1. Optics 光學
1. Reflection of Light

2. Reflection (反射)

3. Light rays
Light is a source of energy.
We detect it with eyes.

4. Experiment : Reflection by plane mirrors
Aims:
Study the reflection of light rays by a plane mirror and also find out how to locate the image formed by a plane mirror.

6. Experiment 1.1 Law of Reflection
1. Direct a ray of light at a plane mirror as shown.
Ray-box
normal
Plane mirror

7. Experiment 1.1 Law of Reflection
1. Direct a ray of light at a plane mirror as shown.
Angle of incidence (入射角) i of the light ray
Angle of reflection (反射角) r

8. Experiment 1.1 Law of Reflection
1. Direct a ray of light at a plane mirror as shown.
2. Measure the angle of incidence and the angle of reflection.

9. Experiment 1.1 Law of Reflection
1. Direct a ray of light at a plane mirror as shown.
2. Measure the angle of incidence and the angle of reflection.
3. Repeat several times by changing the anlge of incidence.

10. 3. Repeat by changing the angle of incidence. Record the results.
Angle of incidence i 0
10
20
30
40
50
Angle of reflection r
How is the angle of incidence related to the angle of reflection?
___________________________________________________________

11. How is the angle of incidence related to the angle of reflection?

12. The angle of reflection is equal to the angle of incidence.

13. Laws of reflection : There are two laws of reflection
1. The incident ray, the reflected ray and the normal all lie in the same plane. This means that all three can be drawn on a flat sheet of paper
2. The angle of incident (i) is equal to the angle of reflection (r). The ray is reflected from the mirror at the same angle as it arrives.

14. Example 1.
(a) What is the angle of incidence?
The angle of incidence = 40°
Plane mirror
(b) Complete the following ray diagram by drawing all the reflected rays.
50°
50°
40°
(c) What is the relation between the original incident ray and final reflected ray
40°
50°
50°
Plane mirror
They are parallel.

15. Arrange light rays to hit the plane mirror as shown in figure
Arrange light rays to hit the plane mirror as shown in figure. Study how the rays are reflected. Look at the rays from above. Also bend down and look along the rays
behind
The reflected rays seem to come from a point __________ (in front of / behind) the mirror.

23. reflection abc perpendicular Conclusion:
1. In reflection, the angle of incidence is equal to the angle of _________________.
2. An image formed by a plane mirror is as far behind the mirror as the object is ____________.
3. The line joining the object and the image is _____________ to the mirror.
reflection
abc
perpendicular

24. Laws of reflection

26. Object
Image

27. Object
Image

28. Object
Image

29. Complete the rays and located the image
Object
Image

30. A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 m
Above the ground.
What is the minimum length of a mirror that allows him to see his whole body?
At what height should the mirror be hung?
Eye
Man
Image

31. A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 m
Above the ground.
What is the minimum length of a mirror that allows him to see his whole body?
At what height should the mirror be hung?
Eye
Man
Image

32. A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 m
Above the ground.
What is the minimum length of a mirror that allows him to see his whole body?
At what height should the mirror be hung?
Eye
Man
Image

33. There are two similar triangles
A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 m
Above the ground.
What is the minimum length of a mirror that allows him to see his whole body?
At what height should the mirror be hung?
Image
0.9m
1.8m
(a) Minimum length = 0.9m
There are two similar triangles

34. A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 m
Above the ground.
What is the minimum length of a mirror that allows him to see his whole body?
At what height should the mirror be hung?
Eye
Man
Image

35. A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 m
Above the ground.
What is the minimum length of a mirror that allows him to see his whole body?
At what height should the mirror be hung?
Eye
Man
Image

36. There are two similar triangles
A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 m
Above the ground.
What is the minimum length of a mirror that allows him to see his whole body?
At what height should the mirror be hung?
Eye
Man
There are two similar triangles
(b) It should 0.85 above the ground
1.7m
0.85m

37. Object

38. Uses of plane mirrors
To help use look at ourselves,
To make a room look bigger,
In periscopes