1. 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

2. 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

3. 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

4. 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

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

6. 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

7. 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

8. 1 Laws of reflection
12.2 Reflection of light

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

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

17. 3 How a plane mirror forms an image
Rays from object O become divergent after reflection.
mirror
12.2 Reflection of light

18. 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

19. 3 How a plane mirror forms an image
Simulation
12.2 Reflection of light

20. 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

21. 3 How a plane mirror forms an image
Simulation
12.2 Reflection of light

22. 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

23. 3 How a plane mirror forms an image
12.2 Reflection of light

24. 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

25. 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

26. 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

27. 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

28. 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

29. The End
12.2 Reflection of light

30. 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

31. 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

32. Return
12.2 Reflection of light

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. Return
12.2 Reflection of light

40. 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

41. 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

42. 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?
= 5.3 m X
12.2 Reflection of light

43. 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

44. 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

45. Return
12.2 Reflection of light