P4
Waves
Can be thought as vibrations or oscillations about a fixed point
Each individual particle in a wave moves back and forth in place, such that overall energy is transferred
Example of oscillation
When a speaker produces music, it vibrates back and forth
As it moves forward, it pushes on closest air particle. This air particle in turn pushes on next and so on.
Each individual particle is oscillating in place, but transfer energy to the next one, and so on along the line
Transverse wave
Vibrate at 90 degrees from the direction the wave is travelling
Wave moves left to right, but particles in medium move up and down.
Thus motion of particles is transverse (or perpendicular) to motion of wave and direction of energy flow
Crests - high points
Troughs (low points)
Examples: Light waves, ripples on water, and vibrations on guitar string
Can be produced by attaching string to a wall and moving it up and down
Longitudinal waves
Vibrations of these waves are in the same direction as they travel
Propagates by changing pressure of medium.
Made up of areas of compressions and rarefactions
Compression wave - reigon where many particles come together and there is higher than normal pressure
Rarefaction wave - Reigon where many particles are spread apart and there is lower than normal pressure
Examples : Sound waves, ultrasound
Can be produced in slinky by moving it towards and away from you
Displacement-time graph
Wave can be described in terms of displacement from equilibrium line. One complete wave cycle is marked on the diagram
Crest - High point (max displacement) of a wave
Trough - The low point(Min displacement) of a wave
Amplitude - Max or min displacement from equilibrium
Wavelength of a wave - distance between any two consecutive points on a wave
In the diagram it is between two troughs, but could be equally be between two crests, or any two other corresponding points
Wavefront
Imaginary line connecting all points reached by the wave at the same time
Usually we connect all of the wave crests creating a wavefront, as each crest will reach the same distance from source in given time.
We can do this for any other crests in the wave, generating series of wavefronts
Distance between each wavefront is the same as distance between each wave crest.
Distance between each wave crest is the wave's wavelength (which is constant).
Meaning distance between each wavefront must be constant and have same value as wave's wavelength
Waves move energy without moving matter
Can be seen in simple experiments with ripple tanks
If object causing waves to be formed is linear, they will appear linear and travel in straight lines
If object causing waves to be formed is spherical, then they will radiate out from object that makes them
How to measure speed of wave
v = f x λ
λ = v/f
velocity = frequency x wavelenggth
Wavelength = velocity/frequency
Frequency - determined by number of cycles per second (Hz)
Particles vibrate parallel to direction of wave propagation
Amplitude also determines the volume of a sound wave
Also determines the pitch of a musical note
Wave properties
Reflection
Waves will change direction when they reach a boundary such as a mirror.
Law of reflection
Whatever angle the light strikes the mirror at, it will reflect from it at the same angle
If boundary is not smooth like a mirror surface, then diffuse reflection occurs
Law of reflection still applies, can still draw a normal line perpendicular to the surface where the ray strikes it and determine the angle of reflection
As surface is irregular, normal line at each point on the surface will have a different angle.
Each incident ray will reflect in a different direction, so light reflected will be spread out overall
Refraction
Medium is any substance through which waves can travel.
Change in medium could involve light passing from air to glass, or water, or crystal- or going in the other direction
Diffraction
Process when waves spread out due to either passing through a gap or passing an edge
Commonly seen when waves pass through an opening smaller than the wavelength.
This results in a spread of circular waves
Wave of diffracted wave is the same as before
Wavelengths
If wave passes a point or edge, the waves which interact with the edge start to bend.
This allows for radio and TV signals to be detected in hilly areas but do not have the same effects on waves which have smaller wavelengths
Large wavelength = diffract more
Sound waves that we can hear have wavelengths of 17cm and 17m
Light waves have wavelengths approximately ten thousand times smaller than a millimetre, meaning they will not diffract around objects at everyday scales
When a wave hits a boundary, such as a ray of light hitting a mirror, it is reflected
Line perpendicular to the mirror, angle between normal and incident ray is the angle of incidence
Angle of incidence is always equal to angle of reflection (i = r)!!!
Angle of incidence (i) - ray of light coming towards mirror, angle between incident ray and normal
Angle of reflection (r) - Angle between reflected ray and normal, ray of light which leaves the mirror after being reflected
The normal is a line perpendicular to the mirror
These angles are measured from a line 90degrees to the boundary called the normal
Waves undergo changes because they change the medium in traveling.
When a wave changes medium, it will change speed and wavelength. but frequency remains the same
When light changes mediums and slows down, it will bend towards the normal
When light changes mediums and speeds up, it will bend away from the normal
If the gap that the wave passes through is larger than the wavelength nothing happens
The waves continue in a straight line but may show bending at edges
Electromagnetic radiation (EMR)
EMR travels through the vacuum of space.
Most waves need a medium to travel in but not EMR
Exists on a spectrum of different types of wave
Light we see can exist in the middle of this spectrum; because we can see it, it is referred to as visible light
Other types : Infrared, microwaves, radio waves, ultraviolet, x-rays and gamma rays
Transverse waves
When light moves through a medium the energy vibrates at right angles to the direction it travels in
Light and other EMR waves are transverse ways.
Light travels with a velocity, and has a frequency and wavelength
Light doesn't need a medium
which is why light from the sun can travel through space to reach the earth
Light
Travels at 3.0 x 10^8 m/s in a vacuum (basically 300,000km/s)
Speed limit of the universe, impossible for anything to travel faster than the speed of light
Wiggles up and down (or back and forth) as it moves forwards
Light has a very short wavelength
When light moves through a medium it bounces around the particles of the medium.
