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)

Resonance Natural frequency Resonance Determined by elasticity and shape Bells, violin strings, idling cars Resonance Dramatic increase in amplitude at natural frequency Pumping a swing (Tacoma Narrows Bridge Video) (Web Link)

Resonance and Black Holes If a normal object (e.g. pizza dough) rotates fast enough what happens to it? What about a loose car tire on a fast moving car? What is the shape of the event horizon of: a nonrotating black hole? a rotating black hole?

Resonance and Black Holes If a normal object (e.g. pizza dough) rotates fast enough what happens to it? it will be torn apart Will the same thing happen to the black hole? A loose tire can borrow rotational energy and wobble at its natural frequency Enough energy => car loses tire! What is the shape of the event horizon of: a nonrotating black hole? spherical a rotating black hole? elongated (bulges at equator) A rotating black hole can “pulsate” like loose tire Can a black hole destroy its own event horizon?

Cosmic Censorship A black hole cannot shed its event horizon by any known mechanism Cosmic censorship theorem There can exist no “naked” singularities Nature requires that a singularity always be shrouded by an event horizon We can never directly observe a singularity Unfortunate, since offers best clue to quantum gravity

Electric Force - opposites attract, likes repel Oscillating charges radiate All objects have temperatures greater than absolute zero - random thermal motion All ordinary 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)

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? (Link to Demo) What about when you are moving towards or away from the source? at a right angle to 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) How does a change in frequency affect how we perceive visible light?

Frequency Shifted EM Waves Frequency (or wavelength) of light determines color. Red shift: longer wavelength and lower frequency Blue shift: shorter wavelength and higher frequency Gravity can also cause a frequency shift! Energy of EM radiation is proportional to its frequency What happens to EM radiation trying to escape a black hole?

Frequency Shifted EM Waves Frequency (or wavelength) of light determines color. Red shift: longer wavelength and lower frequency Blue shift: shorter wavelength and higher frequency Gravity can also cause a frequency shift! Energy of EM radiation is proportional to its frequency What happens to EM radiation trying to escape a black hole? Radiation is infinitely red-shifted => no energy can escape!