Grade 8 Science Chapter 4 Notes

Transcription

1 Grade 8 Science Chapter 4 Notes Optics the science that deals with the properties of light. Light a form of energy that can be detected by the human eye. The History of Optics (3 Scientists): 1. Pythagoras A Greek philosopher who was born around 580 B.C. Believed that beams of light were made up of tiny particles. When the beams of light came from objects and reached the eye, they carried information about the object to the eye. 2. Galileo The first person to try to measure the speed of light. He proposed that he and an assistant would stand on hilltops about one kilometre apart holding lanterns. He would uncover his lantern first and when his assistant saw this, he would then uncover his lantern. Galileo would measure the time between the moment he uncovered his lantern to the moment he saw the light from his assistants. This did not work. 3. Albert Michelson The first person to measure the speed of light accurately. He used a strong light source, an 8 sided mirror and another mirror. By using the distance light traveled and the speed at which the 8-sided mirror rotated, he calculated the speed of light. Speed of light (c) = 3 x 10 8 m/s Vs. Speed of sound = 330 m/s Ex: Thunder and lightening thunder is the sound lightening makes.

2 Technologies that use light: - use a combination of lenses and tubes to magnify objects. 1. Microscope an instrument for viewing objects too small to be seen with the naked eye. 2. Telescope an instrument for viewing objects far away, such as in space. Properties of Visible Light 1. Rectilinear Propagation when light travels in a straight line. 2. Reflects object shown in a mirror like substance. 3. Refracts light can bend in water or other medium. 4. Disperses visible light can separate into the colors of the visible spectrum 5. Vacuum light travels in a vacuum, does not need a medium. 6. Travels to different degrees through transparent, translucent and opaque materials. The Wave Model of Light - light is a type of wave that travels through empty space and transfers energy from one place to another, such as from the Sun to the Earth. Visible Light a wave that you can see. Diagram of a Wave: Definitions: 1. Wavelength the distance from crest to crest of from trough to trough. 2. Crest the highest point of a wave. 3. Trough the lowest point of a wave. 4. Amplitude the height of a crest or the depth of a trough, measured from rest point. - the amplitude is related to the amount of energy carried by a wave. - the larger the amplitude, the greater energy transported.

3 Frequency Frequency the number of repetitive motions, or oscillations that occur in a given time. - frequency is measured in hertz (Hz) or cycles per second. Frequency and Wavelengths have inverse relationships: High frequency waves have short wavelengths. Low frequency waves have long wavelengths. Frequency Calculations - We calculate frequency by dividing the cycles by the time it takes for those cycles to pass a certain point (cycles per second) F = cycles Time Ex: A pendulum swings 24 times in 6 seconds. Calculate the frequency. Frequency = cycles/s = 24 swings/6 s = 4 Hz Ex2: A wave passes a wharf 10 times in 2 seconds. Calculate the frequency. Frequency = cycles/s = 10 passes/2 s = 5 Hz

4 The Electromagnetic Spectrum Electromagnetic Radiation the transmission of energy in the form of waves that extend from the longest radio waves to the shortest gamma rays. - described by different names depending on their frequency and wavelength. Labelling the Electromagnetic Spectrum Radio Waves Microwaves Infrared Waves Visible Spectrum UV X-Rays Gamma Rays Wavelengths longer than visible light 1. Radio waves have the longest wavelength and the lowest energy and frequency. Microwaves, MRI and radar are types of radio waves. 2. Microwaves Have the shortest wavelength and the highest frequency of all the radio waves. Water particles in food absorb these waves and begin to vibrate quickly and heat up. Only food that contain water can be heated using microwaves. 3. Infrared waves Also known as heat radiation Every object emits some infrared waves because all objects contain energy.

5 Wavelengths shorter than visible light 1. Ultraviolet waves UV rays help your body to make vitamin D. But too much UV radiation will cause your skin to burn. 2. X-rays Doctors use x-rays to detect broken bones. 3. Gamma rays Have the highest energy and frequency and shortest wavelength of the electromagnetic spectrum. Result from nuclear reactions and are produced by the hottest regions of the universe. Also used in radiation therapy to kill cancer cells. Electromagnetic Positive Effects Negative Effects Radiation X-rays Medical Detection Over exposure can lead to cancer Ultraviolet Used to treat Jaundice in Cancer babies Radio Waves Improved communications Uncertain