Topic 8 - Waves Flashcards
Transverse wave
A wave that vibrates or oscillates at right angles (perpendicular) to the direction in which energy is transferred.
e.g. light
Longitudinal wave
A wave that vibrates or oscillates parallel to (along) the direction in which energy is transferred.
e.g. sound
Amplitude
The distance from the undisturbed position to the peak or trough of a wave
Wavelength
The distance between a particular point on one cycle of the wave and the same point on the next cycle
Frequency
The number of waves passing a particular point per second
Time period
The time it takes for one complete wave to pass a particular point
What waves transfer
Waves transfer energy and information WITHOUT transferring matter
Relationship between the speed, frequency and wavelength of a wave
wave speed = frequency × wavelength
v = f × λ
Relationship between frequency and time period
frequency = 1/time period
f = 1/T
Doppler Effect
A change in the observed frequency and wavelength of a wave when its source is moving relative to an observer:
- If a car is moving, wavefronts of the sound are no longer evenly spaced
- Ahead of the car wavefronts are compressed as the car is moving in the same direction as the wavefronts - This creates a shorter wavelength and a higher frequency
- Behind the car wavefronts are more spread out as the car is moving away from the previous wavefronts
- This creates a longer wavelength and a lower frequency
Reflection
The change in direction of a wavefront at a boundary between two different media so that the wavefront returns into the original medium
Refraction
Process by which a wave changes speed and sometimes direction upon entering a denser or less dense medium
EM spectrum of light
Lower energy
Long wavelength
Low frequency
Radio waves
Microwaves
Infrared
Visible light
Ultraviolet
X-rays
Gamma rays
-
Higher energy
Short wavelength
High frequency
All these waves travel at the same speed in free space
*refer to diagram for colours
Uses of radio waves
- broadcasting
- communications
Uses of microwaves
- cooking
- satellite transmissions
Uses of infrared radiation
- heaters
- night vision equipment
Uses of visible light
- optical fibres
- photography
Uses of ultraviolet radiation
- fluorescent lamps
- killing bacteria
Uses of X-rays
- observing the internal structure of objects and materials
- medical applications (e.g., detects dental problems, diagnose cancer)
Uses of gamma rays
- Sterilising food and medical equipment
- Treatment and detection of cancer
Detrimental effects of overexposure to microwaves
- internal heating of body tissue
- skin burns
- cataracts
Detrimental effects of overexposure to infrared radiation
- skin burns
- eye damage
Detrimental effects of overexposure to UV radiation
- damage to surface cells
- blindness
Detrimental effects of overexposure to gamma rays
- cancer
- mutation
protection
- shielding: barriers of lead, concrete, or water
Law of reflection
when a ray hits a plane mirror, angle of incidence = angle of reflection
θi = θr
Describe experiments to investigate the refraction of light, using rectangular blocks, semicircular blocks and triangular prisms
- Set up your apparatus using a rectangular block and trace around it on a piece of paper
- Shine the light ray through the glass block
- Trace the incident and emergent rays onto the piece of paper and remove the block
- Draw the refracted ray in by joining the ends of the other two rays with a straight line
- Comment on how the speed of the light has changed as the light moves between the mediums
- Repeat this for different angles of incidence and different glass prisms
Snell’s Law
n = sin(i)/sin(r)
Practical: investigate the refractive index of glass using a glass block
- Draw around a rectangular glass block on a piece of paper and direct a ray of light through it at an angle
- Trace the incident and emergent rays and remove the block
- Draw in the reflected ray between them
- Draw in the normal at 90˚ to the edge of the block next to the incident ray
- Use a protractor to measure the angle of incidence and the angle of refraction based on the normal
- Calculate the refractive index using Snell’s Law
Total internal reflection
When a light ray hits the boundary between two materials of different densities, and is reflected rather than refracted.
Conditions:
- Angle of incidence > critical angle
- The incident material is denser than the second material
Total internal reflection in optical fibres
An optical fibre consists of a thin core of high quality glass. The core is covered by a second layer (cladding).
The core is more dense than the cladding (which has a lower refractive index), so the light ray passes along the core-cladding boundary at an angle greater than the critical angle. This then means the light ray is continuously reflected along the lengths of the optical fibre core.
Critical angle
The angle of incidence which produces an angle of refraction of 90˚ (refracted ray is along the boundary of the surface)
Relationship between critical angle and refractive index
n = 1/sin(c)
Frequency range for human hearing
20Hz – 20,000Hz
Practical: investigate the frequency of sound wave using an oscilloscope
- attach a microphone to an oscilloscope
- emit a sound into the microphone for one second
- find how long it took for one wave to oscillate by using f = 1/T to work out the frequency
How pitch relates to frequency
high frequency = high pitch
Velocity of sound practicals
- Microphones and data logger
Data logger measures and records time taken for sound to reach two microphones
Speed of sound = measured distance/time on computer
- Clap-echo method
1. stand a long distance from a wall
2. clap and listen for the echo
3. distance travelled = twice the distance from you to the wall because sound has to travel to the wall and back
4. s = 2d/t
How to calculate refractive index for a specific medium
n = c/v
where
n is refractive index
c is the speed of light in a vacuum (3.0 * 10^8 m/s)
v is the speed of light in that medium (m/s)
