Unit 6, Chapter 17

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Unit 6, Chapter 17
CPO Science Foundations of Physics
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Unit 6 Light and Optics
Chapter 17 Light and Color

• 17.1 Reflection and Refraction
• 17.2 Mirrors, Lenses, and Images
• 17.3 Optical Systems

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Chapter 17 Objectives

1. Describe the functions of convex and concave
   lenses, a prism, and a flat mirror.
2. Describe how light rays form an image.
3. Calculate the angles of reflection and refraction
   for a single light ray.
4. Draw the ray diagram for a lens and a mirror
   showing the object and image.
5. Explain how a fiber-optic circuit acts like a
   pipe for light.
6. Describe the difference between a real image and
   a virtual image and give an example of each.

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Chapter 17 Vocabulary Terms

• lens
• mirror
• prism
• optics
• geometric optics
• specular reflection
• diffuse
• converging
• diverging
• law of reflection
• normal line
• ray diagram
• magnification
• object

• index of refraction
• focal point
• focal length
• optical axis
• light ray
• magnification
• critical angle
• Snells law
• real image
• virtual image
• chromatic aberration
• refraction
• fiber optics
• dispersion

• magnifying glass
• spherical aberration
• reflection
• diffraction
• telescope
• focus
• total internal reflection
• resolution
• pixel image
• focal plane
• thin lens formula

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17.1 Reflection and Refraction

• Key Question
• How do we describe the reflection and refraction
  of light?

Students read Section 17.1 AFTER Investigation
17.1
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17.1 Reflection and Refraction

• The overall study of how light behaves is called
  optics.
• The branch of optics that focuses on the creation
  of images is called geometric optics, because it
  is based on relationships between angles and
  lines that describe light rays.

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17.1 Reflection and Refraction

• A lens is an optical device that is used to bend
  light in a specific way.
• A converging lens bends light so that the light
  rays come together to a point.
• A diverging lens bends light so it spreads light
  apart instead of coming together.

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17.1 Reflection and Refraction

• Mirrors reflect light and allow us to see
  ourselves.
• A prism is another optical device that can cause
  light to change directions.
• A prism is a solid piece of glass with flat
  polished surfaces.

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17.1 Reflection

• Images appear in mirrors because of how light is
  reflected by mirrors.
• The incident ray follows the light falling onto
  the mirror.
• The reflected ray follows the light bouncing off
  the mirror.

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17.1 Reflection

• In specular reflection each incident ray bounces
  off in a single direction.
• A surface that is not shiny creates diffuse
  reflection.
• In diffuse reflection, a single ray of light
  scatters into many directions.

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Law of Reflection
The incident ray strikes the mirror. The
reflected ray bounces off.
The angle of incidence equals the angle of
reflection.
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17.1 Law of reflection
30o
30o

• A light ray is incident on a plane mirror with a
  30 degree angle of incidence.
• Sketch the incident and reflected rays and
  determine the angle of reflection.

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17.1 Refraction

• Light rays may bend as they cross a boundary from
  one material to another, like from air to water.
• This bending of light rays is known as
  refraction.
• The light rays from the straw are refracted (or
  bent) when they cross from water back into air
  before reaching your eyes.

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17.1 Refraction

• When a ray of light crosses from one material to
  another, the amount it bends depends on the
  difference in index of refraction between the two
  materials.

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17.1 Index of refraction

• The ability of a material to bend rays of light
  is described by the index of refraction (n).

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17.1 Snell's law of refraction

• Snells law is the relationship between the
  angles of incidence and refraction and the index
  of refraction of both materials.

Angle of refraction (degrees)
Angle of incidence (degrees)
ni sin Qi nr sin Qr
Index of refraction of incident material
Index of refraction of refractive material
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17.1 Calculate the angle of refraction

• A ray of light traveling through air is incident
  on a smooth surface of water at an angle of 30
  to the normal.
• Calculate the angle of refraction for the ray as
  it enters the water.

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17.1 Dispersion and prisms

• When white light passes through a glass prism,
  blue is bent more than red.
• Colors between blue and red are bent proportional
  to their position in the spectrum.

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17.1 Dispersion and prisms

• The variation in refractive index with color is
  called dispersion.
• A rainbow is an example of dispersion in nature.
• Tiny rain droplets act as prisms separating the
  colors in the white light rays from the sun.

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17.2 Mirrors, Lenses, and Images

• Key Question
• How does a lens or mirror form an image?

Students read Section 17.2 AFTER Investigation
17.2
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17.2 Mirrors, Lenses, and Images

• We see a world of images created on the retina of
  the eye by the lens in the front of the eye.

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17.2 Mirrors, Lenses, and Images

• Objects are real physical things that give off or
  reflect light rays.
• Images are pictures of objects that are formed
  in space where light rays meet.

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17.2 Mirrors, Lenses, and Images

• The most common image we see every day is our own
  reflection in a mirror.
• The image in a mirror is called a virtual image
  because the light rays do not actually come
  together.

• The virtual image in a flat mirror is created by
  the eye and brain.

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17.2 Mirrors, Lenses, and Images

• Light rays that enter a converging lens parallel
  to its axis bend to meet at a point called the
  focal point.
• The distance from the center of the lens to the
  focal point is called the focal length.
• The optical axis usually goes through the center
  of the lens.

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17.2 The image formed by a lens

• A lens can form a virtual image just as a mirror
  does.
• Rays from the same point on an object are bent by
  the lens so that they appear to come from a much
  larger object.

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17.2 The image formed by a lens

• A converging lens can also form a real image.
• In a real image, light rays from the object
  actually come back together.

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17.2 Drawing ray diagrams

• A ray diagram is the best way to understand what
  type of image is formed by a lens, and whether
  the image is magnified or inverted.
• These three rays follow the rules for how light
  rays are bent by the lens
• A light ray passing through the center of the
  lens is not deflected at all (A).
• A light ray parallel to the axis passes through
  the far focal point (B).
• A light ray passing through the near focal point
  emerges parallel to the axis (C).

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17.3 Optical Systems

• Key Question
• How are the properties of images determine?

Students read Section 17.3 AFTER Investigation
17.3
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17.3 Optical Systems

• An optical system is a collection of mirrors,
  lenses, prisms, or other optical elements that
  performs a useful function with light.
• Characteristics of optical systems are
• The location, type, and magnification of the
  image.
• The amount of light that is collected.
• The accuracy of the image in terms of sharpness,
  color, and distortion.
• The ability to change the image, like a telephoto
  lens on a camera.
• The ability to record the image on film or
  electronically.

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17.3 The sharpness of an image

• Defects in the image are called aberrations and
  can come from several sources.

• Chromatic aberration is caused by dispersion,
  when different colors focus at different
  distances from the lens.

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17.3 The sharpness of an image

• Spherical aberration causes a blurry image
  because light rays farther from the axis focus to
  a different point than rays near the axis.

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17.3 The sharpness of an image

• Diffraction causes a point on an object to focus
  as a series of concentric rings around a bright
  spot.

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17.3 Thin lens formula

• The thin lens formula is a mathematical way to do
  ray diagrams with algebra instead of drawing
  lines on graph paper.

1 1 1 do di df
Object distance (cm)
focal length (cm)
Image distance (cm)
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17.3 Use the thin lens formula

• Calculate the location of the image if the object
  is 6 cm in front of a converging lens with a
  focal length of 4 cm.

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17.3 Image relay

• A technique known as image relay is used to
  analyze an optical system made of two or more
  lenses.

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Application The Telescope