Reflection of Light
12
When light rays are reflected from surfaces, the rays are found to be reflected in such a way that
the angle of reflection (θr) is equal to the angle of incidence (θi). This is the Law of Reflection.
Both angles are measured from an imaginary line perpendicular to the surface at the point
where the ray is reflected. This line is called the normal.
Learning Targets:


Student will locate an image formed by a plane mirror using a technique called
a ray tracing.
Student will observe characteristics of plane mirror images.
Procedure:
A. Locating an Image and Comparing Angles
1. Place the last sheet of paper in this lab on the piece of cardboard. The Mirror Line (ML)
across the middle of the paper is where you will line up the mirror. Support the mirror
vertically by using the wooden block and a rubber band. Place the back edge of the
mirror on line ML.
2. About 4 cm in front of the mirror, place a pin. We will call this point P.
Arrangement of apparatus for Procedure A:
Line ML
Incident Ray, Reflected Ray, and Normal all lie in the same plane. Shoot the laser
to hit the image.
3. Using the laser, shoot a beam and hit the image in the mirror from the left side of the
pin. Have your partner trace both the incident and reflected beam.
a. Incident Ray is denoted by line PX
b. Reflected ray is denoted by line AX
Reflected Ray
Incident Ray
Laser
4. Extend the reflected ray behind the mirror line (ML) with a dotted line.
a. Denoted by line XI
5. Using your protractor, draw the normal with a dotted line. Remember, the normal is
perpendicular to the reflecting surface. At this point, your paper should look like Figure
B above.
6. Duplicate Steps 12-5 for a position to the right of the pin.
7. Images are always located where two perceived light rays converge and intersect. This
occurs where your 2 dotted rays that you traced behind the mirror intersect. Label that
point as I. Measure the perpendicular distance from the mirror to both the Pin and the
Image. Record in Table 12-1.
8. Measure the Angles of Incidence and the Angles of Reflection for both the left and right
side of the pin. Record these values in Table 12-1.
Table 12-1
Object Distance (cm)
Image Distance (cm)
Left Side of Pin
Angle of Incidence (˚)
Angle of Reflection (˚)
Right Side of Pin
Angle of Incidence (˚)
Angle of Reflection (˚)
B. Image Orientation
1. On a new sheet of paper, draw another line ML across the short side and plce the
mirror’s back edge on that line. Draw a triangle in front of the mirror labeling the
verticies A, B, and C.
a. Try to have each vertex of your triangle be a different distance away from the
mirror.
2. Place the pin at point A. Using the ray tracing tehniques you learned in steps 12-4 in Part
A of this lab. Duplicate the “W” you created in Part A ofthis lab for point A. Call the left
side A1 and the right side A2.
a. You need not draw the normal because we are not measuring any angles.
3. Duplicate step 2 above for points B and C. Call the left sides B1 and C1. Call the right sides
B2 and C2.
4. Having extended the reflected rays behind the mirror with a straight edge, you should
now have found the images of A, B and C. We will call them A’, B’, and C’. Connect these
points with a straight edge and you should notice that you get an image triangle that is
“Right/Left Reversed” from the object triangle.
5. Measure the perpendicular distances from the mirror line ML to each of the points A
and A’, B and B’, and C and C’. Record these distances in Table 12-2.
Table 12-2
Point
A
A’
B
B’
C
C’
Distance to ML (cm)
Interpretation Questions
1. From your observations, what do you conclude about the angle of incidence and the
angle of reflection?
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2. A ray of light is incident upon a mirror at an angle of 30˚. What is the angle between the
incident ray and the reflected ray?
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3. How far behind the mirror is the image of an object that is located in front of the
mirror?
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4. Compare the size of your original object triangle in Part B with the size of you
constructed image of the triangle.
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5. From your observations, summarize the general characteristics of images produced by
plane mirrors.
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6. Why do you think the image produced by a plane mirror is called a virtual image?
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Mirror Line
(ML)
Mirror Line
(ML)