Chapter 19

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Chapter 19 Optics

• Jennie L. Borders

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Section 19.1 - Mirrors

• Optics is the study of how mirrors and lenses
  form images.
• A ray diagram shows how rays change direction
  when they strike mirrors and pass through lenses.
• The incoming ray, called the incident ray,
  approaches the mirror.

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The Law of Reflection

• The angle of incidence is the angle the incident
  ray makes with a line drawn perpendicular to the
  surface of the mirror.
• The angle of reflection is the angle the
  reflected ray makes with the perpendicular line.
• The law of reflection states that the angle of
  reflection is equal to the angle of incidence.

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Ray Diagram
exit ray reflected ray
incident ray
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Plane Mirrors

• A mirror with a flat shiny surface is a plane
  mirror.
• When you look into a plane mirror, you see your
  reversed reflection, a right-left reversed image
  of yourself.
• An image is a copy of an object formed by rays of
  light.

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Plane Mirrors

• To produce your image in a mirror, rays of light
  strike you and reflect. These reflected rays then
  strike the mirror and are reflected into your
  eyes.
• Your image appears the same distance behind the
  mirror as you are in front, and the image is
  right side up.

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Plane Mirrors

• A plane mirror always produces a virtual image.
• Although you can see a virtual image, this type
  of image cannot be projected onto any surface.
• A virtual image is a copy of an object formed at
  the location from which the light rays appear to
  come.

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Concave Mirrors

• When the inside surface of a curved mirror is the
  reflecting surface, the mirror is a concave
  mirror.
• The curvature of the reflecting surface causes
  the rays to come together.
• The point at which the light rays meet is called
  the focal point.

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Concave Mirrors

• A real image is the copy of an object formed at
  the point where light rays actually meet.
• Unlike a virtual image, a real image can be
  viewed on a surface such as a screen.
• Concave mirrors can form either real or virtual
  images.

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Real Image

• When the object is farther from the mirror than
  the focal point, the reflected rays meet in front
  of the mirror, making a real image.

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Virtual Image

• When the object is closer to the mirror than the
  focal point is the reflected rays spread out and
  appear to come from behind the mirror, making a
  virtual image.

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Convex Mirrors

• When the outside surface of a curved mirror is
  the reflecting surface, the mirror is a convex
  mirror.
• The curvature of the convex mirror causes the
  reflected rays to spread out.

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Convex Mirrors

• Convex mirrors always cause light rays to spread
  out and can only form virtual images.
• The image formed by a convex mirror is always
  upright and smaller than the object.

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Section 19.1 Assessment

• How is the angle of incidence of a light ray
  related to the angle of reflection?
• What type of image does a plane mirror form?
• What types of image can be produced by a concave
  mirror? A convex mirror?
• How are real images different from virtual images?

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Section 19.1 Assessment

• If you place an object 10 cm from a particular
  concave mirror, a virtual image forms behind the
  mirror. What can you infer about the focal point
  of the mirror?
• If you look inside the bowl of a shiny metal
  spoon, your image is upside down. If you look at
  the outside of the spoons bowl, your image is
  right side up. Explain.

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Section 19.2 - Lenses

• Light usually travels in straight lines.
• When light enters a new medium at an angle, the
  change in speed causes the light to bend, or
  refract.

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Index of Refraction

• How much the speed of a light ray slows as it
  enters a new material depends on the materials
  index of refraction.
• The index of refraction for a material is the
  ratio of the speed of light in a vacuum to the
  speed of light in the material.
• A material with a low index of refraction causes
  light to slow and refract very little.

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Index of Refraction
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Lenses

• A lens is an object made of transparent material
  that has one or two curved surfaces to refract
  light.

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Concave Lenses

• A concave lens (diverging lens) is curved inward
  at the center and is thickest at the outside
  edges.
• Concave lenses cause incoming parallel rays to
  spread out, or diverge.

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Concave Lenses

• Concave lenses always cause light rays to spread
  out and can only form virtual images.
• The image formed by a concave lens is always
  smaller than the object.

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Convex Lenses

• A convex lens (converging lens) is curved outward
  at the center and is thinnest at the outer edges.
• Convex lenses cause incoming parallel rays to
  come together, or converge.

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Convex Lenses

• The converging rays meet at a single point, the
  focal point, on one side of the lens opposite the
  object.
• Convex lenses form either real or virtual images.

Virtual image
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Convex Lenses
Real image
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Concave vs. Convex
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Fiber Optics

• Light rays are generally unable to exit through
  the sides of the curving fiber optic strands.
• Because of this, fiber optics are very useful for
  carrying information in the form of light.

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Total Internal Reflection

• The critical angle is the angle of incidence that
  produces an angle of refraction of 90 degrees.
• Total internal reflection is the complete
  reflection of a light ray back into its original
  medium.

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Total Internal Reflection

• Materials that have small critical angles are
  likely to cause most of the light entering them
  to be totally internally reflected.
• Such materials include diamond and the type of
  glass used in fiber optics.

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Section 19.2 Assessment

• What causes light rays to bend?
• Why can concave lenses only form one type of
  image?
• What type of images are formed by concave lenses?
  By convex lenses?
• How is a convex lens different from a concave
  lens? How are they the same?