1.3 Images formed by a plane mirror

1.3 Images formed by a plane mirror
Candle flame in water Properties of the images formed by a plane mirror Check-point 3 Constructing images Viewing an image formed by a plane mirror Check-point 4 1 2 3 Book 3A Section 1.3 Images formed by a plane mirror

Book 3A Section 1.3 Images formed by a plane mirror
Candle flame in water Look! A candle is burning in water! How does the above magic work? Video 1.2 Candle flame in water Book 3A Section 1.3 Images formed by a plane mirror

1 Properties of the images formed by a plane mirror
We perceive light rays entering our eyes as coming straight to the eyes. E.g. We perceive the divergent rays as coming out from point I , where the rays meet when extended backwards. Book 3A Section 1.3 Images formed by a plane mirror

Reflection: Diverging reflected rays seem to come from point I.  I is the image of O Book 3A Section 1.3 Images formed by a plane mirror

a Position of the image Expt 1b Images formed by a plane mirror Book 3A Section 1.3 Images formed by a plane mirror

Experiment 1b Images formed by a plane mirror Direct a set of divergent rays at a plane mirror on a piece of paper. Draw the reflected rays on the paper and trace them back to meet at a point. 1.3 Expt 1b - Images formed by a plane mirror Video Book 3A Section 1.3 Images formed by a plane mirror

a Position of the image In Expt 1b, the reflected rays appear to come from a point behind the mirror. The image is seen at this point, but no rays actually come from it. The image cannot be projected on a screen  virtual image Rays appears to come from this point Simulation 1.3 Images formed by a plane mirror Book 3A Section 1.3 Images formed by a plane mirror

b Distance between the image and the mirror Steps in finding the position of image I : Image Several rays of light from object O strike the plane mirror MN Draw the reflected rays using the laws of reflection. Trace the reflected rays backwards until they meet at a point I. Book 3A Section 1.3 Images formed by a plane mirror

b Distance between the image and the mirror
Findings from the figure: The image is as far behind the mirror as the object is in front (ON = NI ). The line joining the image and the object is perpendicular to the mirror (OI ⊥ MN ). Distance between observer and image Example 2 Book 3A Section 1.3 Images formed by a plane mirror

Example 2 Distance between observer and image May and Peter are standing in front of a plane mirror. May: 3 m from the mirror Peter: 1 m in front of May Distance between Peter and the image of May = ? Book 3A Section 1.3 Images formed by a plane mirror

Example 2 Distance between observer and image Distance of the image of May from the mirror = 3 m Distance of Peter from the mirror = 3 – 1 = 2 m Distance between Peter and the image of May = = 5 m Book 3A Section 1.3 Images formed by a plane mirror

c Size of the image Distance between image and mirror Distance between object and mirror =  Any two points on the image are of the same distance as the corresponding points on the object.  image: same size as the object Example 3 Image size Book 3A Section 1.3 Images formed by a plane mirror

Example 3 Image size A girl 1.4 m tall stands 3 m in front of a plane mirror. (a) What is the height of her image? Height of the image = height of the girl = 1.4 m (b) What will happen to the size of the image if the girl walks 1 m towards the mirror? Explain your answer briefly. Will be unchanged since it depends only on the size of the girl, not her distance from the mirror. Book 3A Section 1.3 Images formed by a plane mirror

d Orientation of the image If you raise your right hand in front of a plane mirror, your image will appear to raise its left hand.  Mirror images are laterally inverted and erect. Simulation 1.4 Laterally inverted images Book 3A Section 1.3 Images formed by a plane mirror

Image formed by a plane mirror is • virtual • as far behind the mirror as the object is in front • of the same size as the object • erect but laterally inverted Video 1.4 Strawberry floating in mid-air Book 3A Section 1.3 Images formed by a plane mirror

Check-point 3 – Q1 Which of the following about the image formed by a plane mirror is incorrect? A Light rays come from the image to our eyes. B The image cannot be projected on a screen. C It is as far away from mirror as the object is in front. D It is virtual. Book 3A Section 1.3 Images formed by a plane mirror

Check-point 3 – Q2 A boy of height 1.5 m stands 1.8 m in front of a plane mirror. His image is ______ m tall and ______ m away from him. 1.5 3.6 Book 3A Section 1.3 Images formed by a plane mirror

Check-point 3 – Q3 Letters P and Q are printed on a card. Sketch the image of the letters on the plane mirror. Book 3A Section 1.3 Images formed by a plane mirror

