Download Mirrors & Lenses

Chapter 20
Mirrors and Lenses
20-1 The Optics of Mirrors
• Plane Mirror – a mirror with a flat surface; a
piece of glass that has 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.
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.
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
because the light rays do not converge (meet); ex> car
headlights, flashlights, and spotlights use these mirrors.
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.
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.
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.
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.
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.
• They are also used to correct nearsighted
vision.
20-2 The Optics of Lenses
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 (back part of the eye).
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.
20-3 Lenses & Vision
• Nearsighted – has difficulty seeing distant objects
because 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.
(see Fig 20-10 on p. 520)
• Farsighted - can’t focus clearly on nearby objects
because 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 rays to correct the image at
the retina.
20-3 Lenses & Vision
Astigmatism – causes blurry vision because the
surface of the cornea is curved unevenly. These
corrective lenses have an uneven curvature so as
to “smooth out” the curves.
• 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
20- 4 Optical Instruments
• Telescopes – devices designed to magnify objects
far away.
– early telescopes were made of mirrors and
lenses.
– 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.
20- 4 Optical Instruments
– Refracting telescopes use two convex
lenses to gather and focus light from distant
objects.
• Refracting telescopes must have a very
large, very heavy lens to see faraway stars
& planets.
– Reflecting telescopes use a concave mirror,
a plane mirror and a convex lens to magnify
distant objects.
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.
– The image is upside down and backwards.
20 - 5 Applications of Light
• Polarized light – transverse waves that vibrate in only
one plane.
- Polarized filters – helps reduce glare; ex:
sunglasses, camera lenses.
• Lasers – are beams of light that do not spread out.
• Total internal reflection – causes diamonds and
other gems to sparkle due to the light reflecting inside
the object.
• Optical fibers – are transparent glass that is used in
communications to pipe light from one place to
another.