THE RAY MODEL OF LIGHT

Section 11.4

Light Travels In A Straight Line

Light travels in a straight line.

This fundamental property of light can be used to understand how light behaves when it strikes a mirror or lens.

A Light Ray

A luminous object (e.g. Candle) radiates light in all directions. Spherical

Illumination

You can illustrate

this easily using a light ray diagram.

Light Ray

A light ray is a line and arrow that represents the direction and straight-line path of light.

Light Rays

Geometric Optics

What is geometric optics? The use of light

rays to determine the path of light when it strikes an object.

Incident Light

Light emitted from a source that strikes an object.

TRANSPARENT

When a material transmits all or almost all incident light

Objects can be clearly seen through the material.

TRANSLUCENT

When a material transmits some incident light but absorbs or reflects the rest.

Objects are not clearly seen through the material

OPAQUE

When a material does not transmit any incident light.

All incident light is either absorbed or reflected.

Objects behind the material cannot be seen at all.

FLAT MIRROR

An image is a reproduction of an original object that is produced through the use of light.

Mirror

A mirror is any polished surface that exhibits reflection.

Reflection

Reflection is simply the bouncing back of light from a surface.

Regular Reflection

- Reflection off a smooth, shiny surface

Diffuse Reflection

-Reflection off an irregular, rough, or dull surface making the rays scatter in different directions

Regular vs. Diffuse Reflection

Ray Diagram

- Shows how a single ray bounces off a mirror or other smooth surface

Normal

Incident Ray Reflected Ray

Angle of Incidence (θi)

Angle of Reflection (θr)

Ray Diagrams

Incident Ray An incoming ray that travels from a light source toward a surface

 Normal The line that is perpendicular to the surface where a ray of light

meets the surface  Reflected Ray A ray that bounces off the surface and begins at a point where the

incident ray and the normal meet Angle of Incidence (θi)

Angle between the incident ray and the normal Angle of Reflection (θr)

Angle between the reflected ray and the normal

Laws of Reflection

1. The angle of incidence equals the angle of

reflection (θi = θr)

2. The incident ray, the reflected ray, and the

normal all lie on the same plane

Ray Diagrams

1. Draw the normal (perpendicular to the mirror)

2. Measure the angle of incidence

3. Draw the ray of reflection (same angle as angle of incidence)

62° 62°

Homework:

Read Chapter 11.5

Questions: #1, 2, 3, 4, 5, 6

Important Terms The ray of light which strikes the surface is called the

incident ray. The ray of light which leaves the surface is called the

reflected ray. A line perpendicular to the surface is imagined at the

point of reflection. This line is called a normal. In this context the word normal means perpendicular.

The angle between the incident ray and the normal is called the angle of incidence, or the incident angle.

The angle between the reflected ray and the normal is called the angle of reflection, or the reflected angle

How can we predict how we see images or where we will see them?

RAY MODEL of LIGHT: makes the assumption that light travels a straight-line path as long as it is unhindered. 

The RAY is an ideal extremely narrow beam of light.  To look at something stationary at different angles would translate into your eye “catching” different rays as you moved your head. 

In the ray model of light, one treats light as if it were a particle. 

It describes REFLECTION, REFRACTION, and IMAGE FORMATION by mirrors and lenses.

WHAT HAPPENS WHEN LIGHT HITS an OBJECT?

Light can be ABSORBED and transform into THERMAL ENERGY (Think of that dark colored shirt on a hot day…)

Light can be TRANSMITTED through the object if the material is transparent. (Think of that glass of water on a hot day…)

Light can be REFLECTED (or bounced off and object)

Smooth vs. Rough Surfaces

On smooth surfaces, the reflection is known as specular. (Speculum = mirror (latin))  Your eye must be in the right place to receive the image.

  On rough surfaces, the reflection is known

as diffuse. Light hits the surface at different angles so it will bounce in different directions and no matter where you are you will see some of reflected light even if the rays are not the same.