Change is the law of life. And those who look only to the past or present are certain to miss the future. - John F. Kennedy _

LIGHT

LIGHT IS A FORM OF ENERGY THAT HAS BOTH WAVE AND PARTICLE PROPERTIES. LIGHT (ELECTROMAGNETIC RADIATION) IS TRANSMITTED THROUGH SPACE IN SMALL PACKETS OF ENERGY CALLED PHOTONS. THESE PHOTONS HAVE ALTERNATING ELECTRICAL AND MAGNETIC FIELDS ASSOCIATED WITH THEM WHICH GIVES THEM WAVE PROPERTIES.

THESE WAVE PROPERTIES CAN BE DEMONSTRATED BY DIFFRACTION WITH GRATINGS AND REFRACTION WITH PRISIMS. THE PHOTONS ALSO HAVE PARTICLE PROPERTIES THAT CAN BE DEMONSTRATED BY THE PHOTOELECTRIC EFFECT AND BENDING OF LIGHT WAVES BY GRAVITY.

WE CHARACTERIZE DIFFERENT KINDS OF LIGHT (ELECTROMAGNETIC RADIATION) BY WAVELENGTH AND FREQUENCY. WAVELENGTH WOULD BE THE DISTANCE FROM PEAK TO PEAK. VISIBLE LIGHT WOULD HAVE WAVELENGTHS OF 400 TO 700 NANOMETERS (10 -9 METERS). VISIBLE LIGHT IS ONLY A VERY SMALL PART OF THE ELECTROMAGNETIC SPECTRUM.

LIGHT CAN ALSO BE CHARACTERIZED BY FREQUENCY – THE NUMBER OF WAVES (PHOTONS) PASSING A GIVEN POINT IN UNIT TIME (SECONDS).  frequency = c/  units are s -1 or Hertz, Hz WHERE c = speed of light = wavelength of light

THE SPEED OF LIGHT IS CONSTANT IN A VACUUM 3 X 10 8 meters/second. OR C = SPEED OF LIGHT = 300,000 km/sec THE INTENSITY OF THE LIGHT CAN BE REPRESENTED BY THE AMPLITUDE OF THE WAVE.

AMPLITUDE = INTENSITY = # PHOTONS

IF WE MARKED OUT THE ELECTROMAGNETIC SPECTRUM FROM GAMMA RAYS TO RADIO WAVES ON A PIECE OF TAPE 2,000 MILES LONG, THE VISIBLE PART OF THE SPECTRUM WOULD COVER A SECTION ABOUT AN INCH LONG. THE VISIBLE PART OF THE SPECTRUM IS IMPORTANT BECAUSE THAT REPRESENTS THE WAVELENGTHS THAT OUR EYES CAN DETECT. THE RADIATION THE SUN EMITS PEAKS IN THIS SAME REGION.

IN CHEMISTRY, WE ARE CONCERNED WITH THE INTERACTION OF ENERGY WITH MATTER. UNDERSTANDING THE INTERACTIONS OF LIGHT WITH MATTER IS EXTREMELY USEFUL IN UNDERSTANDING THE NATURE OF MATTER. SOLIDS REFLECT OR TRANSMIT LIGHT IN BROAD BANDS. THE LIGHT SOLIDS GIVE OFF (EMIT) DEPENDS ON TEMPERATURE. THE LIGHT EMITTED FROM AN OPAQUE, NON- REFLECTING SOLID (BLACK BODY) DEPENDS ONLY ON TEMPERATURE.

AT ROOM TEMPERATURE, SUCH A SOLID WOULD APPEAR BLACK BECAUSE MOST OF THE RADIATION EMITTED WOULD BE IN THE INFRARED. AS THE TEMPERATURE INCREASED, THE SOLID WOULD APPEAR DULL RED, AND AS THE TEMPERATURE INCREASED FURTHER, THE SOLID WOULD APPEAR WHITE. WIEN’S DISPLACEMENT LAW GIVES THE WAVELENGTH FOR MAXIMUM EMISSION AT A GIVEN TEMPERATURE: max = b/T where b = 2,897,768 nm o K

MOST SOLIDS ARE NOT BLACK BODIES, BUT THE IDEA OF BLACK BODY RADIATION CAN BE USED AS A FIRST APPROXIMATION FOR RADIATION EMITTED AT A GIVEN TEMPERATURE, AND THE IDEA CAN BE USED TO ESTIMATE THE TEMPERATURE OF OBJECTS. THIS IS ESPECIALLY USEFUL TO ASTRONOMERS IN THE ESTIMATION OF THE SURFACE TEMPERATURE OF STARS AND OTHER OBJECTS IN SPACE. FOR EXAMPLE OUR SUN HAS A max of 502 nm, WHICH WOULD CORRESPOND TO A SURFACE TEMPERATURE OF 5778 o K. MOST MAMMALS HAVE A BODY TEMPERATURE OF AROUND 300 o K, WHICH WOULD GIVE A max OF 10  m. THIS WOULD BE IN THE INFRARED.

