Activity 5
Reflected Light
Background Information
When light strikes an ordinary object, the light is reflected in all directions. This reflected light can be represented by light rays that fan out from each point on the object.
When the object is placed before a mirror, the light rays that fan out from the object can be extended to the mirror. As shown in the drawing on page 218 of the Student Book, the angles of incidence and reflection for these rays are equal. These angles are straightforward to measure, but there is a minor complication to interpreting the results. Most lab mirrors are rear-surface mirrors. That is, the reflecting coating is on the rear of the mirror, so the light must first pass through glass before being reflected, and then must pass through glass again before again entering the air. The light is refracted at each air-glass interface, as shown in the drawing.
The net effect is that the light acts as if it had been reflected from a surface located about in the middle of the lab mirror (where the dotted lines meet in the drawing). Notice that the angle of incidence is still equal to the angle of reflection, by symmetry.
However, when the reflected ray is extended behind the mirror to locate the image, the extension will be shifted away from the point where the incident ray hits the mirror. The size of the shift increases with the angle of incidence (and the shift is zero for a light beam coming in along the normal). The result is that the image is slightly blurred.
Extending the paths of the reflected rays back behind the mirror produces the location of the image. Note that the image is as far behind the mirror as the object is in front.
Also, the image is located on the line that goes through the object and is perpendicular to the mirror surface. If you walk toward a mirror, your reflection seems to walk toward you. If you reach out to shake hands with your reflection, a reflected hand reaches toward you . . . but it’s a left hand. What happened? Move your hand to the left, and the reflection moves to the left. Move your hand to the
right, and the reflection moves to the right. But move your hand toward the mirror, and the reflection moves toward you (in the opposite direction). This is the source of the inversion of your hand. What is inverted in the mirror is not right to left, and not up to down, but rather toward the mirror and away from the mirror. The reflection of an asymmetrical letter like an “R” shows what happens. Note that the reflection of the left hand is a right hand.
Reflected Light
Background Information
When light strikes an ordinary object, the light is reflected in all directions. This reflected light can be represented by light rays that fan out from each point on the object.When the object is placed before a mirror, the light rays that fan out from the object can be extended to the mirror. As shown in the drawing on page 218 of the Student Book, the angles of incidence and reflection for these rays are equal. These angles are straightforward to measure, but there is a minor complication to interpreting the results. Most lab mirrors are rear-surface mirrors. That is, the reflecting coating is on the rear of the mirror, so the light must first pass through glass before being reflected, and then must pass through glass again before again entering the air. The light is refracted at each air-glass interface, as shown in the drawing.
The net effect is that the light acts as if it had been reflected from a surface located about in the middle of the lab mirror (where the dotted lines meet in the drawing). Notice that the angle of incidence is still equal to the angle of reflection, by symmetry.
However, when the reflected ray is extended behind the mirror to locate the image, the extension will be shifted away from the point where the incident ray hits the mirror. The size of the shift increases with the angle of incidence (and the shift is zero for a light beam coming in along the normal). The result is that the image is slightly blurred.Extending the paths of the reflected rays back behind the mirror produces the location of the image. Note that the image is as far behind the mirror as the object is in front. Also, the image is located on the line that goes through the object and is perpendicular to the mirror surface. If you walk toward a mirror, your reflection seems to walk toward you. If you reach out to shake hands with your reflection, a reflected hand reaches toward you . . . but it’s a left hand. What happened? Move your hand to the left, and the reflection moves to the left. Move your hand to the
right, and the reflection moves to the right. But move your hand toward the mirror, and the reflection moves toward you (in the opposite direction). This is the source of the inversion of your hand. What is inverted in the mirror is not right to left, and not up to down, but rather toward the mirror and away from the mirror. The reflection of an asymmetrical letter like an “R” shows what happens. Note that the reflection of the left hand is a right hand.