1. Plane Reflection – Learning Outcomes
Use reflection keywords.
Demonstrate the laws of reflection.
Discuss images formed in plane mirrors.
Differentiate between real and virtual images.
Use ray tracing to locate an image in a plane mirror.
Discuss uses of plane mirrors.
Locate an image by no parallax.

2. Use Reflection Keywords
If a surface is rough, light scatters in all directions and it can be seen from any direction.
This applies to most objects.
This is called diffuse reflection.
by Jeff Dahl – CC-BY-SA-3.0

3. Use Reflection Keywords
If a surface is smooth, light reflects only according to certain laws.
These objects are called mirrors.
This is called regular reflection.
We discuss regular reflection almost exclusively and usually don’t bother saying “regular”.

4. Demonstrate the Laws of Reflection
The incident ray, the normal at the point of incidence, and the reflected ray all lie in the same plane.
The angle of incidence is equal to the angle of reflection. (𝑖=𝑟).

5. Demonstrate the Laws of Reflection
Place a plane mirror upright on a sheet of paper and mark its position.
Aim a ray box with a narrowed beam at the mirror.
Mark its path on the paper before and after striking the mirror.
Remove the mirror and laser.
Draw the incident ray, the normal at the point of incidence, and the reflected ray on the paper.
These all lie in a plane parallel to the paper, verifying the first law.
Measure the angles of incidence and reflection.
These angles are equal, verifying the second law.

6. Use Ray Tracing to Locate an Image in a Plane Mirror
Images in plane mirrors are virtual: – they are formed by the apparent intersection of rays.
Note: we say the image is “behind the mirror”, not “in” it.

7. Shortest Mirror
Standing 1 m from a mirror, what is the shortest the mirror can be so that you can see your whole body?
Draw a diagram.

8. Discuss Uses of Plane Mirrors
Bathroom mirrors are plane – they can be used to check appearance.
Periscopes use two plane mirrors to allow you to see over or around objects.

9. Take a Step Back
Imagine you are standing in front of a mirror just tall enough that you can see from the top of your head to your knees.
When you take a step back, you can see:
Less of yourself
More of yourself
The same amount of yourself

10. Locate an Image by No Parallax
Parallax is a difference in the apparent position of an object due to the position of an observer.
Draw a mark on a nearby wall.
Place a metre stick vertically on your table.
Close one eye, and move the metre stick and your head until the metre stick blocks the mark.
Keeping the metre stick in place, move your head until the mark appears again.
This is parallax – moving your head appears to move the metre stick from in front of the mark.

11. Locate an Image by No Parallax

12. Locate an Image by No Parallax
Stand a plane mirror upright on a sheet of paper on a flat surface.
Stand an “object” pencil upright near the mirror, keeping it in place with blu-tack.
Looking into the mirror, move a second “search” pencil behind the mirror and align it with the image of the first pencil.
By moving your head and the search pencil, find where there is no parallax between the image and the search pencil.
Mark the position of the object and search pencils, and the plane mirror on the paper.
Measure and compare the distances from the plane mirror to the object pencil and the image.

13. Locate and Image by No Parallax