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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
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