence ray and the normal Light seems to travel in straight lines. A picture of light as rays traveling in straight lines from a luminous Angle of : The angle between the source or bounching off objects, or being absorbed or reflected transmitted by them, is very useful one. We can think reflection ray and the normal of light ray as a very thin beam of light. A real beam contains many rays. But by drawing just a few of them we can get a clear idea of what happening to the whole beam. When rays of light strike any surface the rays are reflacted , unless the surface is black, when they are absorbed. The reflection depend on how smooth the surfaces are.Good mirrors reflect well over 90% of the light that reaches them, with only a small amount being absorbed. The surface of most objects are very irregular. Light hitting this sort of surface is scattered in all directions. AO : Incident ray This is called diffuse reflection. ON : Normal On the other hand,the surface of a polished sheet of OB : Reflected ray glass or metal can be made extremely smooth. The i : Angle of incidence reflection from this is the regular. r : Angle of reflection Laws of reflection The first law : The angle of incidence is equal to the angle of reflection. The second law : The incident ray, the normal and the reflected ray all lie in the same plane. 1 S M K Pahi Bidang sains dan matematik Lesson 5.1 - Understanding light reflection Charateristics of an Image Formed by a Plane Mirror by drawing Ray Diagram Example 2 The diagram shows two plane mirrors set an angle 100o to each other. A ray strikes one of the mirrors as shown at angle of incidence of 45o . Redraw and complete the diagram showing the path of the ray and calculate the angle of reflection at which it leaves the second mirrors. (1) Draw two normals at the surface of the mirror. (2) Draw two incident rays and each of them is directed to the normals. (3) Measure the angles of incidence respectively by using protractor. (4) By using the Laws of reflection (The angle of incidence is equal to the angle of reflection draw two reflected rays in front the mirror. (5) Extrapolate the reflected rays behind the with dotted line. The point of intersection is Solution the location of the image . The charateristics of an image formed by a plane mirror are : (1) The size of the image= the size of the object (2) The distance of image = the distance of object (3) Virtual image (appears to be behind the mirror) (4) Laterally inverted (left and right are interchanged) (5) Upright Example 1 A boy is at a distance 5 m from a plane mirror. The mirror is moved away from the boy by 2 m. What is the distance between the boy and his image. Solution 2 S M K Pahi Bidang sains dan matematik Lesson 5.1 - Understanding light reflection Common terminology of reflection of light on a curved mirror Radius of curvature, r : Distance between the pole,P and the centre of curvature,C. Focal length , f : The distance between the principal focus ,F and the pole of the curved mirror,P. Concave mirror Relationship between f and r r = 2f Relationship curvature of mirror and f The more curved the mirror , the shorter its focal length,f. Convex mirror Pole of mirror , P : The centre point on the curved mirror Centre of curvature , C : The geometric centre of a hollow sphere of which the concave or convex mirror is a part. Principal axis ,PFC : A line which passes through the centre of curvature ,C and the pole of a curved mirror,P Charateristics of an Image Formed by a curved Mirror by drawing Ray Diagram Principal focus , F : A point through which all rays We must draw any two of three rays to determine the traveling parallel to position of the image.There are three the principal axis converage to or Ray 1 appear to diverge from after reflection For concave mirror : by the mirror. 3 S M K Pahi Bidang sains dan matematik Lesson 5.1 - Understanding light reflection A ray parallel to the principal axis is reflected to pass through F. Ray 3 For concave mirror : For convex mirror : A ray through C is reflected back along its own path. A ray parallel to the principal axis is reflected as if it came from F. F For convex mirror : A ray towards C is reflected back along its own path. Ray 2 For concave mirror : A ray through F is reflected parallel to the principal axis. Image formed by a Concave Mirror A u < f ( Object between F and P ) For convex mirror : A ray towards F is reflected parallel to the principal axis. 4 S M K Pahi Bidang sains dan matematik Lesson 5.1 - Understanding light reflection Characteristics of image : Application : B u = f ( Object,O is at F ) D u = 2f or u = r ( Object ,O is at C) Characteristics of image : 1.real Characteristics of image : 2.inverted 3. same size Application : Application : Light reflector in a projector C f < u < 2f or f < u < r ( Object O is between E u > 2f or u > r ( Object, O is beyond C ) F and C. Characteristics of image : Characteristics of image : Application : ................................................................................... 5 S M K Pahi Bidang sains dan matematik Lesson 5.1 - Understanding light reflection F u = ∞ ( Object ,O very far from the lens) Characteristics of image : Application : B f < u < 2f or f < u < r ( Object O is between F and C) Characteristics of image : Application : Conclusion : The distance Characteristics of image object,u u < f u = f Characteristics of image : f <u < 2f u = 2f Application : u > 2f u = ∞ The object Characteristics of image Image formed by a Convex Mirror distance,u u < f A u < f ( Object between F and P ) f <u < 2f Conclusion 6 S M K Pahi Bidang sains dan matematik Lesson 5.1 - Understanding light reflection Uses of the reflection of light Plane mirror (1) A mirror periscope can be built by mounting two plane mirrors in a cardboard tube. (2) A rear view mirror and a side mirror of a car in order to enable the driver see objects at the back and side of the car. (3) Plane mirror acts as an anti-parallax error What are the angles of incidence and mirror in electrical instruments such as the reflection? voltmeter and the ammeter. Angle of Angle of Concave mirror incidence reflection (1) A shaving mirror or a make-up mirror used A 40o 40o a wide-aperture concave mirror with a B 40o 50o large radius of curvature. C 50o 40o (2) A torchlight or a headlight of motor D 50o 50o vehicles used concave parabolic mirror to produce a beam of parallel light rays. 2 The diagram shows a mirror is titled at an angle of 40o to the bench. A ray of light is directed so Convex mirror that it hits the mirror at an angle of 10o to the (1) Convex mirror are mounted at a sharp surface of the mirror. corner of a road so that the drivers are able to see oncoming cars on the blind side of the corner. (2) Convex mirror hung on the corner of ceilings in supermarkets serve to provide a wide field of view activities happening in the shopping area. What is the angle of reflection? o o A 10 B 20 C 40o D 500 o E 80 3 The diagram shows an image is formed by a mirror. At which point the object is placed? TUTORIAL 5.1 1 The diagram shows a single ray of light being directed at a plane mirror. 7 S M K Pahi Bidang sains dan matematik Lesson 5.1 - Understanding light reflection 4 The diagram shows a ray of light from a small Which lamp’s image can be seen? bulb strikes a plane mirror. Where is the image of the bulb formed and its 7 The diagram shows a wall clock . characteristic? A At P and virtual B At Q and real C At R and virtual 5 The diagram shows a girl stands at the side of a What is the time shown by the image of the wall boy in front of a mirror. They are both the same clock in a plane mirror as seen by a student. distance from the mirror. Where does the girl see the boy’s image? A 01:40 B 04:50 C 07:10 D 10:20 8 Diagram shows a student looking at a plane mirror.