Investigation 20B: Reflection in a plane mirror
Essential question: How and where does an image form in a mirror?
If you look at yourself in a mirror, then where is your image located? In this investigation, you will use three different techniques for locating the image produced by a flat mirror.
Part 1: Locate an image using parallax
1. Mount a flat mirror vertically on top of a piece of graph paper and trace its location.
2. Hold a tall “object” pencil near the shiny side of the mirror. Mark its location.
3. Move a second “tracer” pencil around the back of the mirror until it always lines up with the reflected image of the “object” pencil for every angle you use to view the reflection.
a. What does the location of the "tracer" pencil signify?
b. Measure the distance from the mirror to the object and from the mirror to the image. Which is larger, or are these distances the same?
Part 2: Locate the image using pins
1. Replace the “object” pencil with a pin.
2. Look at the image of the pin and place two pins on the graph paper in line with this image.
3. Move sideways and repeat with two more pins.
4. Draw lines connecting each pair of pins to each other; extend these lines until they intersect.
a. What does each of the two lines represent?
b. Predict where the image will be located, and why.
c. Does your image location agree with the location from Part 1? Was your prediction correct?
Part 3: Trace the rays using a laser pointer and index card Safety: never look directly at a laser!
1. Mount a laser pointer horizontally using a ring stand and 90-degree rod clamp. Align the laser to point through the object pin and strike the mirror at an angle.
2. Use the edge of an index card to trace the path of the laser light (incident and reflected) directly onto the graph paper.
3. Position the laser at a different angle to the mirror and trace the new path.
a. Label the incident and reflected rays. Do the rays obey the law of reflection?
b. How can you find the image using the light paths you made?
c. Compare the laser pointer method to the other two. Which is better? Why?
Applying new knowledge
1. This figure shows a ray reflecting off a plane mirror.
a. Label the following: normal; incident ray; reflected ray; angle of incidence; and angle of reflection.
b. What is the value of the angle of incidence? Explain or show a calculation.
c. What is the value of the angle of reflection?
2. Define normal and angle of incidence.
3. A student who is 1.75 meters tall is standing 80 cm from a flat mirror. How far apart are the student and his image, and how tall is the image?
4. Locate the image of this star in the flat mirror by constructing a ray diagram.
5. In the painting below, where is the location of the reflected image of the woman's face?