Common terminology of reflection of light on a plane mirror. Normal : A line at right angles to the mirror s surface.

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1 LESSON 5.1 Understanding reflection of light Introduction : Light is a form energy that enables us to see. Light is emitted or given by hot objects, like a candle flame, or the glowing filament of light bulb,or the sun. An object which produces its own light is said to be luminous. Most of the objects we can see do not produce their own light they are non-luminous. What we can see is the light is reflected off them. Light seems to travel in straight lines. A picture of light as rays traveling in straight lines from a luminous source or bounching off objects, or being absorbed or transmitted by them, is very useful one. We can think of light ray as a very thin beam of light. A real beam contains many rays. ut 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. This is called diffuse reflection. On the other hand,the surface of a polished sheet of glass or metal can be made extremely smooth. The reflection from this is the regular. ommon terminology of reflection of light on a plane mirror Normal : A line at right angles to the mirror s surface. Incident ray : A ray of light that is directed onto the mirror s surface. Reflected ray : A ray that is reflected by the mirror s surface. Angle of : The angle between the incident incidence ray and the normal Angle of : The angle between the reflected reflection ray and the normal AO : Incident ray ON : Normal O : Reflected ray i : Angle of incidence r : Angle of reflection Laws of reflection The first law : The angle of incidence is equal to the angle of reflection. The second law : and The incident ray, the normal the reflected ray all lie in the same plane. 1

2 harateristics of an Image Formed by a Plane Mirror by drawing Ray Diagram Example 2 The diagram shows two plane mirrors set an angle 100 o to each other. A ray strikes one of the mirrors as shown at angle of incidence of 45 o. 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) y 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 the location of the image. Solution 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

3 ommon terminology of reflection of light on a curved mirror Radius of curvature, r : Distance between the pole,p and the centre of curvature,. Focal length, f : The distance between the principal focus,f and the pole of the curved mirror,p. oncave 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. onvex mirror Pole of mirror, P : The centre point on the curved mirror entre of curvature, : The geometric centre of a hollow sphere of which the concave or convex mirror is a part. Principal axis,pf : A line which passes through the centre of curvature, and the pole of a curved mirror,p Principal focus, F : A point through which all rays traveling parallel to the principal axis converage to or appear to diverge from after reflection by the mirror. harateristics of an Image Formed by a curved Mirror by drawing Ray Diagram We must draw any two of three rays to determine the position of the image.there are three Ray 1 For concave mirror : 3

4 A ray parallel to the principal axis is reflected to pass through F. Ray 3 For convex mirror : A ray parallel to the principal axis is reflected as if it came from F. F For concave mirror : A ray through is reflected back along its own path. For convex mirror : Ray 2 A ray towards is reflected back along its own path. For concave mirror : A ray through F is reflected parallel to the principal axis. Image formed by a oncave Mirror A u < f ( Object between F and P ) For convex mirror : A ray towards F is reflected parallel to the principal axis. 4

5 u = f ( Object,O is at F ) D u = 2f or u = r ( Object,O is at ) 1.real 2.inverted 3. same size Light reflector in a projector f < u < 2f or f < u < r ( Object O is between F and. E u > 2f or u > r ( Object, O is beyond )... 5

6 F u = ( Object,O very far from the lens) f < u < 2f or f < u < r ( Object O is between F and ) onclusion : The distance object,u u < f u = f f <u < 2f u = 2f u > 2f u = Image formed by a onvex Mirror haracteristics of image A u < f ( Object between F and P ) The object distance,u u < f f <u < 2f haracteristics of image onclusion 6

7 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 mirror in electrical instruments such as the voltmeter and the ammeter. oncave mirror (1) A shaving mirror or a make-up mirror used a wide-aperture concave mirror with a large radius of curvature. (2) A torchlight or a headlight of motor vehicles used concave parabolic mirror to produce a beam of parallel light rays. onvex mirror (1) onvex mirror are mounted at a sharp corner of a road so that the drivers are able to see oncoming cars on the blind side of the corner. (2) onvex mirror hung on the corner of ceilings in supermarkets serve to provide a wide field of view activities happening in the shopping area. What are the angles of incidence and reflection? Angle of Angle of incidence reflection A 40 o 40 o 40 o 50 o 50 o 40 o D 50 o 50 o 2 The diagram shows a mirror is titled at an angle of 40 o to the bench. A ray of light is directed so that it hits the mirror at an angle of 10 o to the surface of the mirror. What is the angle of reflection? A 10 o 20 o 40 o D 50 0 E 80 o TUTORIAL The diagram shows an image is formed by a mirror. At which point the object is placed? 1 The diagram shows a single ray of light being directed at a plane mirror. 7

8 4 The diagram shows a ray of light from a small bulb strikes a plane mirror. Which lamp s image can be seen? Where is the image of the bulb formed and its characteristic? 7 The diagram shows a wall clock. A At P and virtual At Q and real At R and virtual 5 The diagram shows a girl stands at the side of a boy in front of a mirror. They are both the same distance from the mirror. Where does the girl see the boy s image? What is the time shown by the image of the wall clock in a plane mirror as seen by a student. A 01:40 04:50 07:10 D 10:20 8 Diagram shows a student looking at a plane mirror. The image observed by the student is 6 The diagram shows four lamps in front of a plane mirror. The card prevents the observer at M from seeing the lamp directly. 8

