lenses & mirrors ch 18. a plane mirror a flat, smooth surface where light is reflected by...
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A plane mirror
• A flat, smooth surface where light is reflected by regular reflection.
• Image formed by brain where all rays would have met if they were straight (in mirror)
Concave Mirror
• Flashlights and headlights• Curves inward- concave(metal spoon)*Gather light, rays intersect
that is the focal point (light straight on center)
Distance between center of mirror and focal pt is the focal length.
2 rules:1)Ray of principle axis passes focal pt2)Ray through focal is parallel to principle axis
How to determine the focal point
To find focal point (F) you will need to find the focal length(f) . It is half the radius of curvature of the mirror or 2f=r. If mirror has a radius of curvature of 10.0 cm than the f=5cm from the center of the mirror
Images- It depends where the object is located in reference to the mirror and focal point.
• 1) Objects will appear as a virtual image if in FRONT of focal pt (behind the mirror)
• 2) a small inverted image• 3) a large inverted image
Lens/Mirror Equation
• f=focal length of the mirror
• do (p)=distance from object to mirror
• di=(q)distance from image to mirror
• M=magnification• h=height
Calculating a Real Image formed by a concave mirror
• A 4.00-cm tall light bulb is placed a distance of 45.7 cm from a concave mirror having a radius of curvature of 30.4 cm. Determine the image distance and the image size.
• ho = 4.0 cm f= 30.4/2= 15.2cm• do = 45.7 cm• f = 15.2 cm
• 1/f = 1/do + 1/di 1/(15.2 cm) = 1/(45.7 cm) + 1/di
• 0.0658 cm-1 = 0.0219 cm-1 + 1/di
• 0.0439 cm-1 = 1/di
• di = 22.8 cm
Now find the height
• hi/ho = - di/do mag. Equation
• hi /(4.0 cm) = - (22.8 cm)/(45.7 cm)
• hi = - (4.0 cm) • (22.8 cm)/(45.7 cm)
• hi = -1.99 cm (neg so inverted)
Why are some things + and - ?• f is + if the mirror is a concave mirror • f is - if the mirror is a convex mirror • di is + if the image is a real image and located on
the object's side of the mirror. • di is - if the image is a virtual image and located
behind the mirror. • hi is + if the image is an upright image (and
therefore, also virtual) • hi is - if the image an inverted image (and
therefore, also real
What about Lenses?
• A lens is called convex if it is thicker at the center than at the edges
• A lens is called concave if it is thinner at the middle than at the edges
Focusing with Lenses
Focal Length=distance from the lens to the focusFocus= point the light converges
Real or Virtual
Real – rays converge on back side of lens
Virtual – rays diverge on backside of lens
Problem
• A 3.0 cm high object is placed 32.0 cm from a convex lens that has a focal length of 8.0 cmWhere is the image?How high is the image?