By Mike Isley
Product Development Department
Do you want to stimulate some critical thinking in your classroom? Tell your class that we do not see living or inanimate things; we only see the light reflected from those things. This is a perfect introduction to the topic of mirrors. After discussing the validity of the statement, ask your students what they would see if the classroom were totally dark. Remind them that no thing can be seen unless it is luminous (emitting light) or being illuminated (reflecting light). A mirror reflects light, producing a clear image of the object that is in front of it. Then why are not all objects mirrors? Because not all surfaces reflect light in parallel straight lines. Rough surfaces can reflect a lot of light, but no image. This is because the reflected rays travel in random straight lines. A smooth surface reflects light in parallel straight lines that produce a clear image of an object.
Clear, still water was one of the earliest mirrors. The Egyptians invented hand mirrors of polished metal around 1500 BC. Today, flat mirrors are made from plate glass with a thin reflective layer of aluminum or silver sprayed or condensed on the back. Mirrors come in many shapes and sizes.
In Part 1, I discuss the optical characteristics of plane (flat) mirrors. This discussion is followed by a simple demonstration and a lab activity with an extension. In Part 2, to be published next month, I will discuss concave and convex mirrors with a simple demonstration and a brief lab activity following each discussion. These activities are designed for students in middle school through college.
A real image is formed by light reflected from an object. A virtual image is formed by light reflected from the surface of a mirror. A virtual image itself never reflects any light, but for the observer it seems to be formed behind the mirror as the reflected light of the object returns from the mirror’s surface. The virtual image in a plane mirror is an upright, left-to-right reversal of the object’s real image and always appears to be behind the mirror’s surface.
Demonstration: Images in plane mirrors
Students can prove that all images from a plane mirror are virtual ones. Ask a student to hold up his or her right hand in front of a plane mirror. The result is similar to the “mirror image” our hands make when they are placed together palm to palm. Have the student then move his or her hand toward the mirror and then away from it. The virtual image remains the same size as the student’s hand and appears to be at the same distance behind the mirror as the student’s hand is in front of the mirror.
The virtual image in a plane mirror is an upright, left-to-right reversal of the reflected object’s real image. This can be demonstrated by having the student write the word “bib” on a piece of paper and then holding it up to the mirror. The word “bib” becomes “did” in the mirror. Not only is the order of the letters reversed, but also the orientation. Emergency vehicles sometimes have their names written upright and reversed on their hoods. Why?
Lab activity 1: Locating the virtual image behind a plane mirror.
- Sheet of copier paper
- Piece of corrugated cardboard (8½ X 11")
- 5 straight pins
- Metric ruler
- Transparent tape
- Rectangular mirror
- Large book
Note: Refer to the photo below as needed when placing the pins.
- Tape the copier paper to the corrugated cardboard at the corners.
- Using your ruler, draw a line that divides the paper into 2 equal sections. This is the “mirror” line.
- Place the edge of the book just behind the line and center the long side of the mirror on the line by propping it against the book.
- Locate the center of the mirror and insert a pin into the paper 5 cm from the mirror. Draw a circle around the pin and label it “object.”
- Looking from the left front corner of the paper, close one eye and sight toward the pin image in the mirror. Take another pin and line it up vertically with the image in the mirror at around 5 cm from the mirror. Insert the pin when the pin and the image in the mirror appear to be one. Draw a circle around the pin and label it “1.”
- Using another pin, sight with one eye and line this pin up with the “1” pin and the image in the mirror. Insert it 5 cm away from the “1” pin (towards you). Draw a circle around the pin and label it with a “1” also.
- Looking from the right front corner of the paper, repeat the sighting procedure as described in steps 5 and 6 and place 2 pins on the right side at the same distances. Again, try to make both inserted pins coincide with the object image in the mirror. Draw a circle around each pin and label each “2.”
- Remove the mirror, book, and pins. Take the ruler, line up he two “1” circles on the left, and draw a straight line extending behind the “mirror” line. This is the “1” line.
- Line up the two “2” circles and draw a straight line extending behind the “mirror” line until it intersects with the “1” line. This is the “2” line.
- The intersection of the 2 lines behind the “mirror” line represents the position for the virtual image of the “object” pin that was placed in front of the mirror. Measure the distance in centimeters from the “object” circle to the “mirror” line and measure the distance from the virtual image (where the “1” and “2” lines intersect) to the “mirror” line. Theoretically, they should be the same distance. This illustrates why a person’s image in a mirror always seems to be at the same distance behind the mirror as the person is in front of the mirror.
Lab activity 1 extension: Ray diagrams for a plane mirror
- The same sheet of copier paper used in Lab activity 1
- Colored pen or pencil
- Draw a straight line on the copier paper from the “object” circle to the point where the “1” line meets the “mirror” line. On this line mark forward arrows (>) from the “object” circle to the “mirror” line and from the “mirror” line back to the beginning of the “1” line.
- Repeat step 1, except start from the “object” circle and proceed to the point where the “2” line meets the “mirror” line. Mark these lines with forward arrows starting from the “object” circle to the “mirror” line and from the “mirror” line back to the beginning of the “2” line.
- Looking at the ray diagrams that you have drawn, answer the following questions: How did the light travel from the object to your eye as you sighted from each corner of the paper? and Why did the image appear to be located behind the mirror?