Waves, Light, and the EM Spectrum What is a wave? What are the main properties of waves? What two things do all waves transport? Waves are a type of disturbance that can propagate or travel. Waves carry information and energy. Properties of a wave wavelength ( ) amplitude (A) velocity (v) trough Period (T): time between crest (or trough) passages Frequency (f): rate of passage of crests (or troughs), f wave speed = wavelength/period = wavelength * frequency (v = f) 1T1T (units: Hertz or cycles/sec) crest Types of Waves Longitudinal Waves Waves of compression Disturbance propagates along direction of travel Sound waves Transverse Waves Disturbance is perpendicular to the direction of travel Water waves, light How does the medium that supports a wave move? Sound Waves Caused by alternating compressions and rarefactions in some medium, usually air. Frequency of waves same as frequency of source Heard as pitch or tone Human ear ~ 20 to 20,000 hertz Loudness determined by amplitude of waves Requires a medium to propagate (Demo) Resonance Any elastic object will vibrate at its own set of frequencies when disturbed Called natural frequencies Determined by elasticity and shape Bells, violin strings, idling cars Resonance Dramatic increase in amplitude when frequency of forced vibrations matches natural frequency of object Pumping a swing Resonance occurs when compressions and rarefactions are timed to the natural frequency of the tuning fork. Interference Constructive interference When the peaks of two waves coincide Amplitude increases Ex. Sonic Boom Destructive interference When the peak of one wave fills in the trough of another Waves cancel Ex. Noise Cancellation Waves bend when they pass through material of different densities. swimming pool air water prism air glass Refraction All radiation (including visible light) travels as electromagnetic waves. That is, waves of electric and magnetic fields travelling together. What are some examples of objects with magnetic fields: What are some examples of objects with electric fields: All radiation (including visible light) travels as Electromagnetic waves. That is, waves of electric and magnetic fields travelling together. What are some examples of objects with magnetic fields: a bar magnet the Earth the Sun What are some examples of objects with electric fields: protons electrons } "charged" particles that make up atoms. Anything with too many or to few electrons! Electrical appliances Lightning Electric Force - opposites attract, likes repel Oscillating charges radiate All objects have temperatures greater than absolute zero - random thermal motion All objects radiate! Why dont we see the radiation coming from many ordinary objects? Dog whistle analogy Electromagnetic Radiation (How we get information about the cosmos) What are some examples of electromagnetic radiation and what are the different ways in which we might experience them or make use of them? Electromagnetic Radiation (How we get information about the cosmos) What are some examples of electromagnetic radiation and what are the different ways in which we might experience them or make use of them? Light(see) Infrared(heat) Ultraviolet(sunburn) Microwaves(cooking, communication) AM radio(communication) FM radio(communication) TV signals(communication) Cell phone signals(communication) X-rays(medical applications) The speed of all electromagnetic waves is equal to the speed of light. c = 186,000 miles / s or c = 300,000 km / s Sun Earth light takes 8 minutes c = ff Bigger means smaller f A changing electric field creates or induces a magnetic field and vice-versa! Constancy of the Speed of Light What physical principle requires that the speed of EM radiation be constant in vacuum? Constancy of the Speed of Light What physical principle requires that the speed of EM radiation be constant in vacuum? Conservation of energy If light will slow down, the changing electric field would generate a weaker magnetic field and so on... Light wave would die out If light will speed up, then energy of light would increase indefinitely The Doppler Effect How does the pitch or tone of a sound wave change when the source of the sound is moving towards or away from you? What about when you are moving towards or away from the source? Does this effect occur for all types of waves or just for sound waves? The frequency or wavelength of a wave depends on the relative motion of the source and the observer. (Shockwave Demo) (Web Link)Shockwave DemoWeb Link For visible light, the frequency (or wavelength) determines its color. TECHNOLOGY HISTORY demonstration of the laser semiconductor laser and low-loss fiber commercial fiber system deployed- Northeast Corridor commercial WDM - 8x2.5Gb/s commercial 10 Gb/s ff c = 1 nm = m, 1 Angstrom = m The Electromagnetic Spectrum Operating ranges of components Major elements of an optical fiber link