Optics

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Optics

• BuggLady and KEJohnson

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Optics

• The study of the behavior of light rays
• Draw a ray to indicate the light path/direction
• All angles (?) are beside the Normal Line
  (dashed line - to the surface)
• Used to control images with mirrors, membranes,
  and lenses
• Aberration optical defect which causes a
  degradation in image quality

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REFLECTION
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Reflection

• Occurs when a wave hits a rigid medium, through
  which it cannot travel
• Bounces off surface

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Depends on Medium

• Some energy loss (friction absorption)
• Water 2 energy reflected
• Glass 4 energy reflected
• Regular Reflection smooth, rigid surface
• Diffuse Reflection irregular surface (bumpy)
  reflected rays scatter
• Depends on the size
• frequency of the wave
• (p.446-447)
• Example Radio dishes

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Law of Reflection

• The wave will reflect at an angle equal to
  the angle at which it hit.
• Angle of Incidence Angle of
  Reflection
• ?i ?r
• If the light wave hits straight on (0?),
  it bounces straight back

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Mirrors

• Real image is formed by actual light rays
  inverted and can be projected on a screen (your
  eye).
• Virtual image is not formed by actual rays
  right-side-up and cannot be projected on a screen
  (appears to be behind the mirror).

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Focal Point/Length

• Where parallel rays meet (or appear to meet)
  after reflecting from a mirror or refracting
  through a lens
• (1/F) (1/do) (1/di)
• CR2F
• F focal length
• do object distance (always a positive number)
• di image distance
• R radius of curvature of mirror
• C center of curvature of mirror

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Linear Magnification

• Object appears taller through a lens
• Calculated by comparing the
• image to the object
• (either distance or height)
• m di/do hi/ho
• m linear magnification
• h height (m, cm)

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Flat Mirrors

• Virtual image appears erect, reversed
  (left-right), and as the same distance behind the
  mirror as you are in front of the mirror.
• F 0
• di -do

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Diverging Mirrors

• Convex curves outward (back of spoon)
• Virtual image is always upright, smaller and
  closer
• No Real image
• Negative F
• di lt F
• Used to see
• around corners
• at intersections

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Converging Mirror

• Concave - curves inward (bowl of a spoon)
• Image generated depends on location of object!
• Positive F

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do lt F

• Virtual image is upright, enlarged, and closer
• Negative di
• Ex make-up compact

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do F

• No Image is formed!
• Because the reflected rays are parallel

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do gt F

• Real image is inverted
• Positive di
• If object between C and F, image will be enlarged
• If object is at C, image (same size) is also at C
  
• If object beyond center of curvature (C2F),
  image is reduced
• If object is infinite distance away, image at
  Focus (F)

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Combined Mirrors

• Funhouses
• Seeing about corners
• Rooms appear larger

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Rear-View Mirror and the Dual Image
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REFRACTION
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Refraction

• The wave travels from one medium to another
• Caused by change in wave speed, depending on
  frequency
• Ex bending/broken sticks, fish in different
  location, water appears shallower

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Refraction of Light
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Refraction of Light
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• Wave fronts (crests/trough) are seen to shift
  their directions

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Snells Law

• A ray of light bends the ratio of the sine of
  the angle of incidence to the sine of the angle
  of refraction is a constant.
• n1 sin ?1 n2 sin ?2
• n index of refraction of the medium
• ? angle to the normal

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Index of Refraction

• Ratio speed of light in vacuum vs. speed of
  light in the material
• n c / v

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Different Transitions

• Moving into higher n ? lower speed ? bends
  toward the normal
• Moving into lower n ? higher speed ? bends
  away from the normal

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• Visit these websites
• http//www.ndted.org/educationresources/communityc
  ollege/Ultrasonics/Physics/refractionsnells.htm
• http//sol.sci.uop.edu/jfalward/refraction/refrac
  tion.html

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Atmospheric Refraction

• Causes mirages, the pavement to appear wet when
  merely hot, UFO sightings, and lets us see the
  sun after it has already set.
• Cool website of different images
  http//www.wdtv.com/weather/images/Weather_Review/
  sky_optics_and_observations.htm

