reflection and refraction light interacts with matter interaction begins at surface and depends on...
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Reflection Reflection and and
RefractionRefraction
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Light interacts with matter
• Interaction begins at surface and depends on– Smoothness of surface– Nature of the material– Angle of incidence
• Possible interactions– Reflection– Refraction – Absorption– Transmission
Transparent materials transmit light
Opaque materials do not allow transmission of light
Reflect, absorb or combination
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ReflectionReflection• Most objects we see reflect light rather than
emit their own light.
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Principle of Least TimePrinciple of Least Time
• Fermat's principle - light travels in straight lines and will take the path of least time
MIRROR
A B
Wrong PathTrue Path
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Law of ReflectionLaw of Reflection
• “The angle of incidence equals the angle of reflection.”
• This is true for both flat mirrors and curved mirrors.
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Reflection
• Angles measured with respect to the “surface normal”– Line perpendicular
to the surface
i = r
Law of ReflectionAngle of incidence = Angle of reflection
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Image formation
• Real image– Can be viewed or
displayed at its location– Example - movie image
on a screen• Virtual image
– Appears to come from a location where it is not directly visible
– Examples: plane mirror, convex mirror, concave mirror
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Types of ReflectionTypes of Reflection
• Specular Reflection - images seen on smooth surfaces (e.g. plane mirrors)
• Diffuse Reflection - diffuse light coming from a rough surface (cannot see a reflection of yourself)
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RefractionRefraction• Refraction is the bending of light when it
passes from one transparent medium to another.
• This bending is caused by differences in the speed of light in the media.
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RefractionRefraction
WATER
AIR
Normal LineFast
Slow
Light Beam
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Refraction • Light crossing a boundary
surface and changing direction • Reason: change in light
propagation speed– Moving to a medium with a
slower propagation speed
• Light bends toward surface normal
– Moving to a medium with a faster propagation speed
• Light bends away from the normal
Magnitude of refraction depends on: •Angle that light strikes surface•Ratio of speed of light in the two transparent materials
Incident ray perpendicular to surface not refracted
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Refraction ExamplesRefraction Examples
• Light slows down when it goes from air into water and bends toward the normal.
• An Analogy: A car slows down when it goes from pavement onto gravel and turns toward the normal.
• An Illusion : Fish in the water appear closer and nearer the surface. (See Figure 28.27)
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RefractionRefraction
WATER
AIR
Observer
True Fish
False Fish
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Atmospheric RefractionAtmospheric Refraction
• Our atmosphere can bend light and create distorted images called mirages.
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Refraction• Mirages • Critical angle
– Light refracted parallel to surface
– No light passes through surface - “total internal reflection”
– Applications - fiber optics, gemstone brilliance
• Index of refraction– A measure of light
speed
Substance Index of refraction
Light speed
Air Approx. 1 ~c
Water 1.333 0.75c
Glass 1.5 0.67c
Diamond 2.4 0.42c
BE condensate
18,000,000 38 mph!
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Total Internal Reflection...Total Internal Reflection...
• …is the total reflection of light traveling in a medium when it strikes a surface of a less dense medium
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WATER
AIR
Light Source
Critical Angle
Total Internal Reflection
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LensesLenses• Converging Lens
• A lens that is thicker in the middle and refracts parallel light rays passing through to a focus.
• Diverging Lens
• A lens that is thinner in the middle than at the edges, causing parallel light rays to diverge.
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Optics
• The use of lenses to form images
• Concave lenses
– Diverging lenses
– Vision correction/in association with other lenses
• Convex lenses
– Converging lenses
– Most commonly used lens
– Magnifiers, cameras, eyeglasses, telescopes, …
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Myopia (Near-Sightedness)
People with near-sightedness cannot see clearly at distance.
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Hyperopia (Farsightedness)
People with far-sightedness cannot see clearly up close.
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The human eye
• Uses convex lens with muscularly controlled curvature to change focal distance
• Nearsightedness (myopia) - images form in front of retina• Farsightedness (hyperopia) - images form behind retina• Correction - lenses (glasses, contacts) used to move images onto retina
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• Total Internal Reflection occurs when the angle of incidence is greater than the critical angle.
• The critical angle for an air-water interface is 48°.
• Fish can see everything above the water’s surface in a 96° cone of vision.
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Dispersion...Dispersion...• …is the separation of white light into pure colors (ROY G. BIV).
• Dispersion Examples: • Prisms• Diffraction Gratings• CD’s• Raindrops
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Dispersion and colors
• White light
– Mixture of colors in sunlight– Separated with a prism
• Dispersion
– Index of refraction varies with wavelength
– Different wavelengths refract at different angles
– Violet refracted most (blue sky)
– Red refracted least (red sunsets)
– Example: rainbows• Wavelength/frequency related
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RainbowsRainbows• Raindrops refract, reflect and disperse sunlight.
• Rainbows will always appear opposite of the Sun in the sky.
• You cannot run from or run to a rainbow!
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Rainbows
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The nature of lightWave-like behavior
Interference• Young’s two slit experiment
• Interference pattern - series of bright and dark zones
• Explanation - constructive and destructive interference
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Wave-like behavior - polarization
• Alignment of electromagnetic fields
• Unpolarized light - mixture of randomly oriented fields
• Polarized light - electric fields oscillating on one direction
• Two filters - passage depends on alignment
• Reflection polarization
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PolarizationPolarization
• The alignment of the electric vectors that make up electromagnetic radiation.
• See Figure 29.31.
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PolarizationPolarization
• Light can be polarized by:
– (1) passing light though a polarizer material or
– (2) reflecting light off of a solid or liquid surface.
• For example, reflected light from a lake is mostly horizontally polarized.