BC SCIENCE 8

CHAPTER 5

5.1 THE RAY MODEL OF LIGHT

·  Particle Model of Light - a model that represents light

(Sir Isaac Newton) as a stream of fast moving,

particles that travel in a straight

line to the eye, where they are

absorbed to form an image

·  Ray Model of Light - light is represented as a

straight line, or ray, that shows

the direction the light wave is

traveling

Figure 5.2

Light and Matter

3 things happen when light strikes a material:

1.  The light may be transmitted (pass straight through it)

2.  The light may be reflected (bounce off it)

3.  The light may be absorbed (become “trapped” in it)

·  Transparent - if all or most of the light is

transmitted then the material is transparent. Clear glass, air, and water are transparent because light passes through them.

·  Opaque - if all or most of the light is absorbed

or reflected so that none of the light

passes through, the material is opaque. A book, a metal can, and a wall are opaque because they block light from passing through them.

·  Translucent - if only some of the light is

transmitted, the material is translucent. Wax paper, clouds, and lampshades are translucent because they scatter the light that passes through them.

Figure 5.4

Figure 5.5

Shadows

Figure 5.6

Figure 5.7A figure 5.7B

Light Can Be Reflected

Example:

Black letters printed on white paper:

·  Black ink is opaque because all the light falling on the ink is absorbed, but the white paper reflects all the light that falls on it.

·  To act as a mirror, the surface needs to be smooth compared to the wavelength of the light striking the surface.

·  Even though the page may feel smooth, a photograph taken through a microscope reveals the surface is actually not very smooth at all

·  The ray diagram below shows that the light rays bounce off randomly at all angles, giving the paper the appearance of being translucent.

Diagram 5.8

The Law of Reflection

·  Incident ray - a light ray that strikes a reflecting or

refracting material

·  Reflected ray - light that is bounced back from a

reflecting surface

·  Normal - an imaginary line drawn

perpendicular to a reflecting or refracting surface at the point where an incident ray strikes the surface

·  Angle of Incidence - the angle formed by the incidence

(i) beam and the normal

·  Angle of Reflection - the angle formed by the reflected

(r) beam and the normal

·  Law of Reflection - the angle of reflection equals the

angle of incidence

Figure 5.9

·  Angle of Refraction - is the angle of a ray of light

(R) emerging from the boundary between

two materials, measured between the

refracted ray and the normal

Figure 5.11A figure 5.11B

Refraction of Light in Water

·  Light rays from an object (ex. fish) bend away from the normal as they pass from water to air. This makes the object (ex. fish) seem closer to the surface than it really is.

Diagram 5.12

Refraction of Light in Air

·  Refraction -can occur when light travels through

air at different temperatures. Warm air is less dense than cold air. Light bends as it travels through different densities of air. The refraction of light through air can result in a mirage

·  Mirage - a misleading appearance or

illusion

Figure 5.13

5.2 Using Mirrors to Form Images

·  Plane Mirror - a flat, smooth mirror that reflects

Light

Diagram 5.14

·  Concave Mirror - is a mirror that curves inward and

reflect light rays to form images. (Parallel light rays bounce off the curved surface of a concave mirror and then meet at a single point or focal point)

- (ex. spotlights, flashlights, lighthouses, and car headlights)

Diagram 5.17

·  Focal Point - the point at which converging

light rays meet or from which light rays diverge

·  Converging - light rays that come together at

a focal point

Diagram 5.18

·  Convex Mirror - is a mirror that curves outwards and

reflect light rays to form an image (reflects parallel light rays as if they came from a focal point behind the mirror)

-  the reflection from a convex mirror has 2 main characteristics:

1.  objects appear to be smaller than they are

figure 5.21

2.  more objects can be seen in a convex mirror than in a plane mirror or the same size

ex. security mirrors, and side-view mirrors in a vehicle

5.3 Using Lenses to Form Images

·  Lens - is a curved piece of transparent

material that can bend, or refract, light rays in useful ways to help form a well-focussed image. (ex. glass or plastic)

-  the image that a lens forms depends

on the shape of the lens

- a lens can be convex or concave

·  Concave Lenses - are lenses that are thinner in the

middle than at the edge

-  light rays that pass through a concave lens diverge, refracting the rays outward and never meeting at a focal point

figure 5.22

-  the image formed is upright and

smaller than the actual object

-  ex. some types of eyeglasses and

telescopes

Figure 5.23

·  Convex Lenses - a lens that is thicker in the

middle than around the edges; focuses light rays at a focal point

- light rays converge when they pass through a convex lens

- the image formed by a convex

lens depends on the positions

of the lens and the object

Images Formed by Convex Lenses

Distance of Object from Lens / Type of Image Formed
More than two focal lengths
Object at focal point
Less than one focal length / Smaller, inverted
No image
Larger upright

Figure 5.25

·  Focal Length - the distance from the centre of

a lens or mirror to the focal point

·  Gravitational Lenses - when gravity causes many

light rays to come together at one point

diagram

1

Eric Hamber Secondary

BC Science 8 – Chapter 5

Learning Strategies Centre