Transcript Chapter 22: Optics

Light and Optics
Unit 8: Light and Optics
Chapter 22: Optics
 22.1
Optics and Reflection
 22.2
Refraction
 22.3
Mirrors, Lenses, and Images
22.1 Investigation: Reflection
Key Question:
How do we describe the reflection
and refraction of light?
Objectives:

Identify the range of frequencies humans can hear.

Describe the how perception influences the sound humans hear.

Make and analyze a histogram of class data.
Optics
 Optics
is the study of how
light behaves.
 Explaining
how magnification
occurs is part of the science
of optics.
 Diagrams
of light use one or
more imaginary lines called
light rays to show how light
travels.
Optics
A ray diagram is an
accurately-drawn sketch
showing how light rays
interact with mirrors,
lenses, and other optical
devices.
 The
curved surface of a
magnifying glass bends
light so that it appears to
come from a much larger
thumb.
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.
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.
Light rays
 Reflection
occurs when light bounces off a surface
and when light bends while crossing through
materials.
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.
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.
The law of reflection
 The
law of reflection
states that the angle of
incidence equals the
angle of reflection.
 Light
rays reflect from a
mirror at the same angle
at which they arrive.
Law of Reflection
The incident ray
strikes the mirror.
The reflected ray
bounces off.
The angle of
incidence equals
the angle of
reflection.
Unit 8: Light and Optics
Chapter 22: Optics
 22.1
Optics and Reflection
 22.2
Refraction
 22.3
Mirrors, Lenses, and Images
22.2 Investigation: Refraction
Key Question:
How do we describe the refraction of light?
Objectives:

Listen to beats and explain how the presence of beats is
evidence that sound is a wave.

Create interference of sound waves and explain how the
interference is evidence for the wave nature of sound.
Refraction
 A straw
in a glass of water
makes a good example of
refraction.
 The
illusion is caused by
refracted light rays when
they cross from water back
into air before reaching your
eyes.
Refraction
 Materials
with a higher index of refraction bend
light by a large angle.
 Refraction
occurs when light rays cross a surface
between two materials that have a different index
of refraction.
What direction does bend light?
 A light
ray going from a low index of refraction into a
higher index bends toward the normal line.
 A light
ray going from a high index of refraction to a
low index bends away from the normal line.
Refraction
 Vegetable
oil and glass
have almost the same
index of refraction.
 If
you put a glass rod into
a glass cup containing
vegetable oil, the rod
disappears because light
is NOT refracted!
Total internal reflection
 The
angle of incidence at which light begins
reflecting back into a refractive material is called the
critical angle.
 Total
internal reflection happens when the angle of
refraction becomes greater than the critical angle.
Fiber optics
 A solid
glass rod can become
a pipe that carries light.
 This
happens if light enters
the rod at an angle of
incidence greater than the
critical angle.
 Inside
the rod, light reflects off
of the inside walls and
bounces back into the rod
because of total internal
reflection.
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.
Unit 8: Light and Optics
Chapter 22: Optics
 22.1
Optics and Reflection
 22.2
Refraction
 22.3
Mirrors, Lenses, and Images
22.3 Investigation: Images from Mirrors and
Lenses
Key Question:

How do mirrors and lenses
form images?
Objectives:
Use the laser flashlight to trace light rays from a lens to
determine its focal length.
 Show how ray diagrams are used to predict where images form
with lenses and mirrors.
 Use the thin lens formula to predict the locations of projected
images.

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.
Mirrors, Lenses, and Images
 Objects
are real physical things that give off or
reflect light rays.
are “pictures” of objects that are formed in
space where light rays meet.
 Images
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.
Virtual images
 Because
the light rays do
not actually meet, a virtual
image cannot be projected
onto a screen or on film.
 Virtual
images are illusions
created by your eye and
brain.
Lenses
 An
ordinary lens is a
polished, transparent disc,
usually made of glass.
 The
shape of a converging
lens is described as being
“convex” because the
surfaces curve outward.
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.
Converging lenses
 For
a converging lens, the first surface (air to
glass) bends light rays toward the normal.
 At
the second surface (glass to air), the rays bend
away from the normal line.
Diverging lenses
 A diverging
lens bends the rays outward, away from
the focal point.
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:
1.
2.
3.
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).
The image formed by a lens
 A converging
 In
lens can also form a real image.
a real image, light rays from the object actually
come back together.
Real images
 To
make an image of any
object, a lens collects rays
from every point on an
object.
 Rays
from each point on
the object are brought back
together again to make
each point of the image.
 Even
when you cover half
the lens, you still see the
whole image.
Magnification
 The
magnification of an image is
the ratio of the size of the image
divided by the size of the object.
 A lens
with a magnification of 4
creates an image that appears four
times larger than the real-life object.
The Telescope
 When
people think of a telescope, most of them think
of a refracting telescope.
 An
astronomical refracting telescope is constructed
of two converging lenses with different focal lengths.
 The
lens with the longest focal length is called the
objective and the shorter-focal-length lens is the
eyepiece.
The refracting telescope
 The
image from this refracting telescope is inverted
which is usually fine for looking at objects in space.
The reflecting telescope
 Because
high-quality, large
lenses are difficult to
make, reflecting
telescopes use a concave
mirror instead of one lens.
 The
diagram shows a
reflecting telescope, much
like the one used by the
Hubble Space Telescope.
Optical systems
 Optical
systems are built from lenses, mirrors, and
prisms.
 Optical
systems do two things:
— collects light rays
— changes/processes the light rays to form an
image.
 A camera
is an optical system that collects light to
record an image.
Pin-hole camera
 A simple
optical system
can be made with a
pinhole in a box.
 The
image inside the
box forms because light
rays that reach a point
on the box surface are
restricted by the pinhole
to come from only a
pinhole-sized point on
the object.
Pin-hole camera
 The
larger the lens, the
brighter the image. This
is because a larger lens
collects more light rays.
 Multiple
lenses are
useful because they
allow an optical system
to change the size of an
image.
A telephoto camera lens
uses two or more lenses
that move relative to one
another.
Recording images
 There
are two basic techniques
for recording images.
— Film records an image by
using special inks that
respond to light.
— A digital camera uses a tiny
sensor called a CCD.
Recording images
 There
are separate light sensors for red
light, blue light, and green light.
 A color
image is recorded as a table of
numbers.
 Each
point on the image has three
numbers corresponding to the amount of
red light, blue light, and green light.
Recording images
 The
resolution of a digital camera is
the number of points, called pixels,
that can be recorded by the CCD.
 A 2-megapixel
camera stores 2
million pixels per image.
 Since
each pixel is three numbers, a
2-megapixel image requires 6 million
stored numbers.
Hope for the Blind

Mike, a 28-year-old blind man,
lies awake on an operating table,
while surgeons place a threemillimeter square electrode
panel on the retina of his
anesthetized eye.

Soon, a medical research team
will stimulate the electrodes, in
hopes that these electrical
impulses will do what the rods
and cone cells in his eye once
did.