Waves Flashcards
What is a wave?
A transfer of energy from one place to another without the transfer of matter
What is the substance that a wave passes through called?
The medium
What is the equilibrium position?
Where the particle is found when no energy is passing through
What are the two types of waves?
Transverse and longitudinal
How do particles in the medium of a transverse wave vibrate?
Perpendicular (at right angles) to the direction of energy. (If the energy is going to the left, the particles go up and down)
What is the top end position of a particle’s vibration? (transverse)
Crest
What is the bottom end position of a particle’s vibration? (transverse)
Trough
Examples of transverse waves
Light waves and earthquake S waves
How do particles in the medium of a longitudinal wave vibrate?
Parallel to the direction of the energy transfer.
What is it called when the particles of a longitudinal wave are close together?
Compression
What is it called when the particles of a longitudinal wave are spread apart?
Rarefaction.
Examples of longitudinal waves
Sound waves, earthquake P waves
Amplitude
The maximum displacement of a particle in the medium from its equilibrium position. The more energy (louder, brighter), the large the wave amplitude. Measured in meters (m), symbol (A)
Wavelength
The distance between two successive particles undergoing the same motion. Ie distance from a trough to a trough, or rarefaction to a rarefaction. Measured in meters (m), symbol λ
Period (T)
The time taken for one complete wave to pass a point. Ie - the time for two crests to pass by. Measured in seconds (s), symbol (T)
Frequency (f)
The number of complete waves passing a point per second. Measured in hertz (Hz). Symbol (f). Higher frequency, higher pitch. Different colours have different frequencies.
Echo
When sound waves are reflected and bounced back, we hear the reflected sound as an echo.
Reflection
When waves hit a surface and bounce off
Ray
A ray is an arrow drawn that represents the direction in which energy is being transferred by a wave
What two things happen when a wave hits a different medium?
Some of the energy is reflected off the surface, some of the energy enters the new medium
When a wave is reflected, what changes?
The amplitude will decrease as some energy will be transferred to the medium
What is the ray called that goes into the other medium?
Incident ray (with the angle of incidence)
What is the ray called that goes out of the other medium?
The reflected ray (with the angle of reflection)
Refraction
The change in speed of a wave when a wave travels from one medium to another
Refractive index symbol
n, no units
If a wave is slowing down when entering a new medium, it will bend …
Towards the normal
If a wave is speeding up when entering the new medium, it will bend …
Away from the normal
What does the speed of light depend on in a transparent medium?
The optical density
What is the refractive index?
The measure of the speed a wave will travel in a medium. The higher n, the slower the wave will travel
Refractive Index units
No units but symbol n
What do we see as a wave?
The vibration of the particles in the medium
Where is the normal?
90 degrees from the surface
What is the law of reflection?
That the angle of incidence and the angle of reflection are the same
What direction does a wave reflect on a smooth even surface and a uneven surface?
Smooth surface - regular reflection, parallel rays
Rough surface - diffuse reflection - all at different directions
What is a plane mirror?
A flat mirror
What happens when light is reflected off a plane mirror?
An image of the original object will be formed in the mirror
Images in plane mirrors are what distance, size etc
Laterally inverted, same size, same distance behind the mirror as the object is in front of the mirror, virtual
What is the refractive index of light in the air?
n = 1.00
Snell’s law is used to find the refractive index (n) of a medium. When is it used?
When light is travelling from air into another medium.
Why does an object under water appear shallower than it actually is?
Our brain thinks that light waves travel in straight line. The refraction of the ray due to the change in medium. Our brain follows the refraction ray into the new medium so the lines meet at a shallower depth and the coin appears closer.
What is a wavefront?
A line drawn to represent the part of a wave where the medium is undergoing the same motion (crests or compressions etc).
What is the distance between wavefronts?
The wavelength
When would a wavefront not be straight lines?
When a stone is dropped in water.- if the wave speed is the same in all directions, circular wavefronts will be formed
When a wavefront is refracted, where do you draw the wavefronts?
At 90 degrees to the refracted ray
When a wave slows down (wavefront), what changes/stays the same?
Wave speed decreases, frequency stays the same so wavelength decreases
What is total internal reflection?
Where light becomes trapped within a medium, occurs when light is travelling from a more to less optically dense medium. The angle of incidence is greater than the critical angle.
What happens when the angle of incidence is equal to the critical angle?
The light travels along the medium.
The lower the refractive index…
The lower the number on the index, the faster light travels through the medium, the less the light is bent
Diffraction
When waves move past an object or gap and tend to spread out
When are circular wavefronts created in diffraction?
When the gap is approximately equal to the wavelengths
Can a sound/light wave travel without a medium?
Light waves can travel through a vacuum but sound waves need a medium.
What is dispersion?
When white light enters a prism, a rainbow comes out on the other side. White light is made up of different colours of light in the rainbow with different wavelengths. When they enter the prism, shorter wavelengths slow down more and so refract at the greatest angle, longer wavelengths speed up so refract at a smaller angle. The differing refraction separates them into a spectrum (rainbow). (splitting up of white light into separate colours)
How do rainbows occur in the sky?
Water droplets act as tiny prisms, light stays constant so rainbow stays constant
Why does a red shirt appear red?
It absorbs all colours but reflects red
White light absorbs or reflects light?
White light reflects all light
Blacklight absorbs or reflects light?
Blacklight absorbs all light
What are the primary colours of light?
Red, green, blue
What is a mechanical wave?
A wave that needs a medium for energy to travel through. Eg sound, seismic earthquake waves.
What is an electromagnetic wave?
A wave that doesn’t need a medium for energy to travel. They can still travel through a medium, however. Eg visible light, radio waves
All electromagnetic waves are what?
Transverse waves, can travel through a vacuum, travel at 3 x 10^8.
What is the critical angle?
The smallest angle of incidence at which total internal reflection happens.
Optical density
Optical density is a measure of how fast light can travel through a substance. The higher the optical density, the slower the wave
