Transcript File

Lesson 6
MIRRORS 1 – PLANE MIRRORS
• A village in Italy does not receive sunlight
from November 11th – February 2nd because it
is in a valley.
The Use of Mirrors
• In 2006 they spent 170,000 $ on a 5m x 8m mirror
that reflects light down to the village in the winter.
• The mirror is controlled by computers that follow
the Sun across the sky
 The science of how light reflects and bends is
called geometric optics.
The Law of Reflection
 Smooth, shiny surfaces like calm water,
mirrors, glass, and even polished metal
allow you to see an image.
 The smoother the surface is, the better
the image will be.
 An image is a reproduction of an object
produced by an optical device like a
mirror.
 An optical device is any technology that
uses light
A ray diagram demonstrating
the law of reflection
 Normal – Dashed line drawn perpendicular
to the mirror at point of reflection.
 Incident Ray – The Incoming Ray of light
 Reflected Ray – The outgoing Ray of light
A ray diagram demonstrating
the law of reflection
 Angle of Incidence – The angle between the
incident ray and the normal. (i)
 Angle of Reflection – The angle between the
normal and the reflected ray. (r)
 Same as incident
ray
Law of Reflection
 When light reflects off a surface, the angle of
incidence is always equal to the angle of
reflection.
Using the Law of Reflection
 The law of reflection can be written using
mathematical symbols.
 The Greek letter theta, θ, is commonly used
as the symbol for an angle
 Subscripts are used to identify the angle.
 Angle of incidence = θ i
 Angle of reflection = θ r
Using the Law of Reflection
 Law of reflection is: θi = θr The angle of
incidence and the angle of reflection are
always measured from the normal and not
from the surface of the object. This is because
some surfaces are curved, making it difficult
to measure an angle from the surface.
Using the Law of Reflection
 The law of reflection is widely used in many
everyday use items such as reflectors
 Two plane mirrors placed at a of 90° to each other
can reflect light back to its source from a wide
variety of angles.
 In each reflection the angle of incidence is equal to
that of the angle of reflection
Plane Mirrors
 The law of reflection applies to light rays
reflected from both flat mirrors and curved
mirrors.
 Plane Mirror - Any mirror that has a flat
reflective surface
 The image in a plane mirror appears to come
from behind the mirror. It is not possible to
catch this image on a piece of paper placed
behind the mirror, since no light from the
object reaches this point.
 1. Consider the diagram at the right. Which one of
the angles (A, B, C, or D) is the angle of incidence?
B
______
Which one of the angles is the angle of
C
reflection? ______
 2. A ray of light is incident towards a plane mirror
at an angle of 30 degrees with the mirror
surface. What will be the angle of reflection?
90 – 30 = 60 degrees
Law of reflection states that angle of
incidence = angle of reflection
Therefore, the angle of reflection will
be 60 degrees
 3. Perhaps you have observed the image of the sun
in the windows of distant buildings near the time
that the sun is rising or setting. However, the image
of the sun is not seen in the windows of distant
building during midday. Use the diagram below to
explain, drawing appropriate light rays on the
diagram.
 4. A ray of light is approaching a set of three mirrors as
shown in the diagram. The light ray is approaching the
first mirror at an angle of 45-degrees with the mirror
surface. Trace the path of the light ray as it bounces off
the mirror. Continue tracing the ray until it finally exits
from the mirror system. How many times will the ray
reflect before it finally exits?
Plane Mirrors
 Because the light rays are not coming from
where your image appears to be, it is
called a virtual image.
 Virtual image - any image formed by rays
that do not actually pass through the location
of the image
 Real Images can be projected on to a screen
and touched.
Image Orientation in a Plane
Mirror
 The law of reflection also applies to looking
at objects in a mirror.
 The image in a plane mirror is an exact
reflection of the actual image.
 To view an image of a object in a mirror, you
must sight along a line at the image location.
As you sight at the image, light travels to
your eye along the path shown in the diagram
below.
Image Orientation in a Plane
Mirror
 The diagram shows that the light reflects off
the mirror in such a manner that the angle of
incidence is equal to the angle of reflection.
 In order to see the image, incident rays from
the object must bounce off the mirror at the
same angle as the reflected rays. If the eye
does not sight on the point of incidence on
the mirror the image will not be visible.
 When the image is sighted, it is visible that
the drawing an extended line of the reflected
ray through the back of the mirror will lead to
the image of the object.
Plane Mirror drawings
 Images are always drawn in dotted lines
 the point at which the extended reflected
rays converge (come together) behind the
mirror indicates the size and distance of the
object
 Note:
 Converge - come together
 Diverge - spread apart
 to locate any point on the image, you need to
draw only two light rays
 a ray from the top of the object parallel to the
base line and a ray drawn from the top of the
object to the point where the base line meets the
mirror
Plane mirror examples
 Plane Mirror Lab
Questions
 1. What is the difference between a reflected ray
and an incident ray? I (1)
 2. (a) What does the law of reflection state in




words? C (1)
(b) Write the law of reflection using
mathematical symbols. C(1)
3. What is a plane mirror? I (1)
4. What is a virtual image? I (1)
5. Draw the image of the letter B and N in a plane
mirror drawing. C (2)