P4 Flashcards
What is a wave?
An oscillation or vibration about a fixed point
Only transfers energy/information not matter
Describe the properties of transverse waves
Movement of energy -> forward/backward
Vibration -> up/down
They are perpendicular to each other
Can move through solids, surfaces of liquids but not inside liquids/gases
(but em waves can (also only ones that can travel in a vacuum))
Constant density
Constant pressure
Examples:
Water ripples
Vibrating string
Em waves
Describe the properties of longitudinal waves
Movement of energy -> forward/backward
Vibration -> forward/backward
They are parallel to each other
Can move through solids, liquids and gases but cannot travel in a vacuum (needs a medium to travel through)
Changes in density
Changes in pressure
Examples:
Sound
How can frequency, wavelength and wave speed be seen in a ripple tank?
(Basically the equation for each one)
Frequency:
Time for a give # of waves to pass / time taken
Wavelength:
Length of screen / # of wavefronts
Wave speed:
Frequency x wavelength
What is a wavefront? (How do they help find the wavelength?)
Each wavefront is a single wave
(Represented as a line and arrow denoting direction)
The space between each line is the wavelength
Define crest and trough
Crest/peak:
The highest point on a wave above the ‘rest’
Trough:
The lowest point on a wave below the ‘rest’
Define amplitude (including its symbol and the unit of measurement)
The distance from the undisturbed position (rest) to the peak or trough of a wave
-> the min/max displacement from the undisturbed position
Symbol: A
Measured in: meters
Define wavelength (including its symbol and the unit of measurement)
The distance from one point on the wave to the same point on the next wave
Transverse: peak to peak
Longitudinal: start of compression time start of compression
Symbol: lambda (λ)
Measured in: meters
Define frequency (including its symbol and the unit of measurement)
The number of waves passing a point in a second
Symbol: f
Measured in: Hertz (Hz)
Define wave speed (including its symbol and the unit of measurement)
The speed at which energy is transferred through a medium
-> distance travelled/second
Symbol: v
Measured in: meters/second (m/s)
Define reflection
When a wave hits a boundary between two media and doesn’t pass though, it instead stays in the original medium
What is the law of reflection?
Angle of incidence (i) = angle of reflection (r)
Define refraction
When a wave enters a different medium, their speed can change
Refraction occurs when a wave passes a boundary between 2 different transparent media
Which characteristics of a wave stays the same when refracted?
Wavelength
Direction
Frequency
Frequency is the only one that stays the same
Which direction will the wave go when it slows of speeds up? What does this do to the wavelength?
Wave slows -> waves bunch together -> shorter wavelength
Towards the normal
Wave speeds -> wave spread -> longer wavelength
Away from normal
Define diffraction
When waves pass through a narrow gap, the wave spreads out
As the gap becomes bigger the effect is less pronounced
It also happens on edges
What is the equation for wave speed?
Frequency x wavelength
What is refraction caused by?
A change in speed
How can refraction be shown in a ripple tank?
Place a glass block in the tank below the surface
-> partially covers the floor
As speed depends on depth the ripples slow over the block (shallower)
What is a normal?
The normal is an imaginary line that is 90° to the boundary
What are the characteristic of the image created by a plane mirror?
Same size as object
Same distance behind the mirror as the object is in front of the mirror
-> in line with the object
Virtual
-> cannot be projected onto a screen usually appearing to be behind the lens
How do the rays relate to the place where the virtual image will form?
Where the virtual rays converge
Explain an experiment that can be done to investigate reflection by plane mirrors
Aim:
Investigate reflection by a plane mirror
Variables:
Independent -> angle of incidence
Dependent -> angle of reflection
Control -> distance of ray box from mirror, width of beam, wavelength+frequency of light
Materials:
Ray box
Plane mirror (+stand)
Protractor
Method:
>middle of paper make 10cm line
>use protractor make 90° line that bisects 10cm line
>place mirror on first line
>switch ray box on -> aim at cross section of the two lines
>mark 2 postions: one on incident ray one on reflected ray
>remove/turn off box and mirror
>use ruler to join 2 points to where the cross section is
>use protractor to measure i and r
>repeat and make a table
Result:
Law of reflection observed
Revaluation:
Errors:
90° line not correct
Mirror distorted
Points marked inaccurately
Protractor resolution
Safety:
Ray box -> could cause burns/eye damage
Liquids + electrical/paper
Mirror damage
What is the equation for the refractive index?
Refractive index = speed of light in vacuum/speed of light in material
Always 1<
Relate the change in direction (refraction) of light to the density of media
Less dense to more dense -> towards normal
r < i
More dense to less dense -> away from normal
r > i
Along normal -> no change
What is total internal reflection?
Sometimes when light goes from denser -> less dense instead of being refracted light is reflected
When: i > critical angle
Where is TIR used?
Prisms (optical instruments)
Periscopes
Binoculars
Telescopes
Cameras
Safety reflectors:
Bikes
Cars road signs
Also optical fibers
What is the critical angle?
