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Chapter 17: Lesson 2 - Mirrors
Chapter 17: Lesson 2 - Mirrors
Objects and Images in Plane Mirrors
Objects and Images in Plane Mirrors
A plane mirror is a flat, smooth surface area that reflects light by specular refection. The mirror you have in your bathroom is a plane mirror.
An object is the source of diverging light rays. An object may be luminous or illuminated. If you are standing in front of your bathroom mirror, you are the object and what you see in the mirror is the image. An image is the combination of image points in a plane mirror from which the reflected object seems to originate.
The image produced by a plane mirror is behind the mirror. The rays reflected from a plane mirror never actually converge (come together) but appear to diverge (spread apart) from a point behind the mirror.
Real vs Virtual Images
Real vs Virtual Images
An image is a real image if the rays actually converge and pass through the image. A real image can be seen on a piece of paper or screen. A virtual image cannot be projected on a screen or captured on a piece of paper because the light rays do not converge at a virtual image.
Images from the projector in class are real images because the image is projected onto the screen. Watch the video to see an example of a virtual image.
Check Your Understanding:
Check Your Understanding:
Click on the down arrow when you have your answer to check to see if you are correct.
Does a mirror produce a real or a virtual image?
A mirror produces a virtual image. You cannot project the image you see in a mirror onto a piece of paper.
Plane Mirror Properties
Plane Mirror Properties
With a plane mirror, the image position is equal to the negative of the object position. The negative sign indicates that the image is virtual. This means that however far away from the mirror the object is, the image is the same distance behind the mirror.
-di=do
In a plane mirror, the image height and the object height are the same.
hi=ho
Concave and Convex Mirrors
Concave and Convex Mirrors
Concave Mirror
Concave Mirror
A concave mirror reflects light from its inner "caved in" surface. Concave mirrors converge, meaning that the light rays come together. A concave mirror would be the inside part of a spoon where you would scoop up soup.
Convex Mirror
Convex Mirror
A convex mirror reflects light from its outer surface. Convex mirrors diverge, meaning that the rays move away from each other. A convex mirror would be the bottom part of a spoon. Rays of light that reflect from a convex mirror always diverge (meaning the rays spread away from each other.)
Real Life Concave Mirror
Real Life Concave Mirror
Concave mirrors are used in flashlights to produce parallel beams of light.
Real Life Convex Mirror
Real Life Convex Mirror
Convex mirrors are used to enlarge objects. Side view mirrors, rear view mirrors, mirrors in the corner of stores are all convex mirrors. By forming smaller images convex mirrors enlarge the area, or field of view, that an observer sees. The center of this field of view is visible from any angle of an observer off the principal axis of the mirror, thus the field of view is visible from a wide perspective.
Parts of a Mirror
Parts of a Mirror
The principal axis is the straight line that is perpendicular to the surface of the mirror at its center that divides the mirror in half. You can see the principal axis in both of the pictures of a concave and convex mirrors.
The focal point is the location at which the rays parallel to the principal axis of a mirror or lens converge to a point. In the pictures, you can locate the focal point (F).
The focal length is the distance from the focal point to the mirror along the principal axis. It is abbreviated with a fancy f, ƒ.
The focal length is half the radius of curvature of the mirror. If you were to continue the curvature of the mirror into a circle, the center of curvature is the center of the circle and the radius of curvature is the distance from the center of the curvature to the mirror.
We abbreviate the center of curvature with a C. The radius of curvature is abbreviate with a r.
Radius of Curvature
Radius of Curvature
To find the radius of curvature, r, of a concave mirror, you use the equation, r=2ƒ. The focal length is half of the radius of curvature.
Spherical Aberration
Spherical Aberration
Spherical aberration is a defect that almost all curved mirrors have. Spherical aberration results when light passes through the edges of a mirror and focuses at a slightly different place from the light passing through the center of the mirror.
The light at the edges of the mirror are the problem, so to fix this, we just cut off the edges of the mirror or cover them up to make sure that the image is in focus.
Light rays leaving in parallel from a parabolic mirror.
Mirrors ground to a perfect parabolic shape, parabolic mirrors, have no spherical aberration. All the rays converge (come together) at the focal point.
Parabolic mirrors can also produce parallel beams of light needed in flashlights, car headlights, and searchlights. The light source is placed at the focal point and the reflected rays leave in parallel beams.
Light rays leaving in parallel from a parabolic mirror where the light bulb is placed at the focal point.
You have finished Lesson 2!
You have finished Lesson 2!
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