1. Chapter 13 Light and Reflection
Ms. Hanan Anabusi

2. 13-2 Flat Mirrors Objectives:
Distinguish between specular and diffuse reflection of light.
Apply the law of reflection for flat mirrors.
Describe the nature of images formed by flat mirrors.

3. Reflection of light
Reflection – the turning back of an electromagnetic wave at the surface of a substance.
Most substances absorb at least some of the incoming light and reflect the rest.
A mirror reflects almost all of incoming light.

4. Angle of incidence: the angle between a ray that strikes a surface and the normal to that surface at the point of contact
Angle of reflection: the angle formed by the line normal to a surface and the direction in which a reflected ray moves.

5. Laws of Reflection According to the Laws of Reflection,
angle of incidence = angle of reflection (θi = θr )
Incident light ray
Reflected light ray
Normal θi θr

6. Notice the difference between a smooth and rough surface and how the light rays are reflected.
Angle of incidence (qi) and angle of reflection (qr) angles are equal.

7. Specular and Diffuse Reflection

8. Notation for Mirrors and Lenses
The object distance is the distance from the object to the mirror or lens
Denoted by p
The image distance is the distance from the image to the mirror or lens
Denoted by q
The lateral magnification of the mirror or lens is the ratio of the image height to the object height
Denoted by M

9. Images
Images are always located by extending diverging rays back to a point at which they intersect
Images are located either at a point from which the rays of light actually diverge or at a point from which they appear to diverge

10. Types of Images
A real image is formed when light rays pass through and diverge from the image point
Real images can be displayed on screens
A virtual image is formed when light rays do not pass through the image point but only appear to diverge from that point
Virtual images cannot be displayed on screens

11. Images Formed by Flat Mirrors
Simplest possible mirror
Light rays leave the source and are reflected from the mirror
Point I is called the image of the object at point O
The image is virtual

12. Images Formed by Flat Mirrors, 2
A flat mirror always produces a virtual image
Geometry can be used to determine the properties of the image
There are an infinite number of choices of direction in which light rays could leave each point on the object
Two rays are needed to determine where an image is formed

13. Images Formed by Flat Mirrors, 3
One ray starts at point P, travels to Q and reflects back on itself
Another ray follows the path PR and reflects according to the Law of Reflection
The triangles PQR and P’QR are congruent

14. Images Formed by Flat Mirrors, 4
To observe the image, the observer would trace back the two reflected rays to P'
Point P' is the point where the rays appear to have originated
The image formed by an object placed in front of a flat mirror is as far behind the mirror as the object is in front of the mirror
p = |q|

15. Lateral Magnification
Lateral magnification, M, is defined as
This is the general magnification for any type of mirror
It is also valid for images formed by lenses
Magnification does not always mean bigger, the size can either increase or decrease
M can be less than or greater than 1

16. Lateral Magnification of a Flat Mirror
The lateral magnification of a flat mirror is 1
This means that h' = h for all images

17. Reversals in a Flat Mirror
A flat mirror produces an image that has an apparent left-right reversal
For example, if you raise your right hand the image you see raises its left hand

18. Reversals, cont The reversal is not actually a left-right reversal
The reversal is actually a front-back reversal
It is caused by the light rays going forward toward the mirror and then reflecting back from it

19. Properties of the Image Formed by a Flat Mirror – Summary
The image is as far behind the mirror as the object is in front
p = |q|
The image is unmagnified
The image height is the same as the object height
h' = h and M = 1
The image is virtual
The image is upright
It has the same orientation as the object
There is a front-back reversal in the image

20. Ray Diagrams - are diagrams that use simple geometry to locate an image formed by a mirror.
Let’s try to draw a ray diagram for a pencil in front of a flat mirror. Draw p and q. Let’s use the tip of the pencil.
1st draw a ray from the tip of the pencil perpendicular to the mirror.
The angle is 0 with the normal, so qi or q’ is 0.

21. 2nd draw a ray that is not perpendicular with the mirror.
Draw in the incidence, and the reflected ray.
Remember qi = qr .
Now use dotted lines to trace both rays behind the mirror. Where these dotted lines meet is the image point, which in this case is where the image of the pencil’s tip forms.

22. Flat Mirrors Can you explain this?
To view the entire body of a person, a mirror that is half the person’s height is all that is needed.

23. Flat Mirror All you see is what reaches your eyes
You think object’s location is where rays appear to come from.
Smooth Mirror qr qi
All rays originating from peak will appear to come from same point behind mirror!
Image
Object

24. Flat Mirror
(1) Draw first ray perpendicular to mirror 0 = qi = qr
(2) Draw second ray at angle. qi = qr
(3) Lines appear to intersect a distance d behind mirror. This is the image location.
Example
Light rays don’t really converge there, so it’s a “Virtual Image”
Here can do cool light with ½ silvered mirror demo (1041)
Virtual: No light actually gets here qr qi d d

25. Flat Mirror Summary Image appears: Upright Same size
Located same distance from, but behind, mirror
Facing opposite direction: Left/Right inverted
Virtual Image: Light rays don’t actually intersect at image location.
Why do ambulances have “AMBULANCE” written backwards?

26. Assignments Class-work: Homework:
Section Review, page 454, odd questions.
Homework:
Section Review, page 454, even questions.
11-3 Properties of Waves

27. Activity 1

28. Activity 2

29. Diagram Skills – Flat Mirrors
θi = θr = 15 deg
θi = θr = 32 deg
θi = θr = 32 deg
θi = θr = 43 deg
Mirror front
Mirror front A B C

30. Diagram Skills – Flat Mirrors
θi = 15 deg
θi = 43 deg
θi = 32 deg
θi = 32 deg
Mirror front
Mirror front A B C