2. What do we know already?

3. Light always travels in a straight line
When Light hits a surface it does one of two things
Get absorbed by the surface
Reflect off the surface
Reflection – the change in direction of an EM wave at a surface that causes it to move away from the surface.

4. Reflection Depends on the Surface
A ray of light striking a highly reflective surface such as a mirror and reflecting
A ray of light striking a rough surface and being scattered and not reflected

5. Three Main Types of Mirrors
Flat Concave Convex

6. Flat Mirrors θi θr normal Incident ray Reflected ray
Light strikes the surface at an angle
Θi = incident angle: the angle between a ray that strikes a surface and the line perpendicular to that surface at point of contact
Θr = reflected angle: the angle formed by the line perpendicular to a surface and the direction in which a reflected ray moves

7. Θi = θr Angle of incoming light ray = angle of reflected light ray
Law of Reflection θi θr
Θi = θr
Angle of incoming light ray = angle of reflected light ray

8. Why we see an image behind the mirror
What is actually happening:
Light is reflected off the mirror by the Law of Reflection:
θi = 0° = θr
What we see:
Our brain cannot see light bending: It assumes that the reflected ray is traveling directly from something at the spot of the image
Incident Ray
Reflected Ray
The “unbent” ray that our brain makes up h h’
mirror
object
image

9. You can predict Image location by use of Ray Diagramsq p h h’
mirror
object
image

10. Who Can See Who in a Flat Mirror?
Make a mark where the image would be located
Draw a straight line from a person to another’s image
If this line passes through the mirror, they can see each other A B C
A can see B
B can see A
But C can see neither A nor B

11. Curved Mirrors
Concave Convex

12. The Set Up for Concave Mirrors
Object
Principle Axis C F
f : focal length
R: Radius
p = object distance

13. Focal Length (and Focal Point)
Positive for Concave Mirror
Negative for Convex Mirror
Focal Point – The point where every ray that hits the mirror will pass through
There can be no image formed when an object is placed at the focal point
**all rays will

14. How is an Image Formed?
An image is formed when reflected light rays intersect at a common point
If rays never intersect (parallel to each other), no image is formed

15. Two Rules of Note! Why so important????
Any ray that is originally traveling parallel to the principal axis will reflect through the focal point
Any ray that originally travels through the focal point will reflect parallel to the principal axis
Why so important????

16. Rules for Drawing Ray DiagramsC f
1. Draw a parallel ray from the object to the mirror and reflect through focal point
2. Draw a ray from the object through the focal point to the mirror and reflect back parallel to principal axis
3. Draw a ray from the object to the mirror through the center of curvature and reflect back along the same line (usually not used)

17. The point where all three lines meet is where your image will be located
Try this one!
An object is placed 10 cm from a concave mirror with a focal length of 6 cm
**Use a ruler and draw your picture to scale**

18. Convex Mirror Set Up c f
Focal point is NEGATIVE

19. Imaging with a Convex Mirror
Tiny image formed
1) Parallel then through focal point c f
2) Through focal point then parallel
Regardless of where an object is placed…
A Convex Mirror will always produce:
SMALLER, UPRIGHT image

20. Real vs. Virtual Image
Virtual image: refers to an image that only “appears” to be behind the surface of the mirror. The image is not located at this apparent position and can’t be focused on a screen.
Located on the opposite side of the mirror as the object
Real Image: refers to an image that can be focused on a screen placed at a position where rays from a point on an object pass through a common point
Located on same side of the mirror as the object

21. Mirror Equation f = focal length do = object distance
di = image distance
*it does not matter the units for this equation
HOWEVER all distances must be in the SAME units

22. Magnification Equation
hi = Image height
ho = object height
What the sign of the magnification means:
negative
Inverted image
positive
Upright Image

23. Practice Problem!
A 4.00-cm tall light bulb is placed a distance of 45.7 cm from a concave mirror having a focal length of 15.2 cm. Determine the image distance and the image size.
Info we Know:

24. Sign Conventions f is (+) if the mirror is a concave mirror
f is (-) if the mirror is a convex mirror
di is (+) if the image is a real image and located on the object's side of the mirror.
di is (-) if the image is a virtual image and located behind the mirror.
hi is (+) if the image is an upright image
(and therefore, also virtual)
hi is (-) if the image an inverted image
(and therefore, also real)