1. Law of Reflection & Plane Mirrors
PW2 - Reflection

2. Recall the Law of Reflection
The angle of incidence is equal to the angle of reflection.
The incident ray, reflected ray, and normal all line in the same plane.

3. Recall the Law of Reflection
incident ray - a ray approaching the surface
point of incidence - the point where the incident ray strikes the surface.
normal - a perpendicular constructed where the incident ray strikes the surface.
reflected ray - a ray that leaves the surface at the point of incidence.
angle of incidence - the angle between the incident ray and the normal.
angle of reflection - the angle between the reflected ray and the normal.

4. Types of Reflection Specular reflection: Diffuse reflection:
is when all the rays in a beam of light are reflected at the same angle
occurs in smooth surfaces (i.e.: mirrors).
Diffuse reflection:
occurs when the rays in a beam of light are scattered off a surface.
occurs in rough surfaces (which is much more common)

5. Types of Reflection
This is why you see increased glare when driving in the rain or icy conditions.
For the purposes of this unit, we will focus on REGULAR REFLECTION

6. Plane Mirrors Plane Mirror:
a flat smooth surface that reflects light in a regular or specular way.
Most common, found in bathrooms, dressing rooms, automobiles, changing rooms, gyms, dance studios, 1970’s living rooms…

7. Plane Mirrors – Describing Images
Virtual Images:
The images formed in a plane mirror appear to be behind the mirror. This means that the light appears to be coming from behind the mirror.
We call images formed in this fashion virtual images. A virtual image is an image that light does not pass through, but only appears to pass through.

8. Plane Mirrors – Describing Images
The four (4)characteristics of images in a plane mirror are:
1) Same Size as object
The dimensions of the image are equal to the dimensions of the object. Magnification is 1.0.
If a 1.6 metre tall person stands in front of a mirror they will see an image that is 1.6 m tall.
2) Erect (upright)
3) Virtual (behind the mirror and cannot be projected on a screen)
The object distance, do, is equal to the image distance, di, and the image is directly across from the object (perpendicular to mirror). do = di
4) Laterally Inverted (right side of object is left side of image)

9. Finding Images – Point Object
Draw a perpendicular line from the object to the mirror.
Extend the perpendicular line behind the mirror.
Measure the perpendicular distance (do) from the object to the mirror.
Make the perpendicular distance to the image (di) equal to do.

10. Finding Images – How Eyes See a point Object
Draw a perpendicular line from the object to the mirror.
Extend the perpendicular line behind the mirror.
Find the image using do = di.

11. Finding Images – How Eyes See a point Object
Draw a straight line from the image to the top of the eye. (Be sure to use a dotted line one the image side of the mirror and a solid line on the object side of the mirror.)
Draw another line from the image to the bottom of the eye.
Where the “image to eye lines” strike the mirror, draw straight lines from the mirror back to the object.
Finally, draw arrows on the “object to eye ray lines” indicating the direction that light travels.

12. Finding Images – How Eyes See Large objects
Draw perpendicular lines from the top and bottom of the object to the mirror.
Extend the perpendicular lines behind the mirror.
Find the image using do = di. Use dotted lines to draw the image.

13. Finding Images – How Eyes See Large objects
Draw a straight line from the top of the image to the top of the eye. Be sure to use a dotted line on the image side of the mirror and a solid line on the object side of the mirror.
Draw another line from the bottom of the image to the bottom of the eye.

14. Finding Images – How Eyes See Large objects
Where the “image to eye lines” strike the mirror, draw straight lines from the mirror back to the object.
Finally, draw arrows on the “object to eye ray lines” indicating the direction that light travels.

15. Tips for Drawing Ray Diagrams
Use a ruler
Use a protractor
Use a pencil
For rays of light, always indicate the direction by using arrows
Label the mirror clearly, or add thatched lines
For rays behind the mirror, used a dotted line
Recall that the angle of incidence is the angle between the normal and the incident ray, not the mirror!
Likewise, the angle of reflection is the angle between the normal and the reflected ray, not the mirror!