When light strikes an object there are different ways it can be affected.

Light can be transmitted,

reflected,

refracted,

and absorbed,

It depends on the type of matter that it strikes.

For example light can transmit or pass through substances such as air, glass, and water.

Substances that have the ability to transmit light are called a medium.

The amount of transmission will depend on such factors as the

thickness,

composition,

and clarity of the medium.

Most of the light is reflected off of substances like mirrors or white snow. However, mirrors reflect light in a predictable pattern; snow scatters light.

A substance that can absorb a large amount of light energy is a black surface.

For example, the light energy absorbed by black asphalt is converted to heat energy and as a result the asphalt will become warm.

Whereas, a white surface will reflect a great amount of light energy and absorb little light.

For example, white snow will remain cool because it reflects a lot of the light energy and absorbs little light.

Substances can be classified according to the way they transmit, reflect, and absorb light.

The three different types of substances are transparent,

translucent,

and opaque.

transmit or allow light through easily;

allow light through them in straight lines;

reflect some light;

absorb a small amount of light; and

allow objects to be seen clearly through them. Examples of transparent substances are

glass,

air,

plastic wrap,

and clean water.

transmit some light through them;

scatter the light in all directions;

vary in the amount of light reflected; and

not allow objects to be clearly visible through them.

Examples of translucent substances are

frosted glass,

smoke in the air,

and human skin.

not transmit any light;

block the light;

reflect and absorb light; and

not allow any visible object to be seen through them.

Examples of opaque substances are

wood,

metal, and

rocks.

Light travels in all directions from the source of light.

Light travels in straight lines until it hits something or passes through a medium with a different composition.

A light ray can be drawn to represent a narrow beam of light.

When light hits a transparent object, the light rays will pass through the object. Some light rays that strike a translucent object will pass through the object while at the same time other light rays are reflected.

Opaque objects will not allow any of the light to pass through, but they will reflect and absorb light rays depending upon their colour.

Substances that can reflect light to produce an image are called mirrors.

The typical mirror is a piece of glass with a thin coating of silver on the on the back.

Three types of reflecting surfaces are plane mirrors,

convex mirrors,

and concave mirrors.

The incident ray: The ray from the source of light to the point of incidence.

The reflected ray: The ray that bounces off the mirror.

A normal (N): A line drawn at right angles (perpendicular) to the reflecting surface of the mirror.

The angle of incidence: The angle between the incident ray and the normal.

The angle of reflection: The angle between the reflected ray and the normal.

The point of incidence: The point where the incident light ray strikes the mirror.

The First Law of Reflection states: the angle of incidence = the angle of reflection

The Second Law of Reflection states: the incident ray, the reflected ray, and the normal all lie on the same plane

Images formed in a mirror have four main characteristics. These characteristics are:

Size: may be smaller than the object viewed, larger than the object viewed, or the same size as the object viewed.

Orientation: upright (right-side up) or inverted (upside down).

Location : possible choices are behind the mirror, closer to the mirror, etc.

Type : a real image or virtual image.

A. Plane Mirrors

A ray diagram can be drawn to show how the image is formed in a mirror.

The object viewed in a plane mirror appears to be behind the mirror, which is an optical illusion. The image formed in the mirror has the same characteristics as the object.

A ray diagram to locate the image in a plane mirror can be drawn using the following instructions.

1. Draw a line to represent a mirror and shade in one side of the line to represent the reflective coating of the mirror.

2. Draw the object in front of the mirror.

3. Draw in normals from the mirror surface to the ends, middle, or other significant parts of the object.

4. Measure the length of each normal and then draw normals of equal length behind the mirror. The image can be drawn by connecting the ends of the normals on the backside of the mirror.

The images produced in a plane mirror are upright,

the same size as the object,

flipped sideways (laterally inverted),

and the same distance behind the mirror as the object.

The image cannot be projected onto a piece of paper therefore, it is considered virtual.

These characteristics of plane mirrors make them useful for such things as

decorative mirrors in homes,

rear view mirrors in vehicles,

and mirrors to assist dentists.

B. Curved Mirrors

There are some other terms used to study the reflection of light from curved mirrors. These terms are used for convex and concave mirrors.

Centre of curvature (C): The centre of the sphere which determines the shape of a curved mirror.

Radius of curvature: The distance from the centre of curvature of the mirror surface.

Vertex (V): The centre of the mirror.

Principal axis (P.A.): The straight line passing through the vertex and the centre of curvature.

Principal focus (F): Or focal point is the place where reflected rays converge and cross at a single point on the principal axis.

Focal length (f): The distance from the principal focus to the vertex.

Virtual principal focus (F'): A point behind a convex mirror; it is where the extended reflected rays appear to converge.

Objects form an image that is always: smaller

upright

behind the mirror

virtual

The reflecting light rays diverge in front of the mirror so the rays need to be extended behind the mirror to form an image.

Convex mirrors have a wide field of view. These mirrors can also make things appear further away than they actually are.

The wide field of view present in convex mirrors makes them useful surveillance mirrors.

Various uses of convex mirrors include some rear view mirrors in vehicles and security mirrors in department stores, grocery stores, and gasoline stations.

Concave mirrors have a wider range of images compared to convex and plane mirrors.

For example objects at a distance from the mirror (beyond the centre of curvature) have an image that is:

smaller

inverted

real

In this example the light rays converge in front of the mirror to form a real image.

Objects that are very close to the mirror (between the principal focus and the mirror) have an image that is:

larger

upright

virtual

The type of image formed will depend on the location of the object in front of the mirror.

In this example, the light rays diverge in front of the mirror to form a virtual image behind the mirror.

Concave mirrors that are pointed towards the Sun will reflect all of the light energy towards one place, the principal focus.

This light energy can cause things to heat up and ignite.

Some uses of concave mirrors include solar cookers for food,

heating water,

heating homes,

generating electricity,

and forming images in telescopes.

How Stuff Works – Types of Materials

Image Formation in Plane Mirrors

How Stuff Works – Types of Mirrors

Concave Mirrors