Transcript – Optics Chapter 19 Jennie L. Borders

Chapter 19 – Optics
Jennie L. Borders
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.
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.
Ray Diagram
surface normal
incident ray
same
angle
exit ray
reflected ray
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.
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.
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.
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.
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.
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.
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.
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.
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.
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?
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 spoon’s bowl,
your image is right side up. Explain.
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.
Index of Refraction
How much the speed of a light ray slows
as it enters a new material depends on the
material’s 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.
Index of Refraction
Lenses
A lens is an object made of transparent
material that has one or two curved surfaces
to refract light.
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.
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.
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.
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
Convex Lenses
Real image
Concave vs. Convex
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.
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.
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.
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?