Ch 16: Light. Do you see what I see?

Transcription

1 Ch 16: Light Do you see what I see?

2 Light Fundamentals What is light? How do we see? A stream of particles emitted by a source? Wavelike behavior as it bends and reflects Today we know light is dual in nature. As a wave, light travels in a vacuum in a straight line at a frequency range in the electromagnetic spectrum that is visible to the human eye. (Ray models explain this)

3 Wavelengths of visible frequencies are between 400nm and 700nm.

4 Speed of Light Galileo was the first to hypothesize that speed of light is finite. His experiments left him to conclude light is too fast to measure Ole Roemer (Danish astronomer ) made careful observations of Jupiter s moon Io over 6 years and was able to give us the 1 st calculation of earth s orbit diameter concluding light takes 22mins to travel that distance. This proved light s finite speed. Light can circle the globe 7.5 times in 1 second.

5 v=fλ Speed of light defined c=fλ American physicist Albert Michelson ( ) measured the time it took for light to travel 35Km b/t 2 mountains in California. Best result was x 10 8 m/s. Michelson was 1 st American to receive the Nobel prize in science for this work Internat l Committee for Weights and Measures defined speed of light in a vacuum, c, to be 299,792,458 m/s. For most calculations we use 3.00x10 8 m/s.

6 Sources of Light Two types of objects we see in the universe. Luminous body- emits light waves (sun, candle, light bulb filament, stars) Illuminated body- reflects light waves (moon, mirror, bicycle reflector, etc ) Rate at which light is emitted from a source is called luminous flux, P, and is measured in lumens, lm. Rate at which light falls on a surface is called illuminance, E, and is measured in lumens per square meter, lm/m 2.

7 Inverse-square Relationship Light from a source spreads out in all directions. As light travels farther from the source it covers more area, but is dimmer. The illumination of an object is reduced as distance is increased by 1/r 2. If you are twice as far away from a source, the object is illuminated with an area ¼ as intense as it was before, but also 4 times as large of a spread as before. Like gravity, this is the inversesquare law.

8 Light and Matter Transparent materials- allow light to pass through without distorting the images. (glass, water, air) Translucent materials- allow light to pass through, but do not permit objects to be seen clearly (frosted glass, lamp shades, certain plastics) Opaque materials- do not allow light to pass through, but absorb and reflect light. (bricks, metal, etc )

9 In 1666, Isaac Newton did the first scientific experiments to produce colors from white light by passing a beam of light through a prism. Colors fell out in order from violet to red, which he called a spectrum. Color

10 Primary colors of light White light can be formed by mixing the 3 primary frequencies of red, blue, and green. This is color by addition. Compliments: red &? blue &? green &? Mixing pairs of primary colors forms secondary colors of magenta, cyan, and yellow. What color are the dots on your television screen?

11 Primary colors of Pigment A dye or pigment is a chemical that absorbs certain frequencies of light and reflects others to our eyes. Absorption of light forms colors by subtraction from white. Primary colors of pigments are secondary colors of light and vice versa. What is white? Black?

12 Colors from interference The colors seen in a bubble, wet gasoline spill, or film on a pavement result from white light hitting the top and bottom surfaces and some bouncing back interfering at just the right frequency. Each color is reinforced where the film thickness is ¼, ¾, 5/4, etc of the wavelength for that color.

13 Light Polarization Ordinary light vibrates in all directions perpendicular to its travel direction. A film with tiny slits one way can block certain vibrations and allow only parallel vibrations through it. This is polarized light.

14 Your turn to Practice Please do Ch 16 Rev. pg 389 #s 1, 3, 4, 5, 7, 10, 11, &12 Please do Ch 16 Rev p 390 #s 13, 16, 18, 23, 27, 28, 32, 33, & 34 Read Ch 18 Sec 1 on Mirrors and take notes.