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# Mirrors and lenses

## on Apr 12, 2012

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Ch 18 Notes

Ch 18 Notes

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## Mirrors and lensesPresentation Transcript

• Ch 18Mirrors and Lenses
• Plane Mirror  flat, smooth surface that reflects light in a regular way;  it produces an image that is virtual, erect, and the same size as the object;  left and right appear to be interchanged, but front and back are interchanged
• Types of Images Real image - source of converging light rays; can be projected; are always inverted Virtual image - source of diverging light rays; cannot be projected; are always erect
• Concave Mirror reflects light off its inner surface Focal point - (f) point on the principle axis where reflecting light converges; f equals one-half the radius of curvature (C)
• Spherical Aberration defect of all spherical mirrors when the parallel rays don’t all reflect through the focal point; correct this by using a parabolic mirror
• Concave Mirror Characteristics  If the object is outside C, the image is real, inverted, and smaller
• Concave Mirror Characteristics  If the object is at C, the image is real, inverted, and the same size
• Concave Mirror Characteristics  If the object is between C and f, the image is real, inverted, and larger
• Concave Mirror Characteristics  If the object is inside f, the image is virtual, erect, and larger
• Mirror Equation 1 = 1 + 1 or di = f x do f do di do – f f is the focal length do is the object distance di is the image distance magnification - the ratio of the image size (hi) to the object size (ho) m = hi = - di ho do
• Mirror Equation Rules di is positive for real images, negative for virtual images hi is positive for erect images, negative for inverted images f is positive for concave mirrors, negative for convex mirrors
• Convex mirror  reflects light off the outer surface  images are always virtual, erect, and smaller  these are used as safety mirrors because you get a wider angle of vision
• Lens transparent material with a refractive index larger than air The focal length of the lens depends on the shape and refractive index of the material
• Convex Lens Convex lens - (converging lens) thicker in the middle than at the edges; acts like a concave mirror. Chromatic aberration - caused when light passes thru the edge of a lens; the light is dispersed; correct using an achromatic lens
• Convex Lens Characteristics  Outside 2f, image is real, inverted, and smaller
• Convex Lens Characteristics  At 2f, image is real, inverted, and same size
• Convex Lens Characteristics  Between 2f and f, image is real, inverted, and larger
• Convex Lens Characteristics  Inside f, image is virtual, erect, and larger
• Concave lens  (diverging lens) thinner in the middle than at the edges  acts like a convex mirror.  Always produces virtual, erect, and smaller image
• Lens Equation Rules The equation and rules stay the same except f is positive for convex lens and negative for concave lens
• Corrective Lenses
• Near-Sighted People have too short a focal length correct this using a concave (diverging) lens
• Far-Sighted People have too long a focal length, correct this using a convex (converging) lens