Grade 8 Science Unit 2: Optics
Chapter 4: Many properties of light can be understood using a wave model of light.
The History of Light
Pythagoras
*A Greek philosopher
* Believed that beams of light were made of tiny particles.
* The eyes detected these particles and could see the object.
A famous scientist Galileo tried to measure the speed of light but was unsuccessful (diagram page 135)
Albert Michelson
* First person to measure the speed of light (3 x 108 m/s)(300 000 000) (Fig 4.9 page 136)
Speed: Light vs. Sound
Light
* 300 000 000 m/s
Thunder & Lightning
* Both the lightning strike and the roar of thunder happen at the same time.
* You see the lightning first. The sound travels much slower
* If you multiply the time (seconds) between the strike and the roar by the speed of sound, you will find
the approximate distance.
Stars in the Sky
The light takes thousands of years to reach our eyes. The light may be produced when the star dies so
by the time you see it that star no longer exists.
Light Technologies Include...
* Microscope
* Telescope
* Periscope
* Binoculars
* Fibre optics
* Camera
* Prescription contact lenses
* Laser
* Movie projectors
* Overhead projectors
Wavelength and Frequency p.139-141
Wavelength - the distance from one wave crest to the next; length of one unit of a wave that repeats
itself. You would think of a “rolling sea” as having long wavelengths And a “choppy sea” as having
short wavelengths.
Review parts of the wave p. 139 (crest, trough, amplitude, wavelength, wave height)
Frequency – the number of repetitive motions (oscillations) that occur in a wave in a given amount of
time. Measured in cycles per second or Hertz.
Amplitude: the height of a wave crest or depth of a wave trough as measured from the rest position.
* crest height = trough depth
* The larger the amplitude, the greater the energy transported.
Properties of Light p.149-153
1. Travels in a straight line. We call this rectilinear propagation.
2. Reflects – bounces back off an object
3. Refracts – bent stick effect (light bends as it passes through another *medium)
4. Disperses – separates such as when a rainbow disperses into its colors
5. Travels through a vacuum – travels through outer space to get here
6. Travels through transparent, translucent, and opaque materials through varying degrees.
*medium – surroundings. For example: the air is a medium, water is a medium.
Refraction – the bending of light as it travels from one medium to another. The amount of density
difference determines how much it bends
Visible light spectrum p.150, 151
– the array of colors of light that result from the separation or dispersion of white light.
ROY G BIV Not who? What?
This is the visible light spectrum in order of decreasing wavelength and increasing frequency.
RED ORANGE YELLOW GREEN BLUE INDIGO VIOLET
Red – longest wavelength/lowest frequency/least refraction
Violet – smallest wavelength/ greatest frequency/greatest refraction
The other colors fall in between red and violet.
All wavelengths of the electromagnetic spectrum enter our eyes but our brains only recognize the
wavelengths of the visible spectrum.
Why we see only one color
We see wavelength that are reflected. So when light hits an object that we view as green it is because
the other wavelengths were absorbed into the object but green was reflected.
****High frequency waves have short wavelengths/Low frequency waves have long wavelengths
The Wave Model
Explains that light is a type of wave that travels through empty space and transfers energy from one
place to another
A Wave... Refer to page 140 Figure 4.13
Electromagnetic Radiation
The transmission of energy in the form of waves that extend from the longest radio waves to the
shortest gamma rays
Electro Magnetic Radiation Spectrum
Refer to the spectrum on page 158
This diagram is showing the increasing wavelength whereas the one in your book shows decreasing wavelength
1. Radio waves: the longest wavelength and lowest energy and frequency. The uses of are MRI ,
Radio and television broadcasting
2. Microwaves: used to heat food
* Ex. Microwave ovens, telecommunication satellites, radio telescopes, radar (remote sensing)
3. Infrared Waves: * Also called heat radiation
* Ex. Remote controls, heat lamps, motion sensors
4. Visible Light Spectrum
* Can be continually detected by our eyes. ROYGBIV
5. Ultraviolet Waves: used to suntan
* Have the ability to kill bacteria in food and water and medical supplies.
* Ex. Sun, detect fingerprints
6. X-Rays: * Used to photograph teeth, bones and the inside of machines, security screening
X-Ray Imaging
7. Gamma Rays: have the highest energy and frequency and the shortest wavelength.
* Result from nuclear reactions.
Gamma Rays: Medical Treatment concentrated gamma rays used in radiation treatment of cancer.
Electromagnetic Radiation... A Safety Concern?
Generally, higher energy means higher frequency electromagnetic radiation is more harmful to humans.
The Earth’s atmosphere is able to protect us from some of the more dangerous electromagnetic
radiation present in space, making the Earth a safe place for humans.
Changes to present conditions may comprise our safety.
Positive and Negative Effects to Exposure to Electromagnetic Radiation
Positive Effects -Medical detection – X rays,
Ultraviolet- Used to treat jaundice in babies, helps production of vitamen D
Improved telecommunication ,
gamma waves to kill cancer cells.
Negative Effects
Over-exposure of UVrays can lead to skin cancer
Uncertain of long-term exposure of some electromagnetic waves
Gamma rays kill cells