12.2 Reflection of light How do you look in others’ eyes? Laws of reflection Regular and diffuse reflection Check-point 1 How a plane mirror forms an image Check-point 2 ? 1 2 ? 3 12.2 Reflection of light

How do you look in others’ eyes? Make two plane mirrors stand perpendicular to each other and look at yourself at the joints of mirrors. This is the same as what other people see you! 12.2 Reflection of light

1 Laws of reflection A ray of light bounces off a plane mirror. mirror normal angle of incidence angle of reflection incident ray reflected ray mirror This is an example of reflection of light. 12.2 Reflection of light

Experiment 12a Laws of reflection Direct a ray of light at a plane mirror from different angles. full circle protractor ray-box mirror strip Any relations between angle of incidence i and angle of reflection r ? incident ray reflected ray 12.2 Reflection of light

Experiment 12a Video Simulation Laws of reflection 12.2 Reflection of light

1 Laws of reflection angle of incidence = angle of reflection normal incident ray reflected ray mirror Also see Simulation program 1.1 in Physics at Work Multimedia CD-ROM 1. 12.2 Reflection of light

all lie in the same plane. 1 Laws of reflection The incident ray, the reflected ray and the normal all lie in the same plane. normal incident ray reflected ray mirror 12.2 Reflection of light

1 Laws of reflection 12.2 Reflection of light

2 Regular and diffuse reflection Regular reflection on a flat, smooth surface e.g. mirror parallel incident rays parallel reflected rays 12.2 Reflection of light

2 Regular and diffuse reflection on a rough, not perfectly smooth surface e.g. cover of a book parallel incident rays reflected rays in different directions 12.2 Reflection of light

Check-point 1 1 The angle between the incident... 2 True or false: Reflection of… 3 The angle between an incident... 12.2 Reflection of light

Check-point 1 - Q1 The angle between the incident ray and the __________ is called the angle of incidence. normal The laws of reflection state that it is equal to the angle of __________. reflection 12.2 Reflection of light

Check-point 1 - Q2 True of false: Reflection of light by a rough surface does not obey the laws of reflection. (T / F) 12.2 Reflection of light

The angle between an incident ray and the mirror is 20. Check-point 1 - Q3 The angle between an incident ray and the mirror is 20. The angles of incidence and reflection are ____ and ____ respectively. 70 70 The mirror turns the ray by ____. 140 20 70 70 12.2 Reflection of light

Experiment 12b Direct a set of divergent rays at a plane mirror. Images formed by a plane mirror ray-box Direct a set of divergent rays at a plane mirror. Trace the reflected rays back to meet at a point. This is the image point of the ray-box lamp (object point) . How is the image point related to the object point? 12.2 Reflection of light

Experiment 12b Video Images formed by a plane mirror 12.2 Reflection of light

3 How a plane mirror forms an image Rays from object O become divergent after reflection. mirror 12.2 Reflection of light

3 How a plane mirror forms an image Reflected rays appear to come from I behind the mirror. I O I is a virtual image. mirror - cannot be picked up on a screen 12.2 Reflection of light

3 How a plane mirror forms an image Simulation 12.2 Reflection of light

3 How a plane mirror forms an image The image formed by a plane mirror is: 1 virtual 2 as far behind the mirror as the object is in front F F 3 of the same size as the object object image Also see Simulation program 1.3 in Physics at Work Multimedia CD-ROM 1. 4 laterally inverted mirror 12.2 Reflection of light

3 How a plane mirror forms an image Simulation 12.2 Reflection of light

3 How a plane mirror forms an image How to draw ray diagram of image formed by a plane mirror? Dotted rays are also virtual. The image is virtual and is represented by dotted lines. 12.2 Reflection of light

3 How a plane mirror forms an image 12.2 Reflection of light

Check-point 2 1 Which of the following... 2 Leon and Patrick look in a... 3 Letters P, Q are... 4 A boy of height 1.5 m stands... 12.2 Reflection of light

A Light rays come from the image to our eyes. Check-point 2 - Q1 Which of the following descriptions 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. 12.2 Reflection of light

Can they see themselves and each other in the mirror? Check-point 2 - Q2 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. 12.2 Reflection of light

Letters P, Q are printed on a card as shown below. Check-point 2 - Q3 Letters P, Q are printed on a card as shown below. Sketch the image of the letters on the plane mirror. 12.2 Reflection of light

A boy of height 1.5 m stands 5 m in front of a plane mirror. Check-point 2 - Q4 A boy of height 1.5 m stands 5 m in front of a plane mirror. His image is ______ m tall and ______ m from him. 1.5 10 12.2 Reflection of light

The End 12.2 Reflection of light

Example 1 Turning directions of light If a ray of light is incident at one of the mirrors at 45°, complete the path of the ray. 45° 45° 45° 12.2 Reflection of light

Example 1 Turning directions of light How is the direction of the outgoing ray related to that of the incoming ray? The ray is turned back by the device. The outgoing and the incoming ray are parallel but opposite in direction. 12.2 Reflection of light

Return 12.2 Reflection of light

Example 2 A girl can just see her feet at the bottom edge of the Seeing the images A girl can just see her feet at the bottom edge of the mirror. 150 m Her eyes are 10 cm below the top of her head. 150 m (a) What is the distance between the girl and her image in the mirror? 12.2 Reflection of light

Example 2 Seeing the images (a) The image is as far behind the mirror as the object is in front.  Distance = 150  2 = 300 cm 12.2 Reflection of light

Example 2 Seeing the images (b) How high above the floor is the bottom edge of the mirror? 2 h 1 12.2 Reflection of light

The bottom edge of the mirror is 70 cm above the floor. Example 2 Seeing the images (b) How high above the floor is the bottom edge of the mirror? By laws of reflection, 1 = 2 h 1 2  h =  140 1 2 = 70 cm The bottom edge of the mirror is 70 cm above the floor. 12.2 Reflection of light

Example 2 Seeing the images (c) What is the minimum height of the top edge of the mirror if she is able to see the top of her head? 12.2 Reflection of light

Example 2 By laws of reflection, 3 = 4 a 4 1 10 cm  b = a =  10 2 Seeing the images By laws of reflection, 3 = 4 3 4 h 1 2 10 cm a b  b = a =  10 1 2 140 cm = 5 cm Minimum height of top edge of the mirror = b + 140 =145 cm above the floor 12.2 Reflection of light

Return 12.2 Reflection of light

Example 3 Checking back lights The driver in the car below wants to check whether the back lights on his car are both working. 12.2 Reflection of light

Example 3 (a) The image of P in the shop window is X. Checking back lights (a) The image of P in the shop window is X. (i) How far is X from the shop window? 1.3 m X 12.2 Reflection of light

Example 3 (a) The image of P in the shop window is X. Checking back lights (a) The image of P in the shop window is X. (ii) How far is X from rear-view mirror? 1.3 + 4.0 = 5.3 m X 12.2 Reflection of light

Example 3 (b) Where does the rear-view mirror form an image of P ? Checking back lights (b) Where does the rear-view mirror form an image of P ? 5.3 m behind the rear-view mirror. 1.3 m X 12.2 Reflection of light

Example 3 Checking back lights (c) Through reflections by the rear-view mirror and the shop window, the driver can see the number plate at the back of his car. Are the numbers laterally inverted? Since the number plate is laterally inverted twice, it looks normal in the rear-view mirror. 12.2 Reflection of light

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