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... 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 Light travels in a straight line (rectilinear propagation).

2. Light reflects (reflection) MirrorDust

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 ROY G BIV Yellow Green Blue Indigo Violet

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 Translucent 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. 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. Reflected Ray Normal Incident 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 MirrorConcave 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 Focus Principal Axis (C) (F) Reflective Surface

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.