Activity 6
Curved Mirrors
Background Information
A good starting point for thinking about the curved mirror is the plane mirror.
The plane mirror makes an erect, virtual image.
Now imagine the mirror is bowed out to make a convex mirror. How does the image change? The light rays show that the image is still behind the mirror, so the image is still virtual.
Moreover, the image is reduced, since parallel rays are reflected out away from the mirror. Also, the image is closer to the mirror than is the object. The focus is labeled. The more strongly the mirror is curved, the shorter the focal length and the more the virtual image is reduced compared to the object.
Now imagine bowing the plane mirror inward, to make a concave mirror.
Since parallel rays now bend in toward the mirror axis, objects now appear magnified when the object is close to the mirror. Only when the object is within a focal length of the mirror is the object virtual. For such objects, the image is further from the mirror than is the object. If the object is outside the focal length, the reflected rays cross, so a real image is formed, as shown in the drawings in Physics Talk on page 227 in the Student Book.
This image can be seen on a card. It can, of course, also be seen by looking at the mirror, rather than on a card, and this is the basis of the famous “floating coin” illusion made by two parabolic mirrors fastened together.
The concave mirror is exactly analogous to the convex lens, which is a magnifier when the object is up close and which produces a real image otherwise. The lens equation also describes the location of the image and object for mirrors:
1/f = 1/Do + 1/Di
Image formation in curved mirrors, including the mathematical description of the equation above, are exactly analogous to image formation in lenses, the subject of Activity 8.
Curved Mirrors
Background Information
A good starting point for thinking about the curved mirror is the plane mirror.
The plane mirror makes an erect, virtual image. Now imagine the mirror is bowed out to make a convex mirror. How does the image change? The light rays show that the image is still behind the mirror, so the image is still virtual.
Moreover, the image is reduced, since parallel rays are reflected out away from the mirror. Also, the image is closer to the mirror than is the object. The focus is labeled. The more strongly the mirror is curved, the shorter the focal length and the more the virtual image is reduced compared to the object.
Now imagine bowing the plane mirror inward, to make a concave mirror.
Since parallel rays now bend in toward the mirror axis, objects now appear magnified when the object is close to the mirror. Only when the object is within a focal length of the mirror is the object virtual. For such objects, the image is further from the mirror than is the object. If the object is outside the focal length, the reflected rays cross, so a real image is formed, as shown in the drawings in Physics Talk on page 227 in the Student Book.
This image can be seen on a card. It can, of course, also be seen by looking at the mirror, rather than on a card, and this is the basis of the famous “floating coin” illusion made by two parabolic mirrors fastened together.
The concave mirror is exactly analogous to the convex lens, which is a magnifier when the object is up close and which produces a real image otherwise. The lens equation also describes the location of the image and object for mirrors:1/f = 1/Do + 1/Di
Image formation in curved mirrors, including the mathematical description of the equation above, are exactly analogous to image formation in lenses, the subject of Activity 8.