Reflection Lab (Day 1)

If you missed today in 2012-2013 or 2013-2014 (note big picture at the bottom of the page!)

We had students pick a new partner for this two day lab on reflection.

The Reflection Lab handout

We had one group of two students volunteer to show all the other groups how to do part 1 (using two light paths through the same point [labeled object] reflected off a 1st surface mirror [the only correct reflecting side  can be found by putting your pencil tip against it and if the image of the pencil tip touches the object tip you have the correct side] , then tracing these reflected rays back to where they intersect at the image.

Then another group demonstrated part 2 by drawing a scalene triangle ABC with no side parallel to the mirror or against the mirror (draw the triangle back 5 cm or more) then using the two way mirror (I call it the cheating mirror) locate the images A'B'C', remove the 2 way mirror, and draw triangle A'B'C' measuring the sides of both triangles and joining A to A'; B to B'; and C to C' measuring all three distance of objects A,B, and C from the mirror and all three image distances A',B', and C' from mirror (12 measurements total on diagram) then noting if you go alphabetically A,B,C around the original triangle if it is clockwise or counterclockwise and it you do the same with A'B'C' if it is clockwise or counterclockwise.

Part 3 tomorrow


Lab due next Wednesday.

Big Picture:  Switch back and forth between reading the below and looking at this copy of my sketch #1 done in a class during the 2013-14 school year.

After studying the sketch above note that you realize that an infinite number of light rays reflect off a given object (another way to say it is that you can draw an infinite number of lines through a given point).  So if we follow just two of these rays of light to the mirror (as we only have two eyes for receivers of these light rays) and trace their diverging reflection straight in back of the mirror (as the brain always take the incoming rays of light straight back); the rays of light traced back with intersect at the image point of the object point.
The purpose of part 1 is three fold:
1).  To show how our brain uses parallel (the fact that are eyes are separated thus when the brain takes the diverging rays straight in back of the mirror, the brain tells us that the light source of these two rays originated at this image point).
2).  To show that the image distance equals the object distance (distance always means the shortest distance thus the perpendicular distance to the mirror from each of the two points).
3).  Here we have two chances to show that the angle of incidence (between the incoming ray of light and its normal to the mirror) equals the angle of reflection.

Big Picture for Part 2.  Note the 2013-2014 sketch part 2 example done during on of the classes this school year.
Note that a scalene triangle was drawn on the front side of the mirror so that no side was parallel to the mirror and then the two sided mirror was carefully placed along the mirror line so both the front side was along the mirror line and the images were marked (or at least one of the images) and then the back side of the mirror was placed on the mirror line and the images were marked a second time (or at least the same one image point was marked) then the image(s) closest to being the same distance in back of the mirror as the object points were in front of the mirror were kept.  This makes sure the correct side of the mirror was along the mirror line in case you could not tell which side is half silvered to be the side on the mirror line.
There are three purposes to sketch #2 also:
1).  The triangles can be shown to be congruent using Side Side Side (three sides of the object triangle are congruent to the corresponding sides of the image triangle)
2).  The object triangle is the same distance from the mirror as the image triangle.
3).  If you following the vertices A, B, and C on the object triangle, you'll notice that there images A', B', and C' are reverse orientation thus an image always has reverse orientation of the object for a plane mirror!