9 9 Diagram shows an object O at a distance in front of a plane mirror D. The plane mirror is moved by a distance 4 cm at LM. What is the distance between object and the new image of the object? A 12 cm 16 cm 24 cm D 32 cm E 64 cm 10 A boy stands in front of a plane mirror a distance 5 m. When the boy moves toward the mirror by 2 m, what is the distance between the boy and his new image? What is the distance of image of the chart from the patient? A 3 m 4 m 7 m D 10 m E 14 m 12 The diagram shows a half-metre rule has a hole at 15 cm mark and is hung vertically. A plane mirror is placed in front of the ruler. A 2m 4m 6m D 8 m E 10 m 11 The diagram shows a patient sitting in a testing room in front of a chart. What is the region of the image of the ruler seeing by an observer, when the observer look through the hole? A D E between 0 cm mark and 30 cm mark between 0 cm mark and 45 cm mark between 5 cm mark and 35 cm mark between 10 cm mark and 25 cm mark between 0 cm mark and 50 cm mark 13 The diagram shows a point object,o is positioned in front of a plane mirror. 9

10 A D diminished, real and inverted magnified, real, and upright diminished,virtual and upright magnified, virtual and inverted In order to view the image of O in the mirror, you must be between the points A U dan V U dan Y W dan Y D Y dan Z 14 The figure shows a light ray being incident to the surface of plane mirror MN. The mirror is then rotated anticlock wise through an angle 10 o. 17 A concave mirror has a focal length 20 cm. What happen to the size of image when an object is placed at a distance of 40 cm in front of the mirror? A diminished magnified same size of object 18 An object is placed at a distance 18 cm from a concave mirror of focal length 10 cm. What is the characteristics of the image formed? A D real, inverted and magnified virtual, inverted and magnified real, upright and diminished virtual, upright and diminished 19 A concave mirror has a focal length 15 cm. The image formed by the mirror is virtual, upright and magnified. What is the object distance? What is the new angle of reflection of the light ray? A D 5 0 A 8 cm 15 cm 20 cm D 30 cm E 40 cm 20 The figure shows a candle placed in front of a concave mirror of focal length, f. 15 The diagram shows five light rays A,,,D and E from an object,o and is reflected by a concave mirror to form an image I. Which of the light ray is not correct? The image formed is A D real, upright and magnified real, inverted and diminished virtual, inverted and magnified virtual, upright and diminished 16 The characteristics of an image, formed by a convex mirror for all positions of the object are 10

11 21 Which of the following states the right reason for replacing a plane mirror are used as rear- view mirrors in motor vehicles with a convex mirror? (c) Write down the word " REFLETION " when the word is seen by a person in a plane mirror. A D To shine the object To widen the field of view To produce a brighter image To produce a sharper image 23 Figure (a) shows the positions of the eye of a car driver, the wing mirror of the car and the front of a lorry. 22 (a) Figure shows the positons of an object O and the image I of the objects is formed by a plane mirror M. Figure(a) omplete figure (a) showing the path of the ray,including the in Incident ray, reflected ray and normal. (a) (b) Figure (a) Draw rays to indicate the part of the front of the lorry which can be seen in the mirror by the car driver. Figure (b) shows a plane mirror and a curved mirror.the mirrors have the same size. The wing mirror of a car is often curved. Draw rays from observer to the plane mirror and the curved mirror to explain how the curved mirror give more advantage than the plane is used as a wing mirror of a car. (b) Figure(b) shows two plane mirrors JK and LM which have been placed at a right angles to each other. Figure (b) omplete figure (b) to show the ray diagram path until it emerges from the both the mirrors. 24 An object is placed at a distance,u from a concave mirror, M. The table shows the 11

12 characteristics of image, when the distance u is varied. Object distance, u/cm 10 (a) (b) 20 Image harateristics 30 Real,inverted,same size as the object 40 Name the light phenomenon that is involved to form the image by the mirror.... Using the information in the table, determine the focal length of the mirror M. (iii).. What is the characteristics of the image when the concave mirror M is displaced by a convex mirror of focal length same as the mirror M Figures(a) and (b) show the parallel rays of light directed toward the curved mirrors, J and K. P is the radius of curvature, and F is the focal point of the mirrors. (c)... Give the reason for your answer in (b). (d) (e) omplete the table An object is placed at a distance 50 cm from the concave mirror M. (i) Draw a ray diagram in the space below to show the formation of the image. (a) (b) (c) What is meant by focal length. Using Figure(a) and (b), compare the curvanture of the mirrors and their focal lengths. Figure(c) shows a cross section of a parabolic reflector used in a car headlamp. The headlamp has two bulbs, X and Y. ulb X is located at the focal point of the reflector while bulb Y is above bulb X. Figure(c) Explain what happens to the light rays from the bulb when only: (i) bulb X is switched on (ii) bulb Y is switched on (ii) What is the characteristics of the image? You may use diagrams to illustrate your answers. 12

13 (d) You are given two pieces of plane mirror measuring 5 cm x 6 cm to make a periscope. (i) (ii) (iii) Using these mirrors and other materials, describe how you would make the periscope. Explain why the image observed through the periscope using plane mirrors are not very clear. Suggest modification that can be made to the periscope to produce clearer and bigger images. 13