Teaching Notes
When the students measure the angle of incidence vs. the angle of reflection, encourage them to take data throughout the range of the angle of incidence (zero to 90 degrees). Every fifteen degrees is a good interval. Encourage the students to sight carefully as they place the dots on the paper to mark the position of the beam. Point out that they must be directly over the beam when they place their dots. You might mention the effect of parallax—an object seems to shift position when seen from different places. To help students visualize the position of the image, have them hold a meter stick or ruler perpendicular to the mirror, as shown.
If the students use lasers as the light source, each group must keep its laser’s beam under control at all times. Have each group set up a backstop behind its mirrors to block the beam. A large piece of cardboard makes a good backstop.
Safety Precaution: Warn the students not to look into the laser beam. Also emphasize that the laser should only be turned on when it is resting on the tabletop.
Many secondary students have not yet understood the path of light from a light source to an object to their eye. Some may still believe that vision is something that emanates from the eyes, so the reflection of light from the object is not important. It may help to darken the room and illuminate an object with a flashlight. Have the students trace the path of light (or even walk along the path) from the flashlight to their eyes. Ask about the fact that many students, in different locations in the classroom, can see the object, and what that must mean in terms of the reflected light (that it’s reflected in many directions).
There should be good agreement (within a few degrees) between the values of the angles of incidence and angles of refraction, especially if the mirrors are made of polished metal. Remember that if the students are using plastic or glass mirrors, these are usually rear-surfaced. In effect, the reflection in effect happens in the middle of the mirror (see Background Information above), but the angle of reflection is still equal to the angle of incidence. The main effect will be to shift the extension of the beam used to locate the image.
When the students measure the angle of incidence vs. the angle of reflection, encourage them to take data throughout the range of the angle of incidence (zero to 90 degrees). Every fifteen degrees is a good interval. Encourage the students to sight carefully as they place the dots on the paper to mark the position of the beam. Point out that they must be directly over the beam when they place their dots. You might mention the effect of parallax—an object seems to shift position when seen from different places. To help students visualize the position of the image, have them hold a meter stick or ruler perpendicular to the mirror, as shown.
If the students use lasers as the light source, each group must keep its laser’s beam under control at all times. Have each group set up a backstop behind its mirrors to block the beam. A large piece of cardboard makes a good backstop.
Safety Precaution: Warn the students not to look into the laser beam. Also emphasize that the laser should only be turned on when it is resting on the tabletop.
Many secondary students have not yet understood the path of light from a light source to an object to their eye. Some may still believe that vision is something that emanates from the eyes, so the reflection of light from the object is not important. It may help to darken the room and illuminate an object with a flashlight. Have the students trace the path of light (or even walk along the path) from the flashlight to their eyes. Ask about the fact that many students, in different locations in the classroom, can see the object, and what that must mean in terms of the reflected light (that it’s reflected in many directions).
There should be good agreement (within a few degrees) between the values of the angles of incidence and angles of refraction, especially if the mirrors are made of polished metal. Remember that if the students are using plastic or glass mirrors, these are usually rear-surfaced. In effect, the reflection in effect happens in the middle of the mirror (see Background Information above), but the angle of reflection is still equal to the angle of incidence. The main effect will be to shift the extension of the beam used to locate the image.