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Mirages

• Hot air (near ground) / Cool air boundary bends
  light rays back up to the eye
• Virtual image is upside down
• Not trick of mind, trick of light

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Prisms

• Dispersion of white light into the color spectrum
  from the refraction at the two air-glass
  boundaries
• Violet light speed is 1 less than Red light

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Rainbows
Occur when light it dispersed in water droplets
in the atmosphere
Actually makes a circle, we see an arch from the
ground.
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Light is actually absorbed, reflected, and
refracted at boundaries, all at the same time,
to different degrees
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Internal Reflection

• There is a critical angle
• where light at a boundary
• will not be refracted,
• it will only be reflected internally
• The internal reflection will be 100
• This is the reason prisms
• are used instead of mirrors in optical
  instruments

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Critical Angle

• Depends on the material used
• n1sin?c n2 sin90?
• simplified
• ?c sin-1(n2 /n1)

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Fiber Optics

• Total internal reflection allows light to bend
  around corners
• http//webphysics.davidson.edu/applets/optics4/fib
  er_optics.html

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Lenses

• Light refracts (bends) twice
• due to the shape of the glass.
• Parallel rays will eventually
• converge at a focal point
• (just like mirrors)
• Equations for Reflections
• also apply to Lenses

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Diverging (Negative) Lens

• Virtual image is always upright, smaller and on
  the same side of the lens as the object
• No Real image
• Negative F
• Negative di

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Converging (Positive) Lens

• Image depends on location of the object!
• Positive F (on other side of lens)

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do lt F

• Virtual image is upright, enlarged, and on the
  same side as the object
• Negative di

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do F

• No Image is formed!
• Because the reflected rays are parallel

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do gt F

• Real image is inverted on opposite side of lens
• Positive di
• If object between 2F and F, image will be
  enlarged
• If object is at 2F, image (same size) is also at
  2F
• If object beyond 2F,
• image is reduced

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Summary of Converging Lens
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Summary of Mirrors Lenses
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Websites

• http//www.glenbrook.k12.il.us/gbssci/phys/CLass/r
  efrn/u14l5a.html
• http//www.physicsclassroom.com/Class/refrn/U14L5a
  .html

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Types of Telescopes
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Galileo's Refracting Telescope

• Magnification is produced by two lenses
• The Objective Lens (Fo) points toward the object
  and produces a real image
• The Eyepiece (Fe) produces a virtual image seen
  by your eye
• Magnification is the ratio
• of the focal lengths of
• the two lenses
• m Fo/Fe

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The Magnifying Glass

• Angular magnification of the object depends upon
  how close you hold the glass and the near point
  of your eye (normally 25 cm, but changes with
  age)
• M near point / F

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Eye Problems

• Lens in eye doesnt focus
• light on retina
• Farsightedness
• Light focuses behind the retina
• Only objects at distance are seen clearly
• Need lenses that are thicker in the middle
• Nearsightedness
• Light focuses in front of the retina
• Only objects up close are seen clearly
• Need lenses that are thinner in the middle

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Eyeglasses

• Power of the prescription glasses
• P 1/Fglasses 1/do(glasses) 1/do(no
  glasses)
• SI unit diopter (m-1)
• Preferred vision range
• Far point is infinity (8)
• Near point is 25 cm

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Wave behavior of Light

• Diffraction the spreading out of a wave as it
  passes an edge or opening
• Interference when waves overlap and produce new
  larger waves

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Diffraction Grating

• Light passes through multiple closely spaced
  slits and creates only one wavelength of light
• ? d x / L
• ? wavelength of light (nm)
• d slit separation (m)
• x space between bright bands (m)
• L distance from slits to screen (m)

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Polarization
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Lasers

• Light Amplification Stimulated Emission of
  Radiation
• Unidirectional, one color
• Parts
• Laser Substance (solid, liquid, or gas)
• Optical Cavity (tube-like shape with mirrors at
  both ends)
• Pump energy source lamp that shines strong,
  bright light into the cavity
• Uses Surgery, cutting metal, fiber optics,
  surveying
• Types Helium and Neon, CO2, Argon

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