Curved Mirrors

Concave and Convex Mirrors Concave and convex mirrors are curved mirrors similar to portions of a sphere. light rays Concave mirrors reflect light from their inner surface, like the inside of a spoon. Convex mirrors reflect light from their outer surface, like the outside of a spoon.

Concave Mirrors Concave mirrors are approximately spherical and have a principal axis that goes through the center, C, of the imagined sphere and ends at the point at the center of the mirror, A. The principal axis is perpendicular to the surface of the mirror at A. CA is the radius of the sphere,or the radius of curvature of the mirror, R. Halfway between C and A is the focal point of the mirror, F. This is the point where rays parallel to the principal axis will converge when reflected off the mirror. The length of FA is the focal length, f. The focal length is half of the radius of the sphere (proven on next slide).

r = 2 f    CF r f s To prove that the radius of curvature of a concave mirror is twice its focal length, first construct a tangent line at the point of incidence. The normal is perpendicular to the tangent and goes through the center, C. Here, i = r = . By alt. int. angles the angle at C is also , and α = 2 β. s is the arc length from the principle axis to the pt. of incidence. Now imagine a sphere centered at F with radius f. If the incident ray is close to the principle axis, the arc length of the new sphere is about the same as s. From s = r , we have s = r β and s  f α = 2 f β. Thus, r β  2 f β, and r = 2 f. tangent line

Focusing Light with Concave Mirrors Light rays parallel to the principal axis will be reflected through the focus (disregarding spherical aberration, explained on next slide.) In reverse, light rays passing through the focus will be reflected parallel to the principal axis, as in a flood light. Concave mirrors can form both real and virtual images, depending on where the object is located, as will be shown in upcoming slides.

CF C F Spherical MirrorParabolic Mirror Only parallel rays close to the principal axis of a spherical mirror will converge at the focal point. Rays farther away will converge at a point closer to the mirror. The image formed by a large spherical mirror will be a disk, not a point. This is known as spherical aberration. Parabolic mirrors don’t have spherical aberration. They are used to focus rays from stars in a telescope. They can also be used in flashlights and headlights since a light source placed at their focal point will reflect light in parallel beams. However, perfectly parabolic mirrors are hard to make and slight errors could lead to spherical aberration. Spherical Aberration

Spherical vs. Parabolic Mirrors Parallel rays converge at the focal point of a spherical mirror only if they are close to the principal axis. The image formed in a large spherical mirror is a disk, not a point (spherical aberration). Parabolic mirrors have no spherical aberration. The mirror focuses all parallel rays at the focal point. That is why they are used in telescopes and light beams like flashlights and car headlights.

Concave Mirrors: Object beyond C CF object image The image formed when an object is placed beyond C is located between C and F. It is a real, inverted image that is smaller in size than the object.

Concave Mirrors: Object between C and F C F object image The image formed when an object is placed between C and F is located beyond C. It is a real, inverted image that is larger in size than the object.

Concave Mirrors: Object in front of F CF object image The image formed when an object is placed in front of F is located behind the mirror. It is a virtual, upright image that is larger in size than the object. It is virtual since it is formed only where light rays seem to be diverging from.

Concave Mirrors: Object at C or F What happens when an object is placed at C? What happens when an object is placed at F? The image will be formed at C also, but it will be inverted. It will be real and the same size as the object. No image will be formed. All rays will reflect parallel to the principal axis and will never converge. The image is “at infinity.”

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Convex Mirrors A convex mirror has the same basic properties as a concave mirror but its focus and center are located behind the mirror. This means a convex mirror has a negative focal length (used later in the mirror equation). Light rays reflected from convex mirrors always diverge, so only virtual images will be formed. light rays Rays parallel to the principal axis will reflect as if coming from the focus behind the mirror. Rays approaching the mirror on a path toward F will reflect parallel to the principal axis.

Convex Mirror Diagram CF object image The image formed by a convex mirror no matter where the object is placed will be virtual, upright, and smaller than the object. As the object is moved closer to the mirror, the image will approach the size of the object.

Mirror/Lens Equation Derivation From  PCO,  =  + , so 2  = 2  + 2 . From  PCO,  = 2  + , so -  = -2  - . Adding equations yields 2  -  = .  = s r s    didi    s dodo (cont.) C     s object image didi O P T From s = r , we have s = r β, s  d i α, and s  d i α (for rays close to the principle axis). Thus: dodo

Mirror/Lens Equation Derivation (cont.) 2s2s r - s didi = s dodo 1 dodo 2 r = 1 didi + 2 2f2f = 1 dodo 1 didi + 1 f = 1 dodo 1 didi + From the last slide,  = s / r,   s / d 0,   s / d i, and 2 β -  = . Substituting into the last equation yields: C     s object image didi dodo O P T The last equation applies to convex and concave mirrors, as well as to lenses, provided a sign convention is adhered to.

Mirror Sign Convention + for real image - for virtual image + for concave mirrors - for convex mirrors 1 f = 1 dodo 1 didi + f = focal length d i = image distance d o = object distance didi f

Magnification m = magnification h i = image height (negative means inverted) h o = object height m = hihi hoho By definition, Magnification is simply the ratio of image height to object height. A positive magnification means an upright image.

Magnification Identity: m = -di-di dodo hihi hoho = C object image, height = h i didi dodo To derive: One hits the mirror on the axis. The incident and reflected rays each make angle  relative to the axis. A second ray is drawn through the center and is reflected back on top of itself (since a radius is always perpendicular to an tangent line of a  hoho circle). The intersection of the reflected rays determines the location of the tip of the image. Our result follows from similar triangles, with the negative sign a consequence of our sign convention. (In this picture h i is negative and d i is positive.)

Mirror Equation Sample Problem Suppose AllStar, who is 3 and a half feet tall, stands 27 feet in front of a concave mirror with a radius of curvature of 20 feet. Where will his image be reflected and what will its size be? d i = h i = CF feet feet

Mirror Equation Sample Problem 2 CF Casey decides to join in the fun and she finds a convex mirror to stand in front of. She sees her image reflected 7 feet behind the mirror which has a focal length of 11 feet. Her image is 1 foot tall. Where is she standing and how tall is she? d o = h o = feet 2.75 feet