The Ray Model of Light

Lesson 4

Light and Matter• Light is represented as straight

lines called rays, which show the direction that light travels.

• Ray diagrams are drawings that show the path that light takes after it leaves its source. Each ray ends with an arrow to indicate the direction of travel.

Light and Matter• Ray diagrams can help explain why

the brightness of a light changes with distance. The more rays that reach your eyes, the brighter the object appears

Light and Matter• Ray diagrams are used to describe

what happens when light strikes an object.

• Light travels in straight lines until it strikes something.

• Materials have different properties which affects what happens when light strikes them

Light and Matter• Transmit - Light passes through them.

Example: Clear glass

• Absorb – Light is absorbed by the object and turned into heat

• Reflect – Light bounces off the object.

Light and MatterThe properties of materials can then be

further classified

• Transparent materials, such as clear glass or clear plastic, transmit light freely.

• Transparent materials absorb and reflect very little light. Example: a clear window

Light and MatterTranslucent materials transmit some

light, but not enough to see through the material clearly. Example: A frosted window pane. Some light can pass through, but you cannot see what is on the other side of the frosted glass in any detail.

Light and Matter• Opaque objects absorb and reflect

light, but they do not transmit it

Shadows• A shadow occurs when an opaque

object blocks the direct light from a light source

Shadows• Ray diagrams can be used to show

1.How the size of the shadow depends on the objects distance from the light source• A larger object will produce a larger

shadow• An object closer to the light source

will produce a larger shadow

Shadows

Shadows2. How the sharpness of the edges of a

shadow are depend on the objects distance from the light source and the size of the light source.

– The closer the object is to the light source the fuzzier the shadow is.

Shadows• Large light sources provide shadows

with fuzzy edges - penumbra– This is due to the object only partially

blocking the light. • The umbra is the part of the shadow in which

all light rays from the light source are blocked

– Point source (Small light sources) show well defined shadows

TShadows

Drawing Light Ray Diagrams of Shadows

1. Draw a line from the top of the light source to the top of the opaque object and continue onto the wall

2. Draw a line from the top of the light source to the bottom of the opaque object and continue onto the wall

Drawing Light Ray Diagrams of Shadows

3. Draw a line from the bottom of the light source to the top of the opaque object and continue onto the wall

4. Draw a line from the bottom of the light source to the bottom of the opaque object and continue onto the wall

5. Label the umbra and penumbra

Examples

Reflection of light• Even objects that do not reflect an

image still reflect light.

• Everything you can see in the room right now is reflecting light.

Reflection of light• When light rays reflect off a rough

or uneven surface, they do not remain parallel but are scattered in different directions, resulting in a diffuse reflection. Because the light is scattered, the rooms lighting is even without bright spots.

Questions – Hand in• 1. How does a transparent object interact with

light? K (1)• 2. How does a translucent object interact with

light? K (1)• 3. How does an opaque object interact with light?

K (1)• 4. What is diffuse reflection? C (1)• 5. Make two light ray shadow diagrams of your

choice. C (4)