2 Constructing images Object O : 2 m tall, 3 m in front of the mirror MN. Image I must be (1) same size as the object, i.e. 2 m tall (2) as far behind the mirror as the object is in front, i.e. 3 m away from the mirror (3) erect. Book 3A Section 1.3 Images formed by a plane mirror

2 Constructing images Example 4 Constructing an image Book 3A Section 1.3 Images formed by a plane mirror

Example 4 Constructing an image A pencil is lying on the floor in front of a mirror. Draw the image of the pencil that you see in the mirror. Book 3A Section 1.3 Images formed by a plane mirror

Example 4 Constructing an image A is 1 unit in front of the mirror  A ’ is 1 unit behind the mirror.  AA ’ ⊥ mirror. Similarly, B ’ is 3 units behind the mirror  BB ’ ⊥ mirror. Book 3A Section 1.3 Images formed by a plane mirror

3 Viewing an image formed by a plane mirror
A ray diagram also shows how we see the image formed by a plane mirror. Construct the image of the object. Draw a cone of rays from I1 to the eye. Use dotted lines for rays behind the mirror. Book 3A Section 1.3 Images formed by a plane mirror

Draw two rays from O1 to the points where the rays from I1 meet the mirror. Repeat steps 2 and 3 for O2. The rays behind the mirror and the image are virtual  represented by dotted lines Example 5 Seeing the image Book 3A Section 1.3 Images formed by a plane mirror

Example 5 Seeing the image A girl: 170 cm tall, cm from a mirror Her eyes: 10 cm below the top of her head She can just see her feet at the bottom edge of the mirror. Book 3A Section 1.3 Images formed by a plane mirror

Example 5 Seeing the image (a) Distance between the girl and her image = ? The image is as far behind the mirror as the object is in front.  Distance = 150  2 = 300 cm (b) Show how the girl sees her feet in the mirror. Book 3A Section 1.3 Images formed by a plane mirror

Example 5 Seeing the image (c) Height of the bottom edge of the mirror = ? By laws of reflection, 1 = 2 1 2  h =  160 = 80 cm The bottom edge of the mirror is 80 cm above the floor. Book 3A Section 1.3 Images formed by a plane mirror

Example 5 Seeing the image (d) Min height of the top edge of the mirror if she is able to see the top of her head = ? By laws of reflection, 3 = 4 1 2  y = x =  10 = 5 cm Min height of the top edge of the mirror = y = 165 cm (above the floor) Book 3A Section 1.3 Images formed by a plane mirror

Example 6 Disappearance of an elephant Book 3A Section 1.3 Images formed by a plane mirror

Example 6 Disappearance of an elephant (a) Draw a ray diagram to illustrate how the magic works. Book 3A Section 1.3 Images formed by a plane mirror

Example 6 Disappearance of an elephant (b) The background and the side walls are usually painted a plain colour without any patterns. Why? The images of the side walls are laterally inverted. If the wall is colourful or painted with patterns, the audience can easily distinguish the images from the background. Book 3A Section 1.3 Images formed by a plane mirror

Example 7 Checking back lights Book 3A Section 1.3 Images formed by a plane mirror

Example 7 Checking back lights A driver parks his car in front of a shop window. He looks at the reflection of the back lights through the rear-view mirror. 2.7 m Book 3A Section 1.3 Images formed by a plane mirror

Example 7 Checking back lights (a) X is the image of P formed by the shop window. (i) How far is X from the shop window? 1.3 m. (ii) How far is X from the rear-view mirror? Distance of X from rear-view mirror = = 5.3 m (b) Where does the rear-view mirror form the image of X ? 5.3 m behind the rear-view mirror. Book 3A Section 1.3 Images formed by a plane mirror

Example 7 Checking back lights (c) Through reflection by the rear-view mirror and the shop window, the driver can see the image of the number plate at the back of his car. (i) Are the numbers laterally inverted? The number plate is laterally inverted twice  looks normal in the rear-view mirror. (ii) State two other properties of the image. Virtual, of the same size as the object, erect Book 3A Section 1.3 Images formed by a plane mirror

Check-point 4 – Q1 Leon and Patrick look in a plane mirror placed directly in front of Leon. Can they see themselves and each other in the mirror? Leon can see himself but Patrick cannot. They can see each other. Book 3A Section 1.3 Images formed by a plane mirror

Check-point 4 – Q2 Show how an observer sees the image of a pen in a plane mirror. Book 3A Section 1.3 Images formed by a plane mirror

The End Book 3A Section 1.3 Images formed by a plane mirror

1.3 Images formed by a plane mirror

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