Grade 8 Science
Unit 3: Optics
Chapter 4: Properties of Light
Can light be produced in different ways:
1. Natural Light
2. Electric Light
3. Chemical Light
4. Combustion Based
The Nature of Light
1) Pythagoras
- A Greek philosopher
- Believed that beams of light were made up of tiny particles
- The eyes could detect these particles and see the objects
2) Galileo
o He was the first person to try to measure the speed of light
o He used lanterns on hilltops to measure the speed of light
3) Michelson
o He was the first person to accurately measure the speed of light (3×108m/s)
o Shone a light on a rotating mirror that reflected to a distant mirror
o He used the distance and reflection time to calculate the speed of light.
Light: a form of energy that can be seen by the human eye
Speed: Light vs. Sound
o Light = 300 000 000 m/s ( 1 000 000 km/h)
o Sound = 345 m/s ( 1 200 km/h)
1. Explain why you see the lightning before you hear it.
______
2. Is it possible for stars to be burned out even though we are seeing them in our night sky?
Explain.
______
Light technologies include:
o Microscope
o Telescope
o Periscope
o Binoculars
o Fibre Optics
o Camera
o Prescription contact lenses
o Laser
o Movie Projectors
o Over head projectors
Foldable – Tab 1 – The nature of light
Inside tab:
· Give a brief description of the discoveries made by Pythagoras, Galileo, and Michelson
· Define light
· State the speed of light
· List four technologies that involve light
Outside tab:
o Draw a picture of a microscope
Pg 137 #8
Properties of waves
A wave is a disturbance or movement that transfers energy through matter or space, without causing any permanent displacement.
The wave model
o Explains that light is a type of wave that travels through empty space and transfers energy from one place to another
Properties of waves
o A wave is a disturbance or movement that transfers energy through matter or space, without causing any permanent displacement.
The wavelength - is the distance from crest to crest or from trough to trough. It is
measured in metres.
Frequency - to the number of repetitive motions, or vibrations, of a wave that occur in
a given amount of time. It is measured in cycle per second or Hertz (Hz).
The higher the frequency, the faster the wave. High frequency waves have short wavelengths.
Low frequency waves have long wavelengths.
Page 147 # 4 and 8
Foldable- Tab: Properties of Waves
Outside tab:
Draw a typical wave and label the following:
o crest
o trough
o wavelength
Inside tab:
o Define wave, wavelength and frequency
o The longer the wavelength, the ______the frequency.
o The shorter the wavelength, the ______the frequency.
Properties of Light
1. Travels in a straight line (rectilinear propagation)
Example: shadows
2. Light reflects (reflection)
3. Light refracts when travelling through different materials
(refraction)
Example: the bent stick effect
4. Light disperses (dispersion)
Example: white light separates into the colours of the rainbow
5. Light travels through a vacuum (empty space)
- does not require a medium
- no particles are involved
Example: light from the Sun and stars
6. Light travels through transparent, translucent, and opaque materials to different degrees.
Visible Light
o White light is a combination of colours.
Visible light spectrum – A form of energy that our eyes can detect
Can be seen due to the dispersion of light through a prism
The constituent colors of white light are:
Red (Smallest refraction, longest wavelength)
Orange
Yellow
Green
Blue
Indigo
Violet (Greatest refraction, shortest wavelength)
o The colour we see is the colour that is being reflected. All other wavelengths are absorbed.
PG. 155 #2-5, 7, 9, 10
Foldable- Tab: Properties of Visible Light
Inside: State the 6 properties of visible light
Outside: Draw a picture that relates to one property of visible light
Light and the Electromagnetic Spectrum
Electromagnetic Radiation
- The transmission of energy in the form of waves that extend from radio waves (longest) to gamma rays (shortest).
Types of Electromagnetic Radiation (in order of decreasing wavelength)
1. Radio waves
- longest wavelength
- lowest frequency
- lowest energy
- Longer radio waves can help us see inside our bodies and diagnose illnesses. Example: MRI machines
2. Microwaves
- A type of radio wave that has the shortest wavelength and highest frequency.
Examples: microwave ovens, telecommunications, and radar
3. Infrared Waves
Compared to visible light:
o longer wavelength
o lower frequency
o lower energy
- Also called heat radiation
Example: motion sensors, remote control
4. Visible Light
- Can be detected by our eyes
Example: microscopes
5. Ultraviolet Light
Compared to visible light:
o shorter wavelength
o higher frequency
o higher energy
Example: sun tanning
6. X-Rays
Compared to visible light:
o shorter wavelength
o higher frequency
o higher energy
Example: medical detection
7. Gamma Rays
They have:
o Shortest wavelength
o Highest frequency
o Highest energy
Gamma rays result from nuclear reactions.
They are used in radiation therapy to kill cancer cells.
Is electromagnetic Radiation dangerous?
o Higher energy electromagnetic radiation (gamma rays) is more harmful than lower energy (radio waves).
o The atmosphere protects us by reflecting higher energy radiation
Pg 167 # 3, 10, 11 and 12
Foldable – Tab: Light and the Electromagnetic Specturm
Inside tab:
o List the 7 types of electromagnetic radiation in order starting with the
longest wavelength and shortest frequency
o Which type of electromagnetic radiation is the most harmful?
o Which type of electromagnetic radiation causes sunburns?
o Which type of electromagnetic radiation has the lowest energy?
Outside tab:
o Draw two objects that rely on radio waves