Light is scattered, scattering means light takes a lot longer to get where it needs to go
Speed through medium is slower than speed of light
Denser media have more particles, so light scatters more and slows down more
Light travels fastest in a vacuum. There are no particles in a vacuum so light has a straight path from point to point
Meaning there is nothing to scatter light and slow it down, allowing it to travel at the speed of light
Bending of light rays, occurs when light travels through from one medium into a medium with different density
Ray diagrams
Normal
Ordinary line that runs at 90 degrees to boundary of media. Usually represented with a dotted line
Angle between incident ray and normal is angle of incidence
Angle between refracted ray and normal is angle of refraction
Refractive index (n)
Measure how easily light can travel through different media
Denser medium - larger refractive index
This means light ray bends towards normal when travels into medium with higher refractive index, and opposite with low refractive index
Vacuum has a refractive index of 1.00 (lowest possible index)
Calculate refractive index using angles
n = sin i / sin r
n = refractive index
i = angle of incidence
r = angle of refraction
Critical angle
Ray of light has high enough angle of incidence, refracted ray will travel along boundary between two media
When the angle of incidence is equal to critical angle, it means angle of refraction = 90degrees
Total internal reflection
When angle of incidence is larger than criticle angle, light won't be able to refract because there is no way for it to pass into the other substance
Can only occur when light travels into a medium with lower refractive index
because light will bend away from normal
Optical fibres
Use total internal reflection to carry signals
Made out of glass fibers 10micrometres thick, designed so rays of light totally internally reflected back into glass.
This way they can carry a signal over a long distance
Used for phone and internet cables, medical tech to observe regions of body difficult to reach
Lenses
Lens
Curved, transparent piece of material used to focus light to produce an image
This works because rays refract (or bend) as they move into and out of the lens.
Lenses make light change direction- this shrinks or magnifies an image
Two types of lens
Convex
Concave
Bulges out in the middle
Found in glasses and cameras
Cause parallel light rays to diverge. meaning they move apart and do not cross. You still see an image with concave lenses
Example: magnifying glass
When you hold a magnifying glass in front of an object, we see an enlarged image
Curved inwards in the middle
Used in flash lights, telescopes, binoculars
Causes parallel light rays to converge, meaning rays meet and form an image
Example of concave: glasses
Short sightedness - Light entering your eye will focus in front of retina, due to a long eyeball or inability of lens in your eye to focus correctly
Short sightedness is corrected using glasses containing concave lenses, which focus light on your retina resulting in forming sharp images
All lenses have principal focus and length
Principal focus - point where light rays converge (or appear to diverge form) after passing through lens
Focal length - distance between either principal focus and centre of lens
Ray diagrams
Drawing ray diagrams
To draw a ray diagram, you need to know where a lens' principal focus is
Harder to find principal focus on concave lens because rays do not actually cross
Trace the rays backwards behind the lens to find point where they do appear to cross
Principal axis
Principal axis is line that runs directly through center of lens
Where object is placed
Important reference for rays that will be drawing
Three important light rays used to find where an image is formed
First ray
- travels parallel to principal axis and passes through principal focus on the other side of len
Second ray
Goes from object straight through centre of lens without refracting
Third ray
Travels through principal focus closest to object and travels parallel to principal axis on the other side of lens
When the three rays are combined onto a single diagram, an image is formed where the light rays cross
Three properties to describe the nature of image
Real or virtual
Real image is formed when light rays actually cross
Virtual image is formed when the rays only appear to cross
Formed when diverging rays are traced back to where they appear to cross - mirrors are virtual images
Can be larger, smaller or same size of an object
Enlarged image - bigger than the object
Diminished image - smaller than the object
Longer image arrow on diagram = larger object
Oriented the same way as the object or not
Image is upright if image arrow points up
Inverted if the image is pointing down, meaning it is upside down
Type of image
Determined by type of lens + location of object
When the object is beyond or outside focus, the image formed is real, inverted and diminished
When you put an object between focus and lens
First ray acts the same
Second ray goes straight from object through center of lens
Third ray goes through focus on object's side of lens, then goes parallel to principal axis
Concave lenses are always VIRTUAL, DIMINISHED and UPRIGHT
First ray goes towards principal focus on opposite side of lens, travels parallel to principal axes
Second ray extends from object straight through center of lens
Third ray starts parallel to principal axes, then diverges away from parallel as it passes lens
Determine the angle by tracing it backwards
Incident ray travelling from parallel to principal the refracted ray travels from principal focus on the same side of lens
When incident light ray is travelling towards principal focus of other side of lens, refracted rays travel parallel to principal axis
When incident rays travel through centre of lens, it is unaffected by the lens
Electromagnetic spectrum
Radio waves
longest wavelengths (1mm - 100km)
Lowest frequency and least energy of all forms of emr
How is color produced
Light waves of different frequencies and wavelengths
Lowest wavelength and frequency color is red
Ultra violet radiation (UV)
Between visible light and x-rays
Produced by hot objects like the sun or solariums
Short wavelengths ranging 10-400nm
High frequencies and high energy
UV travels through the atmosphere with infrared radiation (felt as heat) and visible light (what we see)
UV exposure is important as it helps produce vitamin D, which is needed to absorb calcium and fight infections and bone growth
Exessive uv exposure can cause skin damage, eye damage and skin cancer
UV radiation is able to deactivate bacteria + viruses
X rays
Sits between UV and gamma rays
Short wavelength ranging 0.01nm to 10nm
High frequency and high energy level
Density of medium determines how many x-rays can pass through it
Commonly used for medical imaging because they can pass through low density tissue like muscle
But is absorbed or scattered by high density tissue like bone
Long term exposure to x-rays can damage cells. can cause mutations which lead to cancer