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Study Guide Grade 8 Science: Chapter 4 Light and the Wave Model Things to Know 1. What were the early theories on the nature of light? Some of the early theories of light were that beams of light were made up of tiny particles and that these beams came from objects and reached the eye. Others believed that the eye sends out fibres or threads allowing the eye to touch or sense an object. 2. Know the role of the following people regarding the study of light: i) ii) Pythagoras – made first attempts to explain the relationship between light and vision. He believed that beams of light were made of tiny particles. These beams came from objects and reached the eye. The Janssens – experimented with the lenses and learned how to magnify things even more. They made tubes with lenses in the end that would slide inside one another. iii) Van Leeuwenhoek- famous for building the microscope and discovering micro organisms. He discovered that he could increase the magnifying power of the lens by increasing the curvature of the lens. iv) Galileo – made the first attempts to measure the speed of light. As well invented the first telescopes to study space. Discovered four of Jupiter’s moons. v) Michelson- first person to accurately measure the speed of light. 3. Who was the first person to accurately measure the speed of light? How did they achieve this? Albert Michelson was the first person to measure the speed of light. He used a strong light source, an eight sided mirror, and another large mirror about 35 km away. He shone a light on the rotating mirror which reflected on a larger mirror. The returning beam reflected off another face of the rotating mirror into the eye of the observer. By measuring the speed of the mirror and the distance to the far away mirror, he could calculate the speed of light. 4. What is the speed of light? The speed of light is 300 000 km/s. It would circle the earth seven and a half times in one second. 5. Explain why there is a delay between when you see lightening and then hear thunder. Thunder and lightening happen at the same time. However the speed of sound is slower that the speed of light. So you see the lightening right away but there is a delay in the thunder because it takes longer for the sound to travel. You can tell how far away lightening is by timing the difference between when you see the flash and then hear the thunder clap. 6. Describe how microscopes and telescopes were developed using knowledge of the properties of light. Both were developed as early scientists discovered the relationship between vision and light. The Janssen’s developed the first compound microscope when they discovered multiple lenses could magnify objects. Microscopes were further developed as Leeuwenhoek discovered that increasing the curvature of the lenses would increase the magnifying power of a lense. When Galileo heard about the experiments with lenses, he built a telescope to magnify objects in space. 7. Describe and /or accurately label a wave, noting such things as wavelength, frequency, and what is carried as a wave moves forward. 8. Describe the relationship between wavelength and frequency. As wavelength increases (the distance between the crests/troughs) , the frequency decreases. As wavelength decreases, frequency increases. There is an inverse relationship between wavelength and frequency. Hertz: a measure of frequency. As hertz increases that means the frequency increases. The greater the frequency ,the greater the hertz. 9. List the 6 properties of light and give an example: i) ii) iii) iv) v) vi) rectilinear propagation reflection (specular and diffuse) refraction dispersion travels in a vacuum travels through different material to different degrees 10. What is the visible light spectrum? This is light waves that you can see. It is the ROY G. BIV 11. List the colours in the visible light spectrum and describe each in terms of wavelength, frequency and refraction red- longest wavelength, lowest frequency, least refraction (700nm) orange yellow green blue indigo violet – shortest wavelength, highest frequency, most refraction (400 nm) 12. Define and give an example of wavelengths longer than visible light: i) Radio waves – a type of electromagnetic radiation that have the longest wavelength and lowest energy and frequency compared to all other types. Used for MRIs. ii) Microwaves- a type of radio wave. The radio wave that has the shortest wavelength and highest frequency. iii) Radar – shorter wavelength microwaves used in remote sensing. Waves are beamed out through the air. The waves reflect from an object and show the location and speed. iv) Infrared waves- are a type of electromagnetic radiation that, relative to light, has a longer wavelength and lower energy and frequency. Used for remote controls/heat lamps. 13. Define and give an example of wavelengths shorter then visible light: i) UV rays – these are waves beyond violet. Radiation energy like UV waves, are very energetic and potentially dangerous. Needed for your body to make vitamin D. They have a higher frequency and shorter wavelength than visible light. ii) X rays – much shorter wavelength and higher frequency than UV rays. Discovered in 1895 by Roentgen. Can have harmful health effects with prolonged exposure. 14. Compare wavelengths longer AND shorter then visible light to visible light. Visible light can be found between waves longer than and wave shorter than visible light. Waves like wavelength Waves like wavelength radio waves, radar etc have a longer and lower frequency that visible light. x-rays and UV rays have a shorter and higher frequency than visible light. 15. Give an example of positive and negative aspects of electromagnetic radiation. Positive and Negative Effects of Electromagnetic Radiation Exposure to high frequency and short wave length electromagnetic radiation is dangerous. Type of Radiation X-rays Positive Effect Ultraviolet Used to treat jaundice in babies Improved Uncertain of telecommunications long term exposure effects Radio waves Medical detection Negative Effect Overexposure can lead to cancer Skin cancer