Law of Reflection

Of Light

When a light wave or any other kind of wave strikes a surface, the angle of incidence (coming to the surface) is equal to the angle of reflection (going away from the surface). The angles are measured from an imaginary line perpendicular to the surface at the point where the wave is reflected. A line perpendicular to the surface is called a normal.

Locating Image Position

1. Place a sheet of paper on a piece of cardboard. Using a ruler, draw a line across the middle of the paper and label it ML. Support a mirror with a block and rubber band and place the silvered side on the line you have drawn. Make sure the mirror is perpendicular to the paper.

2. About 4 cm in front of the mirror, make a dot on the paper with a pencil and label it point P. Place a pin upright in point P.

3. Place your ruler about 5 cm to the left of the pin. Put your eye down at the same level as the ruler and aim the right edge of the ruler at the image of the pin in the mirror. When the edge of the ruler is in line with the image of the pin, draw a line along the ruler such that the line is pointing directly at the image in the mirror. Label this line A.

4. Move your ruler to the right of the pin and look along the right edge of the ruler until you see the image of the pin lined up with the ruler. You are simply seeing the image from a location different from the first location you saw it. Draw a line along the edge of the ruler as you did before, making sure the line points directly at the image of the pin in the mirror. Label it B.

5. Remove the pin and the mirror from the paper. Extend lines A and B to the line ML. Using dotted lines, extend each of the lines beyond the mirror position until they intersect. This is the position of the image, I, in the mirror. Measure the perpendicular distance from I back to the line ML. Measure the perpendicular distance from P to ML. Record the distances in the data table.

6. To show the law of reflection, draw a line from P to point X, where line A meets line ML. Using your protractor, construct a normal, N, (a line perpendicular to ML) at this point. Measure angles PXN and AXN and record. Draw a line from P to point Y, where line B meets line ML. Construct another normal at this point, N1, and measure angles PYN1 and BYN1.

Image Orientation

1. Set up the mirror once more on a line drawn across the center of a fresh piece of paper. Draw a small triangle about 5 cm wide on the paper about 5 cm in front of the mirror. Label the vertices A, B, and C.

2. Place a pin in vertex A. Repeat procedures 3 and 4 from above to produce 2 lines aimed at the image of A in the mirror. Label the lines A and A1.

3. Remove the pin from A and place it in B and repeat the previous step. Label the 2 lines B and B1.

4. Remove the pin from B and repeat the procedure at C. Label the lines C and C1.

5. Remove the mirror and the pin. Extend dotted lines beyond the mirror as before to locate points A1, B1, and C1. Construct the image of a triangle. If your constructed image of the triangle does not resemble the original triangle try procedures 1-4 again. Measure and record the perpendicular distances from points A, B, C, A1, B1, and C1 to the mirror line.

Data Tables

Object distance
Image distance
Angle of incidence (PXN)
Angle of reflection (AXN)
Angle of incidence (PYN1)
Angle of reflection (BYN1)
Point / Distance to mirror line (cm)
A
A1
B
B1

C

C1

1. From your observations, what do you conclude about the angle of incidence and the angle of reflection?

2. A ray of light is incident upon a mirror at an angle of 30o. What is the angle between the incident ray and the reflected ray?

3. How far behind the mirror is the image of an object that is located in front of a flat (plane) mirror?

4. How does the size of your constructed triangle compare with the size of the triangle you drew in front of the mirror?

5. Why do you think the image produce by a flat (plane) mirror is called a virtual image?