Physics Section 3: Waves (COMPLETED)
Physics Section 3: Waves (COMPLETED)
degree (°)- distance
hertz (Hz)- cycles per second
metre (m)- distance
metre/second (m/s)- speed
second (s)- time
Properties of Waves
Vibrations (osculations) go up and down along the line of travel,
Light and electromagnetic waves travel in this way,
If you drop something in water the waves move up and down as they travel outwards,
If you lie a piece of string on a table and move one end up and down, the movement will pass through the object to the other end.
The vibrations are in the same direction as the line of travel,
Sound waves travel in this way,
Compressions are where vibrations are close together, rarefactions are where they are more spread out,
If you push one end of a stretched spring the compression will move down the spring.
Period of a Wave
Time taken for the source to produce one complete wave.
As a wave vibrates to either side of the direction of travel, the amplitude is the distance between the line of the direction of travel and the furthest point the it vibrates away from the line
The number of waves per second, it is measured in Hertz (Hz). You can think of it as how quickly the waves are travelling.
The distance between one point on a wave and the same point on the next wave; usually the point from the top/bottom of one wave (peak/trough) to the top/bottom of the next.
Waves and transfer of energy
A wave is a transfers energy through a space or object, but it does not move the particles in it.
If you stand on one side of a door and say 'Hi' a person on the other side will be able to hear you saying 'Hi'. This is because the vibrations that you made have travelled through the door to the other side, the energy moving from one particle of the door to another: but the door its self has not move, none of its particles have changed position.
v = f × λ
wave speed = frequency × wavelength
Frequency= 1/ time period
when a wave hits an edge, as it carries on it spreads out into the space beyond the edge. This happens with radio waves and hills, and water and islands.
Diffraction can happen through a gap, when waves go through a narrow space, on continuing they spread out again. The smaller the gap, in comparison to the wave length, the larger the diffraction.
The Electro-magnetic spectrum
The electromagnetic spectrum is a range of different frequency waves, one section of the spectrum is visible light (light we can see.) All of the waves in the electromagnetic spectrum travel at the same speed when they are in a vacuum.
As you go up the electromagnetic spectrum wavelength decreases and frequency increases.
The same is true for visible light; with red being the longest wavelength and lowest frequency and violet being the shortest wavelength and highest frequency of all visible light.
radio waves: broadcasting and communications
microwaves: cooking and satellite transmissions
infra-red: heaters and night vision equipment
visible light: optical fibres and photography
ultraviolet: fluorescent lamps
X-Rays: observing the internal structure of objects and materials and medical applications
gamma rays: sterilising food and medical equipment
microwaves: internal heating of body tissue, this can damage cells if they overheat
infra-red: skin burns, skin cells are damaged by overexposiure
ultraviolet: damage to surface cells and blindness, can damage receptor cells in the retina
gamma rays: cancer, mutation; can cause cells to change their arrangement causing cancer
Light and Sound
Light waves change speed when they pass through objects of different densities, this causes them to change direction. When they return to the original density they will continue in the original direction
Experiment (refraction (shape))
Place a block of glass on a piece of paper, drawing an outline.
At one point, draw the normal line.
Draw a line at 30 degrees to the normal line, shine a ray of light down this line.
Draw a line where the light comes out the other side. Connect the two lines, drawing the refracted ray.
Measure the angle of the emergent ray.
Repeat for different shaped glass.
n= sin(i)/ sin(r)
Refractive index= sin (angle of incidence)/ sin (angle of refraction)
Experiment (refractive index)
Shine a ray of light through a glass block, measure the angle of incidence and the angle of refraction.
Do sin(i) divided by sin(r) and you will have the refractive index of glass.
Light hitting a reflective surface will 'bounce' back from the surface (at the same angle they hit the surface.)
Law of Reflection
The angle of incidence is the angle that light hits a mirror; it is taken between 90 degrees from the mirror and the incidence wave (the wave that hits the mirror.)
The angle of reflection is the angle that light leaves the mirror; it is taken between 90 degrees from the mirror and the angle of reflection.
The angle of incidence is always the same as the angle of reflection.
a straight line with hatchings to show the side with the reflective coating on it.
line with arrows pointing towards the mirror.
line with arrows pointing away from the mirror.
(where the reflection appears to be behind the mirror) dashed line.
How to create
The angle of incidence should equal the angle of reflection.
A perpendicular line from the object to the mirror, if repeated the other side of the mirror, shows where the image appears to be in the mirror. Draw a line from the image to the eye, where this passes the mirror is where the angle of incidence should also meet the mirror.
Beyond the critical angle, light will be reflected back into the medium they came from at the same angle. In this way they are trapped in the medium.
By reflecting light past its critical angle you can make it travel through a medium to send information: this is done in optical fibres.
When light travels from one medium to another it is refracted; it changes angle due to change in density.
Past a certain angle the light will simply be refracted back into the medium it is in, this angle is the critical angle.
sin(critical angle)= 1/ refractive index
hen light meats a barrier, it will carry on through the gap and spread out in the area beyond.
The amplitude and/or frequency constantly vary.
Analogue signals are affected more by noise
Consists of pulses with two states: on; off
digital signals any noise picked up is likely to be of a smaller amplitude than that if the on state, this means something receiving it will ignore the noise as it is neither on nor off, this makes them less likely to be distorted.
Sound waves are longitudinal waves (the one that looks like a bar code.) They can be reflected, refracted and diffracted much like light can. For example an echo is a reflection of sound.
The human ear can pick up a limited section of different frequencies of sound: that is between 20 and 20,000 Hertz.
Speed of sound
Measure the distance between two places, have a sound made in one place, as soon as you see the sound has been made start a stop watch, as soon as you hear the sound made stop the stopwatch.
A microphone detects sound waves, it can feed this information into an oscilloscope which will display it as a wave (or straight line.)
Experiment (frequency of sound)
Have an noise made into a microphone attached to an oscilloscope, for example have someone try to sing a note. See how many oscillations there are per second, this will be your frequency. Try changing the pitch of the note and see it the number of oscillations per second changes.
Pitch and frequency
The more something vibrates the higher frequency.
The higher frequency the higher pitch.
So the more vibrations the higher pitch.
Volume and amplitude
The bigger the vibration the higher the amplitude.
The higher the amplitude the louder the sound.