Transcript b. Section 2.2 Mirrors and Lenses

Chapter 5: Mirrors and Lenses
Section 5.1 Ray Model of Light
Ray Model of Light
• Another model for light is that it is made up of
tiny particles called photons
• Photons travel in perfect, straight lines away
from a light source
• This model helps us to understand and predict
light’s behaviour, particularly with mirrors
and lenses.
I. Light and Matter
Transparent – objects that transmit light waves.
Transluscent – objects that transmit some light waves.
Objects are not seen clearly through them.
Opaque –objects that absorb or reflect light waves.
II. Shadows
• Ray diagrams can be used to explain shadows
• Light from a light source is blocked by an
object, casting a shadow behind the object
• The size of the shadow
depends on the distance
from the light source
III. The Law of Reflection
• When light bounces off
an object, we say it
reflects
• The incoming ray is
called the incident ray.
• The outgoing ray, that
bounces off the object,
is called the reflected
ray
• A dotted line can be drawn that is
perpendicular to the surface of the object; this
is called the normal.
• The angle between the incident ray to the
normal is the same as the angle of the
reflected to the normal.
A white surface reflects all light…but
why is it not a mirror?
• A mirror must have a
perfectly smooth
surface
• Paper is not smooth; it is
a rough reflecting
surface.
IV. Light can be Refracted
• When light moves from air
into glass, it slows down and
changes direction because
glass is denser than air.
• The angle of refraction is the
angle between a ray of light
emerging from the boundary
between two materials and
the normal.
• When light passes into more
dense material, it bends
towards the normal (and
slows down)
• When light passes into less
dense material, it bends
away from the normal (and
speeds up)
• Refraction causes optical illusions of objects
under water. They often appear to be closer
to the surface than they really are.
Mirages
• Mirages are formed by the refraction of light
as it passes from less dense hot air to more
dense cool air.
• On a hot day, air near the ground surface gets
very hot compared to the air above.
• Light traveling from the
sky gets bent as it
passes through the hot
air, which then appears
to eye to be pools of
water
5.2 Using Mirrors to Form
Images
I. Reflection
• Mirrors and highly polished objects reflect
light.
Law of Reflection
angle of incidence = angle of reflection
II. Images in a Mirror
III. Plane Mirrors
Plane mirror = flat mirror, like a bathroom mirror
• image appears behind
the plane mirror
• erect and same size
• image is inverted
sideways but not
vertically
• If you move towards the
mirror, the image moves
towards you.
• If you move away from
the mirror, the image
moves away from you.
IV. Concave Mirrors
• curve inward
• reflect light like a plane mirror, but reflected light
rays meet at a single point called a focal point
• these light rays that come together are called
converging rays
Images formed in Concave Mirrors
• distant objects appear to
be smaller and upside
down (‘inverted’)
• As you approach the focal
point of the mirror, the
object gets bigger, but is
still inverted
• If an object is between the focal point and
the mirror, the image is larger and upright.
Uses for Concave Mirrors
• lights placed at the focal point will reflect out
of the mirror in straight lines – used in
headlights and spotlights
• make-up and shaving mirrors are concave so
that your image is enlarged (making it easier
to see)
V. Convex Mirrors
• curve outward
• reflect parallel light as if it were coming from
a focal point behind the mirror
• since the light rays spread apart after hitting
the mirror, they are called diverging rays.
Images in Convex Mirrors
• objects appear
smaller than they
really are
• a wider view be seen
than in a plane mirror
of the same size
Uses for Convex Mirrors
• security mirrors in stores
• side-view mirrors in cars
5.3 Lenses
Lenses
• Light bends as it passes through glass and
then bends again as it passes back into air
• A lens is a curved piece of transparent
material that bends light in a controlled way
• Lenses can cause light rays to converge or
diverge as they pass through them
I. Concave Lenses
• Thinner in the middle than at the edge
• Light rays diverge or spread out as they pass
through the lens
• Image formed is upright and smaller than the
actual object
Uses for Concave Lenses
• some eyeglasses (near-sightedness)
• some telescopes
• spy glasses in doors
II. Convex Lenses
• Thicker in the middle than at the edge
• Light rays converge or come together as they
pass through the lens
• Image formed depends on the position of the
object with respect to the mirror
Focal Length in Convex Lenses
• light rays converge at a point called the
focal point
• The distance from the focal point to the centre
of the lens is the focal length
• An object far away from the lens appears
inverted (upside down) and smaller
An object close to the lens is upright and
magnified
Uses for Convex Lenses
• Magnifying glass
• Overhead projector
• Microscopes
• Eyeglasses (long-sighted)