When r = 90° light is refracted along the boundary
i when r=90° is known as the critical angle
How do optical fibers work? How are optical fibers used?
> light travelling down the fiber is internally reflected
Communication:
The wave carries information
Medicine:
> used to see inside the human body
Light from light source goes through optical fiber -> objective lens -> reflected from object -> into eyepiece lens (image created)
Describe the action of a thin converging lens (convex)
Parallel rays are brought to a focus
> represented by line with arrow on both ends
Focal length is determined buy the curviness of the lens
More curve = small length
Describe the action of a thin diverging lens (concave)
Parallel rays are made to diverge from one point
Represented by: a line with reverse arrows on both ends
Define principal focus/focal point
The point at which rays of light travelling parallel to the principal axis:
> intersect the principal axis and converge
OR
> the point where the rays appear to diverge from
Define focal length
The distance between the center of the lens and the principal focus
Define real image
An image that is formed when the light rays from an object converge and meet each other and can be projected onto a screen
Always convergence towards focus
Always inverted
Define virtual image
An image that is formed when the light rays from an object do not meet but appear to meet behind the lens and cannot be projected onto a screen
Always divergence from a point
Always upright
Explain how you can find the location of a real image created by a converging lens
- Draw a ray from the top of the object through the center of the lens
- Draw a ray from the top of the object travelling parallel to principal axis when ray emerges it will travel directly towards focal point
- Where the rays converge -> position of image
-> REMEMBER -> ALWAYS INVERTED
T or F:
Both converging lenses and diverging lenses can only create real images
F
Converging can create both
Diverging can only create virtual
How is the characteristics image change depending on the objects distance from the lens?
f (focal distance)
2f -> same size image
2f< -> diminished image
f < x < 2f -> enlarged image
Upright -> virtual
Inverted -> real
How can a convex lens be used as a magnifying glass?
> if the object is placed closer to the lens than the focal length the emerging ray diverges -> it is now a VIRTUAL IMAGE
The point is found by extending rays backwards -> creating virtual rays
-> virtual image found where the rays cross
—>virtual, enlarged, upright
For use as a magnifying glass -> must be held close to face
How is sound produced?
Sound waves are produced by vibrating sources
When a sound waves comes into contact with a solid vibrations are transferred
Ex: opera singer and crystal glass
What type of wave is a sound wave?
Longitudinal wave
-> Needs a medium to travel through
-> parallel energy transfer and vibration
Define compression
A region of higher density (molecules are bunched together)
Define rarefaction
A region of lower density (molecules spread out)
T of F: rarefaction and compression do cause a change in pressure
T
This makes sound a pressure wave
When wave hits solid -> variation in pressure causes surface to vibrate (in sync)
What is the range of audible frequencies for a healthy human ear?
20 Hz to 20,000 Hz
T or F
Sound waves do not need a medium to travel through
F
What is the speed of sound in air? How does temperature affect this?
Speed of sound in air -> 340 m/s in room temp
High temp -> faster sound
Varies from 330-350 m/s
Describe an experiment to determine the speed of sound in air
METHOD 1: MEASURING SOUND BETWEEN TWO POINTS
- 2 people stand around 100m apart -> distance measured by trundle wheel
- 1st person -> 2 wooden block which they bang above their head
- 2nd person has stop watch -> starts when see bang block -> stop when sound heard
- Repeat and find the average
Speed of sound: distance traveled/time taken
Describe an experiment to determine the speed of sound in air
METHOD 2: USING ECHOS
- Person 1 stands around 50m from a wall (use trundle wheel to measure distance)
- person 1 bangs 2 wooden bocks together
- Person 2 has stopwatch -> starts when hear clap -> stop when hear echo
- Repeat and average
Speed of sound = 2 x distance to the wall / time taken
Describe an experiment to determine the speed of sound in air
METHOD 3: USING AN OSCILLOSCOPE
- 2 microphones are connected to oscilloscope and placed around 5 m apart (use tape measure)
- Oscilloscope set so it’s triggered when 1st microphone detects sound
-> adjust time base -> sound in both microphones can be seen - 2 wooden block clapped next to 1st microphone
- Oscilloscope used to determine time between the 2 microphones
- Repeat then average
Speed of sound = distance between microphones / time between peaks
Order solids, liquids and gases in the order of which medium sound travels through fastest (+ why)
Gas < liquid < solid
Molecules are closer together
What characteristics of waves related to loudness and pitch?
Frequency = pitch
> high pitch -> high frequency
> low pitch -> low frequency
Amplitude = volume/loudness
> large amplitude -> high volume
> small amplitude -> low volume
What is an echo?
The reflection of sound of off a hard surface
How are echos used?
Used to measure depth/detect objects (usually in the ocean)
> sound waves transmitted from surface
> sound waves reflect off the bottom of the ocean
> time take -> used to calculate depth
> length wave travelled = x2 the depth of the ocean
