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					          Seeing Things
Viewing Images in Plane Mirrors
When we look in the mirror, we see an image of the
object.
In the previous slide show, you saw that the image
appears to be the same distance behind the mirror as
the object is in front of it.
You also saw that a line connecting the object and
image intersects the mirror at 90 °.
The image appears to be behind the mirror. But the
light rays cannot come through the mirror since
mirrors are opaque.
         Question 2)
How can the light rays appear to go
    through an opaque mirror?
Get this graphic from your notes .
Place a MIRA or Reflect View on it as shown .
Place a MIRA or Reflect View on it as shown .
Place a MIRA or Reflect View on it as shown .
Notice that you can see an image in the “mirror”.
If you look over the top, there is no image behind
the mirror. The light rays must come from the
object and reflect off the mirror . How do they do
this?
On the graphic in your notes, the back of the
mirror is identified by diagonal lines. This is where
light is reflected on glass mirrors.
First, find the location of the image by drawing a
line from the top of the object through the mirror
so that it intersects at 90°.
Measure the distance from the top of the object
to the mirror (dO).
Copy this distance (dOtop) to the other side of the
mirror. It becomes (dItop).
Repeat this process for the bottom of the object.
These two points give the location of the top and
bottom of the image. For any point dO = dI.
To make the diagram less cluttered, I’ve removed
the unneeded part of the dotted line
Rays of light must be entering the eye as if they
had come straight from the image. That is the way
the eye sees things.
The line is dotted behind the mirror because light
rays cannot go through an opaque mirror. They
travel in this direction but could not come from
the image’s location.
The light rays that appear to come from the top of
the image really came from the top of the object
and reflect off the mirror into the eye.
The light rays that appear to come from the
bottom of the image really came from the bottom
of the object and reflect off the mirror into the
eye.
The light rays reflect most strongly off the back
of glass mirrors. A metallic coating applied to the
back of the glass causes the reflection.
Reality Check
Use a MIRA or Reflect View to see if the predicted
image that you drew and the real image overlap.
If they do, the light rays must reflect as predicted.
Your Turn
Answer the questions in your notes.
Your Turn
Answer the questions in your notes.
Question answers follow.
Your Turn
3) In your notes, show how light is reflected
   from the mirror so that the eye sees the
   image behind it.
A) should look like this. Try B).
B) should look like this. Try C).
C) should look like this. Try D).
D) should look like this. Try the next questions.
3) Which Eye-Brains can see the object?
3) Which Eye-Brains can see the object?
  C) and D) only. Light travels in a straight line
  from the object to the Eye-Brain. Rectilinear
  Propagation is the term for this.
4) Which Eye-Brains can see the image in the mirror?
4) Which Eye-Brains can see the image in the mirror?

   A) C) and D) only. Light appears to travel in a
   straight line from the image to the Eye-Brain.
   Rectilinear Propagation is the term for this.
Full Height Mirror
4) How big does a mirror have to be in order
   to see yourself from “Head to Toe”? (Hint:
   You see your image.)
As usual, find the location of the Image by
measuring the perpendicular distance from the
object to the mirror.
Copy this distance to the other side of the mirror.
This gives the location of the image.
Draw a ray of light entering the eye as if it had
come straight from the top of the image.
This light ray really came from the top of the
object.
Draw a ray of light entering the eye as if it had
come straight from the bottom of the image.
This light ray really came from the bottom of the
object and reflected off the mirror.
The large orange triangle has a height which is the
distance from the object to the image (dI+dO = 2dO ).

Its base is the height of the person (two arrows).
The smaller light orange triangle has a height which is
the distance from the object to the mirror (1 dO ) or
half the height of the larger triangle.

From similar triangles, the size of the mirror is half
the height of the person.
8) Will the size of the mirror be affected by
   how far away you stand, if you still want to
   see yourself “Head to Toe”?
As usual, find the location of the image by
measuring the perpendicular distance from the
object to the mirror.
Copy this distance to the other side of the mirror.
This gives the location of the image.
Draw a ray of light entering the eye as if it had
come straight from the top of the image.
This light ray really came from the top of the
object.
Draw a ray of light entering the eye as if it had
come straight from the bottom of the image.
This light ray really came from the bottom of the
object and reflected off the mirror.
The large orange triangle has a height which is the
distance from the object to the image (dI+dO = 2dO ).

Its base is the height of the person (two arrows).
As before, the smaller light orange triangle has a
height equal to the distance from the object to the
mirror (1 dO ) or half the height of the larger triangle.

From similar triangles, the size of the mirror is half
the height of the person.
So Where is the Image - Really
Below are two eye charts. One is inverted so that it is
easily read in a mirror; the other is normal.

Each line is exactly half the size of the line above it.
An Inverted Eye Chart is held. Its reflection is
viewed in a mirror.




9) How difficult will it be to read the image of an
   Eye Chart held by the student? Explain your
   choice.
a) Would it be as difficult as reading an Eye Chart
   held half the distance to the mirror?
b) Would it be as difficult as reading an Eye Chart
   held at the distance of the mirror?
c) Would it be as difficult as reading an Eye Chart
   held twice the distance to the mirror?
Answer and Explanation follow.
Before looking at the theoretical (academic)
         solution, do the group lab.
The Image of an Eye Chart held by a student would
be as far behind the mirror as the student is in
front.
Therefore, it would be as difficult as reading an
Eye Chart held twice the distance to the mirror.
10) If you can just read the middle line of
    an Eye Chart taped onto a mirror,
   which line of the same sized Eye Chart
   would you be able to read when viewed in
   a mirror?
   Would it be
   a) one line up (2X’s larger),
   b) the same line,
   c) one line down (smaller by half)?
The size of the image viewed in a mirror is
determined by the angle between the rays coming
from the top and bottom of the Eye Chart image.

This angle is the same as that of an Eye Chart one
half as high as the mirror. The lines would all be
shrunken by one half.

Therefore, you would only be able to read the
larger lines (one up or 2X’s larger, on the image).

				
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posted:4/25/2011
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