1. Reflection

2. Wavefront vs. Ray
To analyze the behavior of light, we use the ray model
Ray is perpendicular to the wavefront
By the time you get far away from the source, the wavefronts are basically parallel- thus the rays are parallel

3. Wavefronts & Rays
We Use A Ray To Indicate The Direction Of Motion Of A Wave Front
The Rays Of A Circular Wavefront Are Radial Lines
The Rays of a Plane Wavefront Are Parallel Lines 3

4. Wavefronts & Rays
We Use Rays To More Easily Track The Direction Of Waves
Consider A Wave Striking A Surface At An Angle, A Process We Know As “Reflection”
It Is Much More Convenient To Refer To The Rays Associated With The Wavefronts 4

5. Reflection
Waves can reflect off a barrier
Ex- echo, mirror

6. (Angles Measured From Normal)
Law of Reflection
Angle Of Incidence (i) = Angle Of Reflection (r)
i = r
(Angles Measured From Normal) 6

7. Types of Reflection
The Law of Reflection Is Always Obeyed, Regardless of the Surface Orientation 7

8. Types of Reflection Specular Reflection Diffuse Reflection
Smooth Surfaces
Usually Produces and Image of The Source
Mirrors
Diffuse Reflection
Rough Surfaces
Scattered Light
Each Individual Ray Follows The Rule of Reflection 8

9. Specular vs. Diffuse 9

10. Plane Mirror Images Your brain assumes light travels in straight lines
You will perceive a virtual image behind the mirror
Image is:
Upright
Same size as object
Located as far behind mirror as object is in front

11. Virtual Image
The image you see appears to be behind the mirror but none of the light rays emanate from the image
Thus it is a virtual image
We use dotted lines to show rays coming from virtual images

12. Plane Mirror Images
Image will appear to have reverse orientation

13. Reverse Orientation
To see the image, I, you “dot back” from your eye through the mirror to the virtual image
Due to the law of reflection, the image appears upright but reversed left to right

14. Example- minimum length
Note how only a 1/2 length mirror is required to see your whole body
Now draw a proof to show that how far away you stand from the mirror doesn’t matter

15. Multiple Reflections
When you set 2 mirrors parallel to each other, you can see infinite images
Note that you alternately see the front and back of the coke can depending on which mirror the image initiated in

16. Right Angle Mirrors

17. Right Angle Mirrors

18. Spherical Mirrors Concave or Convex C=center of curvature
R=Radius of curvature
F= focus
f=Focal length= 1/2 R
f=R/2
Vertex= where axis intersects mirror

19. Concave Mirrors Parallel incident rays reflected through the focus
Incident ray that passes through the focus is reflected parallel to axis
Incident ray that strikes the vertex is reflected at equal angle to the axis

20. Images in concave mirrors
Can be virtual or real
Real images have light passing through them
If we hold a paper there we will see the image

21. locate image in concave mirror
You can locate an image by where these 3 rays intersect
1: runs parallel to axis until the mirror then reflects through the focus

22. locate image in concave mirror
2: runs straight through the center (C) of mirror and reflects back, never bending
3: passes through focal point (F) on way to mirror and reflects parallel to the axis

23. What can image look like?
Image can shrink and turn upside down

24. What can image look like?
Image can be magnified

25. What can image look like?
Image can be virtual- note that real rays never converge so trace them back to find virtual image

26. Concave Images Object>1 focal length in front of mirror produces:
real image
Object =1 focal length produces:
no image b/c all reflected rays are parallel so never converge
Object < 1 focal length produces:
a virtual, enlarged image
CONCAVE mirrors considered positive since the mirrored surface curves towards center

27. Diverging (convex) mirrors
F and C are located behind the mirror
Use the same 3 rays to locate the image
Now rays need to be “dotted back” to locate the image
Since the real rays diverge, all images are virtual, upright, reduced, located in front of F

28. Check:
The 3 types of mirrors we have discussed can be classified by the virtual images they form
Describe the virtual images formed by each of the following:
Plane
Concave
Convex

29. Mirror Equation
If you need to calculate the location of the image or focus of a spherical mirror:
1/so + 1/si = 1/f
Sign conventions:
+ is in front of mirror
So always +
Si is + for real image in front and - for virtual image behind
f is + for concave (F is in front of mirror)
- is behind the mirror
Si is virtual and behind
f is - for convex mirror (F is behind)

30. Magnification Equation
M = hi/ho = si/so
Often, instead of using this as a straight calculation, it is used to compare relative sizes of object and image
For example: if a problem states that the virtual image is twice as large as the object, di=-2do
Note the relationship and the sign