Mirrors

1
Mirrors Lenses

• Jayne Robinson

2
20-1 The Optics of Mirrors

• Plane Mirrors mirrors with a flat surface a
  piece of glass coated with a reflective coating
  on the front or back.
• Virtual image an image in which no light rays
  pass through the image it appears behind the
  mirror, is upright, and appears as far behind the
  mirror as the object is in front of the mirror.

3
20-1 The Optics of Mirrors

• Concave Mirrors curved inward like a spoon. The
  image formed by concave mirrors depends on how
  far the object is located from the mirror.
• Optical axis - straight line through the center
  of the mirror
• Focal point the point on the optical axis
  through which all reflected light rays pass.
• Focal length distance from the center of the
  mirror to the focal point.

4
20-1 The Optics of Mirrors

• Concave Mirror Images
• Real image an image formed where the light rays
  meet and it can be projected onto a screen forms
  when the object is located farther than the focal
  point from a concave mirror it is also enlarged
  and inverted (upside down).
• An object placed at the focal point reflects no
  image b/c the light rays do not converge (meet)
  exgt car headlights, flashlights, and spotlights
  use these mirrors.

5
20-1 The Optics of Mirrors

• Concave Mirror Images
• Virtual image appears behind the mirror, is
  upright and enlarged formed when the object is
  between the focal point and the mirror. Make-up
  mirrors use this type of mirror.

6
20-1 The Optics of Mirrors

• Convex Mirrors curve outward
• The reflected rays never meet so the image is
  always virtual, upright, and smaller than the
  actual object.
• Used in stores and factories to see large areas,
  and used for rear-view or side-view mirrors on
  autos to see a wide view of traffic. However,
  objects are always closer than they appear.

7
20-2 The Optics of Lenses

• Convex Lenses thicker in the middle than at the
  edges.
• Light rays are refracted toward the center of the
  lens and converge at the focal point they are
  capable of forming real images that can be
  projected on a screen.
• The amount of refraction depends on the change in
  the speed of light as it passes through the
  material and the shape of the object.
• Thick lenses with very curved surfaces bend light
  more than ones with less curved surfaces.
• The focal length of thick lenses is shorter than
  those of thin lenses.

8
20-2 The Optics of Lenses

• Cameras and the human eye lens view objects that
  are more than two focal lengths away so the real
  image is smaller and inverted. (The brain
  converts the image to upright.)
• If the object is between one and two focal
  lengths from the lens, the real image is inverted
  and larger than the object. This is the method
  used by movie projectors and overhead projectors.

9
20-2 The Optics of Lenses

• Concave Lenses thinner in the middle than at
  the edges.
• Light rays diverge and never form a real image.
  The image is virtual, upright, and smaller than
  the object.
• Concave lenses are usually used in combination
  with other lenses.
• They are used with convex lenses in telescopes
  and cameras to spread out incoming light and
  extend the focal length to see far-away
  objects.
• The are also used to correct nearsighted vision.

10
20-3 Lenses Vision

• Light enters your eye through the cornea
  (transparent covering), then passes through the
  pupil (opening). The iris (colored part) adjusts
  the pupil size to control how much light reaches
  the lens. The light converges to form an
  inverted image on the retina.

11
20-3 Lenses Vision

• The lens in the eye is soft, and flexible muscles
  in the eye can change its shape.
• When you look at a distant object, you need a
  longer focal length, so your eye muscles adjust
  the lens to a less convex shape.
• When you focus on a near object, the eye muscles
  increase the curvature of the lens to shorten the
  focal length.

12
20-3 Lenses Vision

• Nearsighted has difficulty seeing distant
  objects b/c the eyeball is too long or the
  corneas are too flat to allow the rays to
  converge on the retina. Concave lenses correct
  this problem by diverging the light rays before
  they enter the eye. (Fig 20-10 p. 520)

13
20-3 Lenses Vision

• Farsighted - cant focus clearly on nearby
  objects b/c the eyeball is too short or corneas
  are too flat to allow the rays to converge on the
  retina. The image is focused behind the retina.
  Convex lenses converge the image at the retina.

14
20-3 Lenses Vision

• Astigmatism causes blurry vision b/c the
  surface of the cornea is curved unevenly. These
  corrective lenses have an uneven curvature so as
  to smooth out the curves.

15
20-3 Lenses Vision

• Methods to correct vision currently include
• Eye glasses
• Contact lenses worn on the cornea.
• Laser surgery to change the pull of the muscles
  that control the shape of the eyeball itself,
  making it less elongated or more elongated.

16
20- 4 Optical Instruments

• Telescopes devices designed to magnify objects
  far away.
• Early telescopes were made of mirrors and lenses.
• Around 1600, lensmakers in Holland constructed a
  telescope.
• In 1609, Galileo built and used his own telescope
  to discover the moons of Jupiter, the phases of
  Venus, and some details of the Milky Way.

17
20- 4 Optical Instruments

• Refracting telescopes use two convex lenses to
  gather and focus light from distant objects.
• The objective lens is the larger lens that allows
  light to enter and form a real image of the
  object.
• The eyepiece is a smaller lens that forms an
  enlarged, virtual image of the real image, but
  the image is inverted.
• Refracting telescopes must have a very large,
  very heavy lens to see faraway stars planets.

18
20- 4 Optical Instruments

• Reflecting telescopes use a concave mirror, a
  plane mirror and a convex lens to magnify distant
  objects.
• Light enter the telescopes and is reflected by
  the concave mirror to a plane mirror.
• The plane mirror reflects the rays to form an
  inverted real image in the telescope.
• The convex lens (eyepiece) magnifies the image.
  This lens can be replaced by a camera to
  photograph the image.

19
20- 4 Optical Instruments

• Binoculars and terrestrial telescopes (used for
  bird-watching) use a third lens or reflecting
  prism to invert the upside down image so it
  appears upright.
• Microscopes use two convex lenses with short
  focal lengths to magnify very small, close
  objects.
• Light enters through the objective lens and forms
  a real, enlarged image.
• This image is magnified again to create a
  virtual, enlarged image.
• The image is upside down and backwards.