Physics Waves and Magnetism

What must the conductor do to generate a potential difference?

• has to cut across the magnetic field lines

• if the magnetic field lines are going side to side, the conductor has to move up and down (“cutting” the lines)

• conductor or magnet can move relative to eachother to create a potential difference

Maxwell’s equations

• a changing electric field induces a magnetic field

• a changing magnetic field induces an electric field

• as the electric field changes it induces a magnetic field, as the magnetic field is created it induces an electric field….

• this is called light

transverse waves

• light

• particles vibrate perpendicular to the direction the wave is traveling

• it only happens with transverse/light waves

Longitudinal waves

• move parallel to the direction of the wave

• sound and water

Do electromagnetic waves need a medium to travel through?

• no

• speed of light in a vacuum (no medium)

wavelength

• variable = lambda λ

• unit = meters (is a distance)

• length of one FULL wave

amplitude

• distance from equilibrium to max displacement

• meters

• direct relationship to energy

• more energy, higher wavelength

crest

highest point of a wave

trough

lowest point of a wave

frequency

• number of complete cycles per second

• unit = Hz (hertz), variable= f

• ex. 8 cycles in 4 seconds (number of cycles/ by how many seconds_

• f= 2 Hz

period

how LONG it takes for 1 complete cycle

time it takes for one full cycle to be completed

• is a unit of time (seconds)

• variable = T

• period equation: T= 1/f

wave equation

v=fλ velocity= frequency x wavelength

• velocity will be either speed of sound or light in ref table (usually)

What are waves that are in phase?

• points that are wavelengths apart (one or more)

• 0 or 360 degrees

• split wavelength into fourths (90,180,270,360)

What are waves that are out of phase?

• points are are 180 or 270 or 90 out of phase

• if it goes past 360 then its what is left over

• ex. if it is 450 out of phase, it is really 90 (450-360=90)

wave interference

when two or more wave meet while traveling along the same medium

• there is two types

constructive interference

when two waves overlap another in phase

• their amplitudes are added, a bigger resultant wave

destructive interference

when two waves overlap out of phase, results in smaller amplitude

• if they are not the same shape then the wave flips to the side of the larger wave and the difference is subtracted

standing waves

• produced when two waves of equal amplitude and wavelength pass through one another while traveling in opposite directions

• alternate from going in phase and out of phase

• constructive and destructive interference occurs

nodes

• points that remain stationary on a standing wave

• on the equilibrium line

antinodes

points of maximum amplitude on waves

move the most (on the crest)

• just count the points on the crests not troughs because it is the same point (ossilates)

Wave fronts

• way to make drawing waves easier

• the lines are crests and in between are troughs

• doesn’t show amplitude

• can be represented using a circle of lines

What is the relationship between wavelength and frequency?

• inversely related

• as wavelength increases (gets longer), frequency decreases

• as wavelength decreases, frequency increases (waves get shorter)

Doppler Effect

• a change in frequency due to movement (either object moving or a person is moving relative to an object)

• coming towards you (ambulance), frequency increases, wavelength decrease, pitch is higher

• moving away, —> frequency decreases, wavelength increases, lower pitch

What is the difference between particles and waves?

• particles have mass like marbles (can’t bend)

• waves don’t have mass so they can bend

diffraction

bending of a wave through or around the edge of a barrier

• depends on size of barrier (narrower—> more diffraction, wider—> less diffraction)

Double slit experiment

• waves diffract through both openings and then create an interference pattern

• where they intersect they form a pattern (destructive —> a line)

• constructive —> semi-circles

resonance

• everything has a natural frequency at which it will vibrate if disturbed

• if something is vibrating at the natural frequency of another object, that other object will vibrate

what is the variable for speed of light in a vacuum?

lower case c

• 3 × 10^8 m/s (m/s because it is a speed)

• natures speed limit (nothing can travel faster)

How do different mediums affect the speed of light?

• travels slowest in solids

• fastest in gases

• consider air the same as a vacuum (n=1)

electromagnetic spectrum on reference table

• left to right the frequency decreases so the wavelength is is increasing (getting longer)

• the only values shown are frequency

• have to use v=fxlambda to figure out wavelength of light

reflection

the bouncing back of light as it strikes a medium

• consists of two rays, incident and reflected

• a normal line is drawn perpendicular to the reflecting surface

• all angles of incidence and reflection are measured against the NORMAL

regular reflection

when a beam of light strikes a smooth surface (glass or metal)

• the reflected rays will be parallel, creating an undisturbed mirror image

diffuse reflection

when a beam of light strikes an irregular surface

• each ray obeys the laws of reflection

• reflected rays are not parallel so no mirror image is formed

refraction

change in direction due to a change in velocity when light enters a medium

• FREQUENCY STAYS THE SAME

absolute index of refraction

• variable = n, no units

• ratio of the speed of light in a vacuum to its speed in a medium

• n=c/v

• absolute index of refraction= speed of light in a vacuum x speed of light in a different medium

What does a higher index indicate?

• higher index means the light will travel slower (n=1 for light in a vacuum, so the further, greater you are away from 1 the slower you will travel because nothing can be faster than light)

• llower index—> faster

How does density/n impact the bending of light rays?

• greater the density/ higher n —> bends toward the normal

• lower density/ lower n —> bends away from the normal (y -axis)

• think of marching outside

• if incident ray is against the normal line/ vertical no refraction will occur

Snell’s law

• used to measure how much a ray is refracted/ bends

• n1 x sin1 = n2 x sin2

• MAKE SURE CALCULATOR IS IN DEGREE MODE

• 1 is where the light is coming from, 2 is what its entering

• how to use a protractor

critical angle

• the angle at which light will emerge parallel to the boundary

• bends so much that it doesn’t leave the water/ refracts completely horizontal

• critical angle problem, air will be 90 degrees and n=1

How is index of refraction affected by wavelength and frequency?

• as wavelength increases (longer) —> the index of refraction decreases

• refracted less than light with shorter wavelengths

• as wavelength decreases (shorter) —> n increases (refracted more)

• index of refraction is inversely proportional to WAVELENGTH (not frequency)

• ex. blue light (shorter wavelength) so it refract increases (refracts more), blue is always on the bottom of the rainbow

Which wave has a period twice that of wave W?

period- number of seconds per complete cycle

wave w frequency- 2 cycles/ sec, T=1/2

wave a frequency- 1 wave/ 1 sec, T=1 twice as much)

blue and red light

• blue light has a higher frequency, decreased wavelength (refracts more/ increased refraction)

• red light has a lower frequency, increased wavelength (refracts less, decreased refraction)

What do sound waves and x-rays have in common?

transmit energy without transmitting matter

When does a light ray increase in speed as it travels from one medium to another?

• increases if it travels from a small index of refraction to an even smaller one

• bends AWAY from normal

What does an inverse relationship look like?

• fraction

Differences between sound and light waves?

sound - mechanical (medium), longitudinal (particles parallel)

light- electromagnetic (no medium), transverse (particles perpendicular)