Chapter 5

Science 8

The Ray Model of Light and Images in Plane Mirrors

We will start this section with a review of the two main types of reflection.

1. Specular Reflection

  • Reflection from a smooth, flat surface, which produces an clear image of the surroundings.
  • Example: reflection off water, glossy finish on picture, glazed ceramics, smooth tin foil

2. Diffuse Reflection

  • Reflection from a rough surface, which does not produce a clear image but instead allows you to see what, is on the surface.
  • Example: Reading print from paper, matt finish on a picture, unglazed ceramics. Crumpled tin foil

The Ray Model of Light

  • A light ray similar to a water wave bounces off a solid barrier.
  • Ray diagrams use straight lines to illustrate the path of light rays.
  • Ray diagrams are used to study the behaviour of light when it meets a surface.
  • Light is represented by a straight line or ray that shows the direction the light is traveling.

Ray diagrams show you where the top of an image and the bottom of an image will be

The Law of Reflection

-The ray going into the mirror is called the incident ray.

-The ray that bounces off the mirror is called the
reflected ray.

-Thenormal is an imaginary line that is perpendicular (90o) to the mirror. It is used to help explain how waves reflect.

-The angle formed by the incidence beam and the normal is the angle of incidence.

-The angle formed by the reflected beam and the normal is the angle of reflection.

-Angles are always measured from the normal line to the ray

Law of Reflection

  • The Law of Reflection states that “the angle of reflection equals the angle of incidence.”
  • For example, if the angle of incidence is 60۫then the angle of reflection will be 60۫.

Three main types of mirrors

1) Plane Mirrors

2) Concave Mirrors

3) Convex Mirrors

Plane Mirrors:examples

1) rear view mirror in car

2) dentist mirror

3) periscope

4) inspection mirror ( "c" on page 191)

When are they used?

-plane mirrors are used when you want to see an image that is not distorted in any way- you want to see an image that is the same as the original object.

How Plane Mirrors Work (page 189)

  • Light shines on an object
  • Rays from the object strike the mirror
  • The rays that reach your eye seem to be coming from behind the mirror
  • This is called a virtual image

Steps to producing an image in a plane mirror

Step 1: Draw an incident ray of any angle except 90o going into the mirror from a point on the object.

Step 2: where the incident meets the mirror draw a normal line 90o from the mirror

Step 3: Measure the angle of incidence

Step 4: Measure the angle of reflection (same as incidence angle according to the Law of Reflection) from this you can draw the reflected ray on the other side of the normal.

Step 5: Repeat steps 1-4 on the same point of the object using another incident ray.

Step 6: Extend all the reflected rays until they meet at a point. This is where the image will appear.

Real Image VS Virtual Images

Mirrors can produce either a real image or virtual image.

Virtual Image the image is behind the mirror where the extended rays meet

-An image in which the light rays that reach the eye only appear to be coming from the
object

-The reflected rays do not meet; only the extended rays meet.

-The image lies behind the mirror

-Cannot form on a screen

Ex. Plane Mirror

Real Image the image is in front of the mirror where the reflected rays meet

-Reflected rays meet not the extended rays.

-The image lies in front of the mirror.

-Can form on a screen

Ex. Concave Mirror (some cases)

Describing Images

When we talk about images in mirrors we use the acronym S.P.O.T. to talk about the characteristics of the image.

S = size - is the image bigger or smaller than the actual object

P = position - how far is the image from the mirror

O= orientation - is the image right side up or inverted (upside down)

T= image types - is it a real image or a virtual image

So the image description for Plane Mirrors:

-S-same size

-P -image is same distance from mirror as the object.

-O - upright

-T- virtual as it is the extended rays that are meeting behind

the mirror.

Concave mirrors

Important vocab for this section:

-principal axis: the line perpendicular to the centre of themirror In the concave mirror it acts as the normal

-focal point: the point at which light rays entering parallel to the principal axis converge OR where light rays diverge.

-vertex: the point at which the principal axis meets the mirror.

How Concave Mirrors Work (page 203)

-Because concave mirrors are curved inwards, reflected light rays travel towards each other.

-Examples of this are satellite dishes and flashlights, make up mirrors.

The place where these reflected rays meet is called the focal point.

In a concave mirror the light rays converge in front of the mirror.

Steps to producing an image in a concave mirror

Step 1: Draw an incident ray parallel to the principal axis, the reflected ray of this will always be through the focus point.

Step 2: Draw the next incident ray through the focus and point on the object into the mirror, the reflected ray to this always leaves parallel to the principal axis.

Step 3: Extend the reflected rays until they meet either in front or behind the mirror. This point is the top of the image (the bottom always remains on the principal axis)

Three different images are produced in the concave mirror depending on where the object is. The object can be between the focus and the mirror, the focus and 2 x the focus, and behind the 2 x the focus.

Convex Mirrors

How Convex Mirrors Work (page 204)

-Because convex mirrors are curved outwards, reflected light rays travel away from one another.

-However, if you extend the rays behind the mirror, the extended rays will intersect at the focal point, F. The focal point of a convex mirror is behind the mirror.

-Examples of this are surveillance mirrors, safety mirrors on busses and disco balls.

Steps to producing an image in a convex mirror

Step 1: Draw an incident ray parallel to the principal axis, the reflected ray of this will always be through the focus point. (note that the reflected ray had to be extended in order to go through the focus.)

Step 2: Draw the next incident ray through the focus and point on the object into the mirror, the reflected ray to this always leaves parallel to the principal axis.

Step 3: Extend the reflected rays until they meet either in front or behind the mirror. This point is the top of the image (the bottom always remains on the principal axis)

Only two incident rays are needed to find the image but you can use another by drawing the incident ray through the vertex and measuring the angle of incidence, this in turn tells you the angle of reflection that gives you the reflected ray.

Three different images are produced in the concave mirror depending on where the object is. The object can be between the focus and the mirror, the focus and 2 x the focus, and behind the 2 x the focus.

In a concave mirror, the focal point (the point at which light rays converge) is in front of the mirror. However in convex mirrors, the focal point is in back of the mirror as light rays diverge when they hit this type of mirror.

1.Ray Model of Light
2.Specular reflection
3.Diffuse reflection
4.Ray Diagrams
5.Incident Ray
6.Reflected Ray
7.Normal
8.Angle of Incidence
9.Angle of Reflection
10.Law of Reflection / 11.Refracted Ray
12.Angle of Refraction
13.Image
14.Plane Mirror
15.Extended Ray
16.Virtual Image
17.Upright
18.Inverted
19.Object Distance
20.Image Distance / 21.Concave Mirror
22.Principal Axis
23.Vertex
24.Focal length
25.Focal point
26.Real Image
27.Convex Mirror