Lenses
Foot
lenses-intro-slides.pdf
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Lenses
Chapter 24: Walker
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Brainstorm
• Lenses in everyday life • What we learned last time about reflection and
refraction
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Light Rays • Ray: imaginary line representing the direction of travel
of light beam • Light travels in a straight line until it encounters a
boundary between two different materials • Examples:
o Rays going out in all directions from a light bulb o Parallel rays coming out of a ray box
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Thin lenses
• Converging lenses: o Rays pass through focal point o Convex: thin at edge, thick in
middle
• Diverging lenses: o Rays seem to come from focal
point o Concave: thick at edge, thin in
middle
Lenses can also have one side curved one side flat
• All lenses work by refraction https://phet.colorado.edu/sims/html/bending-light/latest/bending-light_en.html
Place where rays meet (or appear to diverge from) is called the focal point
F
F
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Ray Tracing: Rules for Lenses https://physics.bu.edu/~duffy/HTML5/lenses_revised.html
Rays should not bend in mid-air! • Thin lens approximation: rays bend at center of lens • The intersection of any 2 rays locates an image • Other rays are a check • There are many rays, but the 3 principle ones are:
o Ray 1: comes in parallel to axis, goes out through focal point
o Ray 2: goes straight through center of lens o Ray 3: comes in through focal point, emerges parallel to
axis • Results should agree with distances from equations
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1. Rays parallel to principal axis pass through focal point. 2. Rays through center of lens are not refracted.
3. Rays through F emerge parallel to principal axis.
Converging Lens: Object Far From Focal Point
F
F
Object Principle Axis
Image is (in this case): Real or Virtual Upright or Inverted Reduced or Enlarged
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2
3 do di
f
Image
f
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Equations for Lenses • The object’s location, the image location, and the focal
length are related by the lens equation:
1/do + 1/di = 1/f
• The magnification is given by:
M = -di/do
o do is the ________________ o di is the ________________ o f is the ______________
• Applications: o Important for glasses and contacts o Glasses or contacts prescription is 1/f o For contacts curvature must match your eye
object distance image distance focal length
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Sign Conventions for Lenses
+ when
- when
do Object in front Object in back
di Image in back (real) Image in front (virtual)
f Converging lens Diverging lens
m Image upright Image inverted
