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• Grade 8 Science focusses on Unit 2: Optics
• Chaper 4 covers the many properties of light that can be understood through a wave model of light

The History of Light

• Pythagoras, a Greek philosopher, thought light beams were made of tiny particles
• Pythagoras believed eyes detect these particles to allow sight of objects

The Speed of Light

• Albert Michelson was the first person to measure the speed of light
• Light speed is measured at 3 x 10⁸ m/s
• Light travels at 1 000 000 000 km/h
• Sound travels at 1 200 km/h

Thunder and Lightning

• Lightning and the roar of thunder occur at the same time.
• Lightning is seen first due to the faster speed of light compared to sound
• Multiplying the time in seconds between the lightning strike and thunder by the speed of sound yields the approximate distance to the strike

Orion's Belt

• It is possible to view stars that have burnt out
• The light seen from stars takes thousands of years to reach our eyes

Light Technologies

• Include microscopes, telescopes and periscopes
• Include binoculars, fibre optics, and cameras
• Include prescription contact lenses and lasers
• Include movie projectors and overhead projectors

Light Defined

• Light is form of energy that can be detected by the human eye
• Visible light os a mixture of all the colors of the rainbow

Properties of Visible Light

• Light travels in a straight line, known as rectilinear propagation
• Light reflects, known as reflection
• Light refracts, known as refraction
• Light disperses, known as dispersion
• Light travels through a vacuum, which does not require a medium or particles
• Light travels through objects to different degrees

Visible Light Spectrum

• Can be seen due to the dispersion of light through a prism
• Constituent colors of white light are red, orange, yellow, green, blue, indigo, and violet, remebered as ROY G BIV
• Red has the smallest refraction
• Violet has the greatest refraction
• A laser shone through a prism will refract, not disperse
• Laser light is a single color

The Wave Model

• Light a type of wave that travels through empty space
• Light transfers energy from one place to another

Wave Properties

• A peak is also called the crest
• Frequency, measured in Hertz (Hz),is the number of repetitive motions that occur during a given time
• For example, the number of wavelengths that pass a point in 1 second
• Amplitude is the height of a wave crest or depth of a wave trough as measured from the rest position
• The wave crest height equals the wave trough depth
• The larger the amplitude, the greater the energy transported
• Wavelength is the distance from crest to crest, trough to trough, or the distance covered by one complete crest and one complete trough
• Wavelength is measured in meters
• Longer wavelengths refract the least
• High frequency waves have short wavelengths
• Low frequency waves have long wavelengths

Electromagnetic Radiation

• Electromagnetic radiation is the transmission of energy in the form of waves
• Waves that extend from the longest radio waves to the shortest gamma rays

Types of Electromagnetic Radiation

• Radio waves have the longest wavelength and lowest energy and frequency

• Radio waves can be used to see inside bodies to diagnose illness, for example, MRI

• Microwaves have the shortest wavelength and the highest frequency of all radio waves

• Microwaves are found in microwave ovens, telecommunication satellites, radio telescopes, and radar (remote sensing)

• Microwave ovens use a particular frequency that is strongly absorbed by water molecules in food

• Infrared waves have longer wavelengths and lower energy and frequency

• Infrared is below red

• Infrared is also called heat radiation

• Infrared is found in remote controls, computers, heat lamps, and motion sensors

• Visible light is continually detected by our eyes

• The visible light spectrum ranges from from infrared at 780 nanometers to ultraviolet at 390 nanometers

• Ultraviolet waves have shorter wavelength and higher energy and frequenc

• They are very energetic

• Ultraviolet light has the ability to kill bacteria in food, water, and medical supplies

• Ultraviolet are found in the Sun, or used to detect fingerprints

• X-rays have a shorter wavelength and higher energy and frequency than UV

• Used to photograph teeth, bones and the inside of machines for security screening

• Gamma rays have the highest energy and frequency and the shortest wavelength

• Gamma rays result from nuclear reactions

• Produced by the hottest regions of the universe

Electromagnetic Radiation Safety

• Higher energy electromagnetic radiation is more harmful to humans
• The Earth's atmosphere protects humans from the most dangerous electromagnetic radiation in space

Positive and Negative Effects of Electromagnetic Radiation

• X-Rays have the posotive effect of medical detection, and negative effect of over exposure can lead to cancer
• Ultraviolet light has the positive effect of treatment jaundice in babies, and negative effect of skin cancer
• Radio Waves have the positive effect of imporved tele-communication, and the negative effect of uncertainty of long-term exposure