Transcript Grade-8-Sciencex

Grade 8 Science
Unit 2: Optics
The law of reflection allows mirrors to form images.
Many properties of light can be understood
using a wave model of light.
The History of Light
Pythagoras, Greek philosopher believed that:
• Beams of light were made of tiny particles.
• The eyes detected these particles and could see the object.
Light Technologies Include...
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Microscope
Telescope
Periscope
Binoculars
Fibre optics
Camera
Prescription contact lenses
Laser
Movie projectors
Overhead projectors
Light
Light: a form of energy that can be detected by the human eye.
Visible light: a mixture of all the colors of the rainbow.
Properties of Visible Light...
• 1. Light travels in a straight line (rectilinear propagation).
2. Light reflects (reflection)
Mirror
Dust
3. Light refracts (Refraction)
4. Light Disperses (dispersion)
5. Light travels through a vacuum (does not require a
medium; no particles involved)
6. Travels through objects to different degrees
Visible Spectrum of Light
• Can be seen due to the dispersion of light through a prism.
The constituent colors of white light are:
• Red
• Orange
• Yellow
• Green
• Blue
• Indigo
• Violet
ROY G BIV
The Wave Model
• Explains that light is a type of wave that travels through empty space
and transfers energy from one place to another
Electromagnetic Radiation
• The transmission of energy in the form of waves that extend from the
longest radio waves to the shortest gamma rays.
Types of Electromagnetic Radiation:
1. Radio waves: the longest wavelength and lowest energy and
frequency.
• Can be used to help us see the inside of our bodies to diagnose
illness. Ex. Magnetic Resonance Imaging (MRI)
• 2. Microwaves: have the shortest wavelength and the highest
frequency of all radio waves.
Ex. Microwave ovens, telecommunication satellites, radio telescopes,
radar (remote sensing)
How a Microwave works:
Microwave ovens use a
specific
frequency that is strongly absorbed
by water molecules in food.
3. Infrared Waves: Longer wavelength and lower
energy and frequency.
• Infrared means below red
• Also called heat radiation (Ex. Remote controls, computer, heat lamps,
motion sensors)
4. Visible Light Spectrum
Can be continually detected by our eyes.
5. Ultraviolet Waves: shorter wavelength and higher energy and
frequency.
• Very energetic
• Have the ability to kill bacteria in food and water and medical
supplies.
• Ex. Sun, detect fingerprints
6. X-Rays:
• have a shorter wavelength, and higher energy and frequency than UV.
• Used to photograph teeth, bones and the inside of machines, security
screening
7. Gamma Rays:
Have the highest energy and frequency and the shortest wavelength.
• Result from nuclear reactions.
• Produced by the hottest regions of the universe.
Gamma Rays: Nuclear Explosion
& Medical Treatments
The Ray Model of Light
• Used to study the behaviour of light when it meets a surface.
• Light is represented by a straight line or ray that shows the
direction the light is travelling.
Different Materials
Transparent
Opaque
Allows light to pass
through freely. Only a
small amount of light
is absorbed or
reflected.
Prevents any light from
passing through it. It will
only absorb or reflect light.
Translucent
Most light rays pass
through, but are
scattered in all
directions.
Rectlinear Propagation
Light travels in a straight line.
Solid Object
Shadow
Reflection…
Incident light ray:
The incoming light ray
Reflected light ray:
 The ray that bounces off the barrier
Normal:
An imaginary line that is perpendicular to the barrier.
Normal
Incident Ray
Reflected Ray
Angle of incidence:
The angle formed by the incidence ray and the normal
Angle of reflection:
The angle formed by the reflected ray and the normal
Plane Mirror
Concave Mirror
Specular (regular) vs. Diffuse Reflection
Applications of specular and diffuse reflection:
 Countertop surfaces
 Furniture or car wax
 Glazed vs. unglazed ceramics
 Matte vs. glossy finish on photographs or prints
Determining the Laws of Reflection
• The angle of incidence equals the angle of reflection
Types of Mirrors
1. Plane Mirrors
 A flat, smooth mirror
Characteristics of images using plane mirrors:
•Image size is equal to object size
•Image distance is equal to object
distance
•The image is upright
•The image is virtual
Examples of Plane Mirrors:
 Bathroom
mirrors
Rear-view mirrors
Dentist mirrors for looking at teeth
Periscopes
Concave Mirrors
 Have a reflecting surface that curves inward like the inside of a bowl
Characteristics of images using concave mirrors
(depends on the position of the object):
 it
can be smaller, larger, or the same size
 it can be upright or inverted
It can be real or virtual
Examples of Concave Mirrors:
Inside a metal spoon
Spotlights
Overhead projectors
Flashlights
Car headlights
Lighthouses
Satellite dishes
Convex Mirrors
• Curved outward like the outside of a bowl
Characteristics of images using convex mirrors:
• The image is smaller than the object
• The image distance is smaller than the object
distance
• The image is upright
• The image is virtual
Examples of Convex Mirrors:
• Safety mirrors at the front of a bus
• Side view mirrors of vehicles
• Disco balls
Ray diagrams (parts to know)
Light comes from this side
hard surface
Focal length
Centre of Curvature
(C)
Reflective Surface
Focus
(F)
Principal Axis
Real Vs. Virtual
Real Image:
• Formed when reflected rays (not extended rays) meet
• Located in the front of the mirror
• You need a screen to see the real image
Virtual Image:
• Formed when the reflected rays are extended
• Located behind the mirror
Refraction:
• The bending of a wave when it travels from one medium to another.
Refracted ray:
• Is in the second medium travelling in a different
direction than the incident ray.
Angle of refraction:
• The angle between the normal and the refracted
ray.
What happens…
As light travels from a less dense medium to a more dense medium
(ie. Slows down)?
The ray bends towards the normal.
As light travels from a more dense medium to a less dense medium
(ie. speeds up)?
The ray bends away from the normal.
• Why is the object not where you think it is?
If the light travels through two different media before it reaches your
eyes, it does not travel in a straight line.
The object is not where your brain thinks it is.
Lenses:
• Lenses refract light to form images.
• A curved piece of transparent material that refracts light in a
predictable way.
• Usually made from glass or plastic.
There are two types of lenses:
1. Convex
• Centre of the lens bulges out
• Causes light rays to bend toward each other (converge)
2. Concave
• Centre of the lens is curved in
• Causes light rays to bend away from each other (diverge)
Concave Vs. Convex
Convex Lenses
• Can act as a magnifying glass
• Each lens has its own focal length (the distance from the centre of the
lens to the focal point)
The greater the curvature
of the lens, the shorter the
focal length.
• Lenses have focal points on either side because light shines through
either side.
• The line through the centre of the lens is called the principle axis.
• Optical centre is where the principle axis meets the lens centre.
Concave Lenses
• To find the focal point, you must extend the refracted rays back.
Lenses with the
greater
curvature have the
shortest
focal length.
Corrective Lenses
Vision
Near-sighted Vision
• See objects up close but not at a distance.
• Concave lenses are used to correct this vision.
Far-sighted Vision
• See objects at a distance but not up close.
• Convex lenses are used to correct this vision.