[1] The document summarizes early theories and experiments regarding the nature of light and vision, including challenges in explaining them. [2] Pythagoras originally theorized that light comes from the eyes, but this was disproven. [3] Experiments by al-Haytham, Newton, Romer and Michelson helped establish that light travels in straight lines and can be reflected, and determined the speed of light.

1. Science 8 Unit C: Light and Optical Systems Science InAction 8 Our knowledge about light and vision comes from explanations, inventions, and investigations Section 1.1: Pages 174-181

2. The Challenge of Light 1.1 Learner Outcome Students will identify challenges in explaining the nature of light and vision

3. Light Light is energy Our eyes are able to see an object only if light is emitted from the object or bounces off it Light travels in straight lines and needs a direct path Early Greeks and Chinese used mirrors to refract the light from the sun using mirrors.

4. Early Light Ideas Pythagoras – mathematician – tried to explain how we see light He thought light consisted of beams that came from a persons eyes in straight lines Problem = If light came from a persons eyes we would be able to see in the dark This theory was accepted for many years

5. Euclid Euclid - Greek mathematician Discovered that when you shine a beam of light onto a flat mirror, the angle between the incoming beam and the mirror is equal to the angle between the reflected beam and the mirror Suggested that light travels in straight lines

6. Light Ideas in the Middle Ages A.D. 1000 , Arab Scientist al-Haytham took up the study of light. Wrote a book on optics Discovered that light bounces off objects and then travels to the eye Pythagoras’s theory was abandoned after his discovery

7. Newton’s Light Experiment Rainbow experiment Newton showed that white light is actually a mixture of different colors of light As light is passed through a prism, it is split up into many separate colors Showed that separate colors combined back into white light

8. A Speedy Discovery Ole Romer in 1676 estimated the speed of light Albert A. Michelson refined his measurement in an experiment involving 2 mirrors on top of mountains in California in the 1920’s Page 181

9. Calculating the Speed of Light - c Velocity formula v=d/t Or… d=v*t Or… t=d/v Mirrors apart = 35.4 km Light was calculated to be 299, 798 km/s or ~ 300 000 km/s What was the time it took to travel from one mountain to the other?

10. Galileo’s Experiment: Later refined by Romer and Michelson

12. Michelson–Morley experiment

13. The Properties of Light Travels in straight lines Can be reflected Can bend Is a form of energy

14. Did you… identify challenges in explaining the nature of light and vision?

15. Check and Reflect Questions 2-5, page 181 2. What was Pythagoras’s theory of how we see light? ______________________________________________________________________________________________________________________________________________________________________________________ 3. What was the problem with his theory? ______________________________________________________________________________________________________________________________________________________________________________________

16. Check and Reflect Questions 2-5, page 181 4. What did al-Haytham try to explain? _________________________________________________________________________________________________________________________________________ 5. Based on Michelson’s measurement of the speed of light, if the distance between the sun and the earth is 149 596 000 km, how long does it take light to travel from the sun to the earth?