Does a changing Electric Field produce a magnetic field The Replicating Wave 1 2 Maxwell s equations basically say that a changing magnetic field produces a changing electric field which in turn produces a changing magnetic field and so on Once you get this going these two kinds of fields would just continue replicating themselves moving thru empty space where both J and are 0 he then calculated the speed at which these would propagate to be the know speed of light c Implication is that light must just be an electromagnetic wave First Transatlantic communication in 1901 3 The Nature of Light What causes Electric and Magnetic Fields 4 C 3 x 108 m s Shake a charged rod and you generate electromagnetic waves Shake slowly and you generate radio waves shake quickly and you generate a million times per sec and you can see them 5 Wiggling one charge causes the field lines attached to it to wiggle and after a time the other charge starts to wiggle 6 1 Electromagnetic Waves 7 Electromagnetic Waves are Transverse Waves 8 Polarization The and fields are perpendicular to each other Both fields are perpendicular to the direction of motion Therefore em waves are transverse waves Demo Polaroid 9 Charges and Fields Summary 10 Electromagnetic Spectrum Stationary charges produce only electric fields Charges in uniform motion constant velocity produce electric and magnetic fields Charges that are accelerated or oscillate produce electric and magnetic fields and electromagnetic waves 11 12 2 The EM Spectrum 13 14 How are EM waves produced Is it correct to say that in every case without exception any radio wave travels faster than any sound wave In glass Red is scattered the most violet the least 1 Yes 2 No 15 16 Check yourself 1 Is it correct to say that a radio wave is a low frequency light wave 2 Is a radio wave also a sound wave 3 What is the wavelength of TV station broadcasting at 100 MGHertz Just as sound can force a sound receiver into vibration alight wave can force electrons in materials into vibration For visible light this high frequency will only affect electrons which basically have no mass in the material 17 18 3 Speed of Light Olaus Roemer1675 Earth Io Sun Jupiter Period of Io is 42 5 hr period was shorter by 1000 sec when earth was moving forward than when it was moving away Extra Distance 300 000 000 000 m c Extra Time 1000 sec 19 Transparent materials A wave of visible light incident upon a pane of glass sets up in atoms vibrations that produce a chain of absorptions and re emissions which pass the light energy through the material and out the other side Because of the time delay between absorptions and reemissions the light travels through the glass more slowly than through empty space 20 Light and transparent materials At lower wave frequencies such as those of visible light electrons in the glass atoms are forced into vibration but at less amplitude The atoms hold the energy for less time with less chance of collision with neighboring atoms and less energy transformed to heat The energy of vibrating electrons is re emitted as light Glass is transparent to all the frequencies of visible light The frequency of the re emitted light that is passed from atom to atom is identical to the frequency of the light that produced the vibration in the first place However there is a slight time delay between absorption and re emission 21 22 Only energy having the frequency of blue light is transmitted energy of the other frequencies is absorbed and warms the glass Check yourself 1 Why is glass transparent to visible light but opaque to ultraviolet and infrared 23 24 4 Check yourself 1 When red light shines on a red rose why do the leaves become warmer than the petals Dispersion 2 When green light shines on a rose why do the petals look black Usually a material absorbs light of some frequencies and reflects the rest If a material absorbs most of the visible light that is incident upon it but reflects red for example it appears red That s why the petals of a red rose are red and the stem green The atoms of the petals absorb all visible light except red which they reflect the stem absorbs all except green which it reflects An object that reflects light of all the visible frequencies as the white part of this page does is the same color as the light that shines on it If a material absorbs all the light that shines on it it reflects none and is black 25 26 White Light White light from the sun is a composite of all the visible frequencies is easily demonstrated by passing sunlight through a prism and observing the rainbow colored spectrum The intensity of light from the sun varies with frequency being most intense in the yellow green part of the spectrum The radiation curve of sunlight is a graph of brightness versus frequency Sunlight is brightest in the yellow green region in the middle of the visible range 27 Reflection of Light 28 Mixing Colors Most of the objects around us reflect rather than emit light They reflect only part of the light that is incident upon them the part that gives them their color 29 30 5 Why is the sky blue The tinier the particle the greater amount of higherfrequency light it will re emit Why sunsets are red 31 A sunbeam must travel through more atmosphere at sunset than at noon As a result more blue is scattered from the beam at sunset than at noon By the time a beam of initially white light gets to the ground only light of the lower frequencies survives to produce a red 33 sunset The spaceship at S wishes to touch the surface of the giant planet and proceed to point X in the shortest distance possible To what point P on the planet surface should the spaceship travel A beam of light falls on an atom and increases the vibrational motion of electrons in the atom The vibrating electrons re emit the light in various directions Light is scattered Of the visible frequencies of sunlight violet is scattered the most by nitrogen and oxygen in the atmosphere followed by blue green yellow orange and red in that order Red is scattered only a tenth as much as violet Although violet light is scattered more than blue our eyes are not very sensitive to violet light Therefore the blue scattered light is what predominates in our vision and we see a blue sky 32 Fermat s Principal This idea was formulated by the French scientist Pierre Fermat in about 1650 and it is called Fermat s principle of least time His idea is this Out of all possible paths that light might take to get from one