Waves: Sound/Light Flashcards
Mechanical wave
wave that needs a medium to travel through e.g. sound waves, water waves
electromagnetic wave
does not need a medium to travel through e.g. visable light, x-rays
periodic travelling wave
periodic disturbance that travels outward from a source, transferring energy from the source to other places
transverse wave
wave where the direction of vibration is perpendicular to the direction in which the wave is travelling
longitudinal wave
wave where the direction of vibration is parallel to the direction in which the wave is travelling e.g. sound waves
amplitude of wave
height of wave from undisturbed point to crest
oscillation/cycle
period
1 crest and 1 trough
wavelength
distance from trough to trough/crest to crest
frequency
number of cycles passing any point per second
unit of frequency
Hertz (H) - 1Hz=1 cycle per sec
wave phenomena
- reflection
- refraction
- diffraction
- interference
- polarisation
c=fλ
c=speed
f=frequency
λ=wavelength
reflection
bouncing of waves off an obstacle in their path
refraction
changing of direction of a wave when it enters a region where its speed changes e.g. hear better on cold night
diffraction
sideways spreading of waves into the region beyond a gap or around an obstacle
constructive interference
2 waves meet and resulting amplitude is greater than the amplitude of each individual wave
destructive interference
where 2 waves meet and resulting amplitude is less than the amplitude of each individual wave
note: when trough meets crest the waves completely cancel each other out - completely out of phase
coherent sources
constant phase difference or in phase
they will have same frequency
nodal lines
lines in interference pattern where there is total destructive interference
antinodal lines
where constructive interference occurs
polarisation
restriction of waves to vibrating in a certain plane
stationary waves
amplitude of wave at every point is constant: no net transfer of energy.
occurs when 2 periodic travelling waves of same amp/frequency moving in opposite directions meet
node to node
λ/2
antinode to node
λ/4
doppler effect
apparent change in frequency of waves due to the motion of the source/observer
doppler effect uses
red/blue shift
gardaí speed traps
source moving towards observer
f’=fc/c-u
source moving away from observer
f’=fc/c+u
red shift
tells us if a star is moving away from us (red has longest λ)
blue shift moving towards us
overtones
multiples of a certain frequency
2f=1st overtone
3f=2nd
harmonics
multiples of a certain frequency
f=fundamental frequency/1st harmonic
2f=2nd harmonic
loudness
depends on the amplitude of the sound wave (higer amp. = louder)
pitch
depends on the frequency of the sound wave. as frequency increases so does pitch
quality
depends on the number of overtones present and the relative strengths of the overtones present
frequency limits of audibility
highest and lowest frequencies that can be heard by a normal human ear
20Hz-20kHz
natural frequency
the main frequency that an object will vibrate at if free to do so
resonance
this is when a periodic force is applied to an object and has a frequency the same as or very near to the object’s fundamental frequency. this causes the body to vibrate with a very large amplitude
sound intensity at a point
the rate at which sound energy is passing through unit area at right angles to the direction in which the wave is travelling at that point
I=P/A
unit: Wm^-2
threshold of hearing
smallest sound intensity detectable by the average human ear at a frequency of 1kHz
it is 1×10^-12 Wm^-2
frequency response of the ear
the average human ear is most sensitive between 2kHz and 4kHz
Sound intensity level
scale that measures the amount of energy carried per sec into your ear
unit: decibels (dB)
sound intensity to decibels
when sound intensity doubles sound intensity level (dB) increases by 3dB
fundamental frequency of a string
when a string vibrates with an antinode at its centre and a node at each end
f= 1/2l(√T/µ)
open pipe and harmonics
all harmonics present
closed pipe harmonics
only odd harmonics possible
grating constant
d=1/x where x=lines per mm
distance between 2 adjacent skits on grating
zero order diffracted image
if light is passed through the diffraction grating and then passed through a convex lens all the light is brought together to form a very bright line
first order diffracted image
first bright line to left/right of zero order
monochromatic light
light that only has one frequency e.g. sodium only made up of orange light
dispersion
the breaking up of white light into different wavelengths/colours
recombination
bringing together the different wavelengths of light to make white light
primary colours
red, green, blue
secondary colours
yellow, cyan, magenta
complementary colours
red + cyan
green + magenta
blue + yellow
(mix to give white)
electromagnetic spectrum
from longest to shortest wavelength:
radio waves, infra-red, visable light, ultraviolet, x-rays, gamma rays
UV light properties
- wavelength between 0.4µm and 5nm
- causes fluorescence in certain substances (substance absorbs UV and emits visable light)
- causes sunburn, cancer, harms eyes
- causes photo-emission
infrared light properties
- wavelength between 0.7×10⁶m and 1mm - emitted by all hot bodies
- affects photographic plates: can take photos in dark, fog/mist, used by military rescue and doctors to diagnose abnormalities in body
- causes substances to heat up if it falls on them
diffraction grating formula
nλ=dSinQ
n=order
d=grating constant
stress polarisation uses
used by engineers to find stress in components
