Experiment IV-1

Virtual Image


Success of mechanics led physicists to suspect that the universe consisted of particles in motion.  Isaac Newton and others thought that light too was a collection of particles.  Newton used that idea to explain all the properties of light known to him.

Light has a number of properties, most of which we can observe with our personal light detectors, our eyes.  Most obvious, light travels in straight lines.  We see this as the cause of shadows.  We use this in countless ways from our visual recognition of shapes to surveying our lands. 

But there are a few ways that light can change its direction.  Unfortunately the path of light is not always obvious.  Ray optics is a study of the behavior of light by tracing the paths of beams of light.  It assumes that light always travels in straight lines except when reflected, refracted, diffracted, or scattered.  We are familiar with reflection from mirrors and other shiny objects.  But at very small, glazing angles, a great many materials reflect.

In this experiment we will develop the rules of ray optics for reflection.  Once understood, these rules can be applied to more complicated situations such as curved mirrors and broadened to help understand other properties of light.


In this experiment we wish to trace reflected light rays in order to locate a virtual image.  You will need
  1. a plane (i.e, flat) mirror,
  2. protractor to measure angles,
  3. a small object to look at such as a small toy, chess piece, or cap to a pen
  4. a clean piece of paper for recording locations and drawing rays, and
  5. a way of marking locations.  You might want to use several nails that will stand on their flat heads.  An alternate would be to use pins and a material under the paper that could hold pins upright.
mirror Directions:
  1. Place the mirror in the middle of the paper.  Verify that the mirror is perpendicular to the paper.  The edges of the paper viewed in the mirror should appear as straight extensions of the edges of the real paper.
  2. Determine the reflecting surface of the mirror and accurately mark its location with a line on the paper.  Many mirrors are glass with reflecting silver on the back side (and covered in black paint) while a few have reflecting front surfaces.  If you touch the front of the mirror with a pencil point or other object, a visible gap between the real point and the reflected point indicates the reflection is NOT the front surface.reflection
  3. Locate the object in front of the mirror as shown in diagram 1.  Accurately mark its location.
  4. Holding you head steady and using one eye (closing the other) located so you can see the image of the object in the mirror, align two vertical markers as far apart as possible in front of the mirror so that the markers are exactly between the image and your eye.  See diagram 2.  Draw on the paper a ray from the mirror, passing through the marked points towards you eye.  This should be part of the route the light took getting from the mirror to your eye at this location.
  5. Temporarily remove the mirror.  Extend the straight ray behind the mirror to eventually help located the image.
  6. Draw a line segment from the object to the location where the first light ray intersects the line marking the reflected surface.  See diagram 3.  This completes the ray diagram for the light when your eye was viewing the object in the mirror.  The light ray started at the object, travelled in the straight line towards the mirror, then reflected to your eye.
  7. But many light rays start from the object going in other directions.  Replace the mirror precisely and choose a new location for your eye.  Repeat steps 4 through 6.  Do this for at least a half dozen eye positions.

  8. Use a protractor to construct a perpendicular at each point where a light ray reflects off the mirror.  See diagram 4.  Measure the angle between the perpendicular and the ray the light approached the mirror, called the angle of incidence, and the angle between the perpendicular and the ray leaving the mirror, the angle of reflection.  How does the angle of incidence compare with the angle of reflection?

  9. Notice all the rays that were extended behind the mirror.  Do they all cross at a single location, or nearly so?  This location is the virtual image.  Place a marker at the location of the virtual image, then move your head so you can just see the TOP of the marker over the top of the mirror.  If you found the virtual image correctly, the bottom of the image should be visible just below in the mirror.  Move your head from side to side noting that the virtual image marker stays aligned with the image.

  10. Measure the distance from the virtual image to the mirror and compare that to the distance of the object from the mirror.

The process of locating an image by viewing it from several perspectives is called parallax.  Those of us creatures with two eyes in the front of our heads use parallax to help determine distance to objects, apparently an advantage to predators and animals who live in the trees.  Those animals with eyes on opposite sides of their head have the advantage of nearly 360° vision, but do without effectively determining distances, an advantage for grazing animals who must stay away from predators.

Note that a virtual image SEEMS to be at the location we determined because all the light rays to the various eye observation locations SEEM to originate here.  But in fact none of the light rays actually originated or passed through this location, so the image is VIRTUAL, not REAL.  If you were to place a screen at this location, nothing would appear on the screen.  Real images are created by light actually passing their location so that the image may be shown on a screen.

Record your observations recorded in your journal.  If you need course credit, use the information in your journal to construct a formal report.


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created 1/21/2003
latest revision 1/23/2003
by D Trapp
Mac made