LIGHT HAS BOTH WAVE AND PARTICLE PROPERTIES. THE WAVE PROPERTIES ARE SHOWN BY THE FACT THAT LIGHT CAN BE REFRACTED BY A PRISM (SEPARATED INTO ITS VARIOUS WAVELENGTHS ON PASSING THROUGH A PRISM). DIFFERENT WAVELENGTHS OF LIGHT ARE BENT MORE THAN OTHERS, SO THAT LIGHT WOULD BE SEPARATED BY WAVELENGTH ON PASSING THROUGH A PRISM.

LIGHT IS ALSO DIFFRACTED BY A GRATING INTO ITS VARIOUS WAVELENGTHS. A GRATING CAN BE A TRANSPARENT OR REFLECTIVE MATERIAL WITH MANY THIN PARALLEL LINES DRAWN ON THE SURFACE. A CD OR DVD SURFACE MAKES A GOOD REFLECTION GRATING.

PARTICLE PROPERTIES OF LIGHT CAN BE DEMONSTRATED BY THE PHOTOELECTRIC EFFECT AND BY GRAVITATIONAL LENSING OF LIGHT (BENDING OF LIGHT BY GRAVITY). PHOTONS OF LIGHT WITH SUFFICIENT ENERGY CAN KNOCK ELECTRONS FROM METAL SURFACES – THE PHOTOELECTRIC EFFECT. THE ENERGY OF PHOTONS OF LIGHT IS INVERSELY PROPORTIONAL TO WAVELENGTH (DIRECTLY PROPORTIONAL TO FREQUENCY). E = h = hc/ where = frequency h = Planck’s const. = x J s = wavelength c = speed of light

THERE IS A CERTAIN MINIMUM ENERGY REQUIRED TO KNOCK AN ELECTRON FROM THE SURFACE OF A METAL. THIS WOULD CORRESPOND TO THE THRESHOLD FREQUENCY o. ANY EXCESS ENERGY WOULD SUPPLY KINETIC ENERGY TO THE EJECTED ELECTRON. Electron kinetic energy = photon energy – binding energy E kinetic (electron) = h - h o

THE INTENSITY OF THE LIGHT DOES NOT DETERMINE THE ENERGY OF THE ELECTRONS EJECTED. THE INTENSITY OF LIGHT (NUMBER OF PHOTONS) DETERMINES HOW MANY ELECTRONS ARE EJECTED. EACH DIFFERENT METAL HAS ITS OWN THRESHOLD FREQUENCY.

THE INTERACTION OF LIGHT ENERGY WITH MATTER IS DIFFERENT IN SOLIDS AS COMPARED TO GASES. IN A SOLID, THE ATOMS ARE STACKED CLOSE TOGETHER AND BEHAVE AS A COLLECTION. IN A GAS, THE ATOMS ABSORB AND EMIT LIGHT INDIVIDUALLY. SCIENTISTS NOITCED EARLY ON THAT IF THEY LOOKED A THE SUN’S SPECTRUM THEY FOUND A CONTINUOUS SPECTRUM WITH DARK LINES SUPERIMPOSED ON IT. THE CONTINUOUS LINES REPRESENTED THE THERMAL EMISSION FROM THE SUN’S SURFACE. THE DARK LINES REPRESENTED LIGHT ABSORBED BY ATOMS OF ELEMENTS IN THE SUN’S GASEOUS ATMOSPHERE.

WHAT THIS MEANS IS THAT IF YOU PASS LIGHT IN A CONTINUOUS SPECTRUM THROUGH A GAS COMPOSED OF ATOMS, ATOMS WILL ONLY ABSORB DISTINCT WAVELENGTHS OF LIGHT. DIFFERENT ATOMS WILL ABSORB DIFFERENT WAVELENGTHS OF LIGHT. EACH ELEMENT HAS ITS OWN ABSORPTION SPECTRUM. IF YOU HEAT ATOMS IN A GAS UP TO HIGH ENOUGH TEMPERATURES – GIVE THEM ENOUGH THERMAL ENERGY – ATOMS WILL GIVE OFF LIGHT AT DISCRETE WAVELENGTHS. EACH ELEMENT HAS ITS OWN EMISSION SPECTRUM. Atomic spectrum of hydrogen

Atomic spectrum of iron SINCE THE EMISSION AND ABSORPTION SPECTRA OF ELEMENTS ARE CHARACTERISTIC OF GIVEN ELEMENTS, THE SPECTRA CAN BE USED BY SCIENTISTS TO IDENTIFY ELEMENTS PRESENT IN MATERIALS. THIS IS MOST USEFUL TO ASTRONOMERS IN THE IDENTIFICATION OF THE ELEMENTAL COMPOSITIONS OF STARS AND OTHER BODIES. THE WAVELENGTHS COULD BE USED TO IDENTIFY THE ELEMENTS PRESENT, AND THE INTENSITY AT A GIVEN WAVELENGTH COULD BE USED TO IDENTIFY THE AMOUNT OF A GIVEN ELEMENT PRESENT.

