CHAPTER 5 Optical Systems

Make use of Mirrors and Lenses!

5.1 The Ray Model of Light

Sir Isaac Newton – developed the particle model of light- thought that light was made of tiny particles that travelled in a straight line until they entered the eye.

• Light is made up of both the particle model and the wave model = the ray model of light. • Light is represented as a straight line showing direction of travel

Light and Matter

What you see depends on the amount of light available

In dim light you can no longer tell colours apart The type of matter in an object determines the

amount of light it absorbs, reflects, and transmits

Transparent

• Light can pass through freely, only a small amount

of light is absorbed and reflected (Example: air, water, and glass)

Translucent

• Most light rays get through but they are scattered in all directions (Example: frosted glass)

Opaque

• Prevents light from passing through (Example: cardboard)

Diagrams of How Light is Reflected and Refracted

Shadows

You can predict shadows using the ray model of light

Size of shadows are related to distance from the light source

Light can be Reflected

• To act like a mirror a material must: have a smooth surface compared to the wavelength of the light striking the surface

• If the surface is uneven then the rays will be reflected at different angles

The Law of Reflection• Incoming ray = incident ray• Ray that bounces off = reflected ray• Right angle between the two rays• Normal line is at right angles to the reflecting surface• Angle of incidence (i) = Angle of reflection (r) = Law of Reflection

Light Can Be Refracted • When light is bent, when it changes speed

from one medium into another, it is called refraction

• When light moves between air and glass it slows down because glass is more dense

• Angle of refraction = angle of a ray of light emerging from the boundary between two materials it is measured between the normal and the refracted ray

Refraction of Light in Air

Can occur when light passes through air of different temperatures

Warm air is less dense than cold air

Results in a mirage

Homework Time

Textbook questions Page 181

Questions # 1-7, 9

There will be a homework check!

5.2 Using Mirrors to Form Images

Plane mirror – flat smooth mirror, where you appear to be the same distance behind the mirror as you are in front of the mirror

How Do Reflected Rays Form an Image That We See in a Mirror?

• The reflected light that bounces off the mirror is doing so in all directions but only certain ones reach the pupil of our eye

• Our brain knows that light travels in a straight line so it interprets the image coming from behind the mirror

Image Size, Distance and Orientation

Both will be the same when using a plane mirror

A plane mirror will produce an image that has the same orientation as the object (both upright) but left and right will appear reversed

Concave Mirrors • Curve inwards, and reflect light rays to

form images • Causes light rays to converge and meet

at a focal point • Image produced depends on the

distance form the focal point

Concave Mirrors • Image will be small and upside down

as you get closer the image will get larger to a point where it is between the mirror and the focal point where the image will appear upright (see p.185)

• Used for flashlights, headlights, lighthouses, telescopes, and make-up mirrors

Convex Mirrors • Curves outward, opposite to

concave, diverging rays are given off • Image is always upright and smaller

then the actual image

Convex Mirrors Main Characteristics: 1. Objects appear to be smaller than they are2. More objects can be seen than in a plane mirror of the same size • Used for security mirrors, on car

mirrors

Homework Time!

Textbook questions Page 189

Questions #1-5

5.3 Using Lenses to Form Images

• Lens = a curved piece of transparent material that refracts light rays to either diverge or converge

• Can be either convex or concave

Concave Lenses • Are thinner in the middle than the

edge • Makes light rays diverge and won’t

meet at a focal point • Image is always upright and smaller

than the actual object • Uses: glasses, telescopes

Convex Lenses Thicker in the middle Light rays converge meeting at a focal

point behind the lens Image depends on the distance between

the lens and the object Uses: magnifying glasses, cameras,

telescopes, microscopes, our eye

Focal Length in Convex Lenses

• Focal length = distance between the lens/mirror and the focal point

• This has been mathematically determined for distance of object

Focal Length and Image Formed

Distance of Object from Lens

More than 2 focal lengths

Between 1 and 2 focal lengths

Object at focal length Less than 1 focal length

Type of Image Formed

Smaller, inverted Larger, inverted No image Larger, upright

Work Time !

Textbook QuestionsPage 197

Questions # 2-7, 11