Reflection in Plane MirrorsReflection in Plane Mirrors

Objectives: Investigate reflection in plane mirrors Verify the “1st Law of Reflection” Compare image location to object and mirror locations

Equipment: paper, small plane mirrors, mirror holders, pin, ruler, protractor, cardboard, laser pointer

PART 1: MIRROR IMAGES: ENGAGE ACTIVITIES

A. Carefully poke your pen or pencil through an index card so that you cannot see it on the paper as you write. Try to write your name on your paper so that its image appears “correct” in a mirror placed on the paper above it. For younger students, have them look in the mirror and try to write their names so that they appear “normal” on their papers.

B. Use 3-4 small mirrors, a laser, and a target. Each group attempts to shine the laser beam so that it reflects from mirror to mirror until finally hitting the target. The group with the greatest total distance the beam travels before hitting the target is the winner. One student holds laser, one student measures the distances, and the other students hold mirrors. All students participate.

C. Play the game at http://www.phy.ntnu.edu.tw/oldjava/optics/mirrorgame_e.html.

D. Place a plane mirror on a wall. Line up across the room so that some students are directly in front of the mirror and some are not. Each student should look at the mirror and see how many and which other students’ reflections they can see in the mirror.

Name: _____________________________________ Woodrow Wilson

Summer 2010

PART 2: INCIDENT AND REFLECTED RAYS

1. Place the mirror on the mirror line shown below so that its rear reflecting surface is on the line and the mirror is perfectly vertical.2. Use the laser pointer to shine a beam of light from the Source Point so that its reflected beam hits Target A. 3. Use your ruler to sketch the rays of light passing from the Source Point, to the mirror, and then through Target A. Be sure to place arrowheads on the rays to indicate the directions the laser beams were traveling.4. Repeat this process for Targets B, C, and D.

What do you notice about the locations in which the incident rays hit the mirror in relation to the source and targets?

SourcePoint

Target A Target B Target C Target D

mirror

PART 3: CONSTRUCTING SIGHT LINES TO LOCATE AN IMAGE

Procedure:1. Draw a line across your page.2. Place your mirror so that the reflecting surface is on the line,

making sure that the mirror is straight and vertically upright.3. Place a pin about 10-15 cm in front of the mirror. This is your

object pin. Label it “object.”4. Look at the reflection of this pin from one side. Place your ruler

on the paper so that its edge points straight toward the image you see in the mirror. Trace along the edge of the ruler with your pen or pencil.

5. Repeat this procedure until you have 5 lines on one side of the mirror; then do the same from the other side of the mirror.

6. Remove the mirror and extend these lines until they reach the “mirror line.”

7. Now connect these lines from where they hit the mirror to the hole where the “object pin” was located.

8. Place “arrowheads” on your lines showing how rays of light went 1) from the pin, 2) to the mirror, and then 3) to your eye. The rays going toward the mirror are known as the incident rays and the rays reflecting from the mirror are known as the reflected rays.

9. Extend the reflected rays behind the mirror as dotted lines. These should all intersect in the same place behind the mirror, showing where the image of the pin is located. Label the point of intersection “image”.

Questions:A. Do all of your lines intersect in the same point? What would be reasons why the lines might not all

intersect in the exact same place?

B. Is this image real or virtual? How do you know?

Be sure to indicate on your diagram where the mirror was located.

object

image

mirror line

PART 4: 1st LAW OF REFLECTION & OBJECT/IMAGE DISTANCE RELATIONSHIP

Procedure:1. Set up your mirror and paper with a pin in front as before. Label

the object pin “object.”2. Use your ruler to draw one sight line from each side of the mirror. 3. Remove the mirror and extend these rays to the mirror and then

to the pin as before, showing the direction of the rays with arrowheads.

4. Extend the reflected rays as dotted lines behind the mirror in order to locate the image. Label the image location “image.”

5. Draw dotted lines normal (perpendicular) to the mirror where the rays reflected from the mirror. Label these lines “n” for “normal.”

6. Label and measure the angles of incidence ( i) and reflection ( r) for each ray.

7. Measure the distance from the object to the mirror. Label this “do.”8. Measure the distance from the image to the mirror. Label this “d i.”9. Make sure all distances and angle measurements are recorded on

your ray diagram.

Questions:A. How does the image distance compare with the object distance?

B. What might cause the image distance to be less than the object distance?

What might cause the image distance to be greater than the object distance?

C. Does your diagram verify the 1st Law of Reflection?

D. What are some reasons why your angles may not have been exactly equal?

object

image

n n

i = ? i = ? r = ?

r = ?

do = ?

di = ?

mirror line

Be sure to indicate on your diagram where the mirror was located.

PART 5: MORE IMAGE LOCATING

Procedure: Draw AT LEAST THREE sight lines to locate these images when the object pin (x) and mirror (bold line segment) are in the following configurations:

a) place object pin off-center from angled mirror b) cover central portion of mirror with paper and place object pin directly in front of mirror

Be sure to indicate on your diagrams where the mirror was located.

mirror, with paper covering central section

mirror line

mirror

mirror line

Xobject pin

Xobject pin