KEEP IN MIND THAT LIGHT IS ENERGY. EACH PHOTON OF LIGHT REPRESENTS A CERTAIN DISCRETE AMOUNT OF ENERGY. THAT AMOUNT OF ENERGY IS A FUNCTION OF WAVELENGTH. E = h = h(c/ ) Where = frequency and = wavelength C = 3 x 10 8 m/s h = Plank’s const. = 6.63 x J s

Ground state Energy Photoionization Photon emission Photon emission Energy/photon absorption SCIENTISTS CONCLUDED THAT, DEPENDING ON THE ENERGY OF THE PHOTON, THE ABSORPTION OF PHOTONS BY FREE ATOMS COULD RESULT IN EITHER OF TWO EVENTS. 1)IF THE PHOTON HAS SUFFICIENTLY HIGH ENERGY, PHOTOIONIZATION CAN RESULT. 2)IF THE PHOTON HAS LESS ENERGY, THE PHOTON COULD BE ABSORBED IF ITS ENERGY CORRESPONDED TO THE ENERGY DIFFERENCE BETWEEN LEVELS IN THE ATOM.

WHAT SCIENTISTS REALIZED AS A RESULT OF THE STUDY OF ATOMIC SPECTRA IS THAT THE ELECTRONS COULD ONLY EXIST IN CERTAIN DISCRETE ENERGY LEVELS, AND THESE ENERGY LEVELS REPRESENTED THE ORBITALS OF THE ELECTRONS. THE BOHR THEORY OF ATOMS TALKED ABOUT ORBITS SIMILAR TO THE ORBITS THAT PLANETS HAVE AROUND THE SUN. IN THE NEW ATOMIC THEORY, THE QUANTUM THEORY, ELECTRONS ARE DESCRIBED AS OCCUPYING ORBITALS.

ENERGY LEVELS IN THE HYDROGEN ATOM

ACCORDING TO THIS THEORY, WE CANNOT DESCRIBE THE EXACT ORBITS OF ELECTRONS, BUT WE CAN TALK IN TERMS OF PROBABILITIES. SOME FAIRLY COMPLEX MATHMATICAL EQUATIONS HAVE BEEN DEVELOPED IN THE QUANTUM THEORY THAT DESCRIBE THESE ORBITALS AND THEIR SHAPES. AND, THEY HAVE BEEN VERY USEFUL IN DESCRIBING THE SHAPES OF MOLECULES THAT RESULT WHEN ATOMS REACT.

A FEW STRANGE FACTS ABOUT LIGHT OUR EYES AND MIND DO SOME INTERESTING THINGS WITH LIGHT AND OPTICS. 1)PRINTING IS DONE WITH A SERIES OF VERY SMALL DOTS OF ONLY 4 COLORS. WE DON’T SEE THE SMALL DOTS, BUT OBJECTS AND COLORS. 2)IF STILL IMAGES ARE SHOWN TO US FAST ENOUGH, WE SEE THEM AS MOTION. TELEVISION AND MOTION PICTURES USE 34 FRAMES PER SECOND.

COLORS FROM DOTS - PRINTERS PRINT IMAGES AS A SERIES OF DOTS USING ONLY 4 COLORS, CYAN, MAGENTA, YELLOW, AND BLACK. THE COLORS YOU SEE ARE COMBINATIONS OF DOTS OF THESE 4 COLORS.

CYAN MAGNETA YELLOW

THE DOPPLER EFFECT CAN ALSO CHANGE THE WAVELENGTH OF LIGHT. YOU SHOULD BE FAMILIAR WITH THE DOPPLER EFFECT AND SOUND. WHEN A CAR IS BLOWING ITS HORN AND SPEEDING TOWARDS YOU, THE CAR HORN WILL BE AT A HIGHER FREQUENCY (SHORTER WAVELENGTH) THAT WHEN IT IS MOVING AWAY FROM YOU (LOWER FREQUENCY AND LONGER WAVELENGTH). THE DOPPLER EFFECT IS USED IN ASTRONOMY TO DETERMINE HOW FAST STARS ARE MOVING TOWARDS US OR AWAY FROM US.

THE SPEED OF LIGHT IN A VACUUM IS A CONSTANT. 186,000 MILES PER SECOND OR 300,000,000 m/s NOTHING CAN GO FASTER. LIGHT DOES MOVE SLOWER IN OTHER MATERIALS.

Falcon can see a 10 cm. object from a distance of 1.5 km. Fly's Eye has a flicker fusion rate of 300/s. Humans have a flicker fusion rate of only 60/s in bright light and 24/s in dim light. Penguin has a flat cornea that allows for clear vision underwater. Penguins can also see into the ultraviolet range of the electromagnetic spectrum. With eyes mounted laterally on their heads, pigeons can view 340 degrees … everywhere except in back of their heads.

WHY DO WE SEE IN THE “VISIBLE” REGION OF THE SPECTRUM?

IF WE LIVED ON A PLANET AROUND ANOTHER STAR, OUR VISION MIGHT BE DIFFERENT.