Chapter 27 Properties of Light
Models of Light • Newton introduced the particle theory. He said that light travels in straight lines unlike waves.
Models of Light • Huygen disagreed with Newton and said that light is a wave because it spreads out sometimes.
Models of Light • Einstein realized that light acts not only as waves but as quantum particles now known as photons. • Light has dual properties – Particles and Waves
An event horizon exists • inside which the intense gravitational field of the black hole prevents any type of electromagnetic radiation including light from escaping Black Holes Question: Does light have mass? • Once was a star at least twice the mass of the sun that exhausted its fuel and collapsed to infinite density
A ravenous black hole This is a Hubble Space Telescope image of an 800-light-year-wide spiral-shaped disk of dust fueling a massive black hole in the center of galaxy NGC 4261, located 100 million light-years away in the direction of the constellation Virgo. By measuring the speed of gas swirling around the black hole, astronomers calculate that the object at the center of the disk is 1.2 billion times the mass of our sun.
Animated simulation of gravitational lensing caused by a black hole going past a background galaxy. http://en.wikipedia.org/wiki/File:BlackHole_Lensing.gif
Speed of Light • A. Roemer was the first person to try to measure the speed of light in observations of Io. • His calculation was 300,000 km/s.
Speed of Light • In 1880, Albert Michelson measured the speed of light to be 299,920 km/s. • Used spinning octagonal mirrors and a mirror far away • Adjusted spinning speed to calculate speed of light In 1907, he won the Nobel prize in physics.
Electromagnetic Wave Velocity Light could go around the world 7.5 times each second http://liftoff.msfc.nasa.gov/academy/rocket_sci/satellites/geo-high.html
Electromagnetic Wave Velocity Light takes 8 minutes to travel from the sun to earth.
Electromagnetic Wave Velocity The next closest star is 4 light years away The diameter of our galaxy is 100,000 light years Some galaxies are 14 billion light years away http://bluepoint.egenet.net/sagan/galaxy.gif
Electromagnetic Spectrum • Energy moving at speed of light • Vary by f and l only • Light is only a small part • Below light is infrared, above is ultraviolet
Electromagnetic Spectrum Radio Waves– communicationTV and Radio Microwaves–cooking & cell phones Infrared– “heat waves” Visible Light–detected by your eyes Ultraviolet– causes sunburns X-rays– penetrates tissue Gamma Rays– most energetic
The Visible Spectrum A range of light waves extending in wavelength from about 400 to 700 nanometers Less than 1% of the entire spectrum
Questions • Is it correct to say that radio wave is a low-frequency light wave? • Is a radio wave also a sound wave?
Transparent Materials Transparent - the term applied to materials through which light can pass in straight lines
Visible Light and Glass Visible light maintains the same frequency when it enters glass But the velocity changes: • Light travels 0.75x the original speed in water • In glass – 0.67x. • In diamond – 0.41x When the light comes out it regains its speed
Opaque Materials Opaque - the term applied to materials that absorb EM
Opaque Materials Opaque materials absorb EM • ex.) wood, metal, rocks ,etc. The light energy is transformed into random kinetic energy • makes the objectwarmer because it absorbs the electromagnetic energy
Example Questions • Are clouds transparent or opaque to visible light? • Answer: opaque • Are clouds transparent or opaque to ultraviolet light? • Answer: transparent
Example Questions Are windows transparent or opaque to visible light? • Answer: transparent Are windows transparent or opaque to ultraviolet light? • Answer: opaque • Due to Law of Conservation of Energy the EM is given off as heat
Shadows Umbra - the darker part of a shadow where all the light is blocked Penumbra - a partial shadow These terms also apply to Solar Eclipses and Lunar Eclipses.
Umbra Penumbra Solar Eclipse • A solar eclipse occurs when the Moon passes in front of the Sun.
Questions • Which type of eclipse is dangerous to view with the unprotected eye? • Why are lunar eclipses more commonly seen than solar eclipses?
Seeing the Light – The Eye • Cornea - does most of the focusing • Iris - has the eye color and controls light intensity • Pupil - the hole in the eye • Lens - does remainder of focusing • Retina - location of light sensors, has rods and cones center of vision, predominantly cones • Fovea - • Blind spot - optic nerve exit, no light sensors
Parts of the Eye Detectors on the Fovea • Rods • light intensity and motion sensitive • Cones • color sensitive The blind spot for the eye is cause by the optic nerve.
ColorVision! Colorblindness - about10%ofthepopulation • Red-green is predominant • Yellow-blue - a few • Total – some Mostlymales– X-linked trait
Optical Illusions Are the horizontal lines parallel, or do they slope?
Optical Illusions During the Optical Art (OpArt) Movement of the 1960s, artists would create all sort of puzzling effects with color. This "flashing squares" drawing seems to wobble and flash when you concentrate on one particular area of the image. How many squares can you see in this diagram? Can you feel the "motion" of the image?
Optical Illusions Can you count the black dots?