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 () crest amplitude (A) trough velocity (v) Period (T): time between crest (or trough) passages Frequency (f): rate of passage of crests (or troughs), f wave speed = wavelength/period = wavelength * frequency 1 T (units: Hertz or cycles/sec) (v = f)

Types of Waves Longitudinal Waves Transverse 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? (Shockwave Demo) (Web Link)

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. (Tacoma Narrows Bridge Video) (Web Link)

Interference Constructive interference Destructive 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. Refraction Waves bend when they pass through material of different densities. air water swimming pool prism glass air air

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:

Electric Force - opposites attract, likes repel Oscillating charges radiate All objects have temperatures greater than absolute zero - random thermal motion All objects radiate! Why don’t 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 (medial applications)

Bigger  means smaller f The speed of all electromagnetic waves is equal to the speed of light. c = 186,000 miles / s or c = 300,000 km / s light takes 8 minutes Sun Earth c = f Bigger  means smaller f A changing electric field creates or induces a magnetic field and vice-versa! (Shockwave Demo) (Web Link)

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 were to slow down, the changing electric field would generate a weaker magnetic field and so on... Light wave would die out If light were sped up, then energy of light would increase indefinitely

The Electromagnetic Spectrum 1 nm = 10 -9 m , 1 Angstrom = 10 -10 m c = f

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) For visible light, the frequency (or wavelength) determines its color.