Uses of waves for OCR GCSE Physics
This page covers the following topics:
2. Echo sounding
3. Polarising filters
Ultrasound waves are a type of sound wave which have a frequency which is above the range of human hearing. Ultrasound waves have a number of uses such as a range of uses in medicine and to clean jewellery.
High frequency sound waves are used to detect objects in deep water using the same method as the ultrasound imaging. This is often called SONAR (Sound Navigation and Ranging) and uses waves that are within normal hearing range rather than using ultrasound. They can be manufactured and can be found in the natural world used by some animals, such as bats and dolphins.
Polarisation filters allow light to pass through but only where the waves oscillate in ONE specific direction. For example, if an unpolarised light beam is sent through a filter then the resulting beam of light is vertically polarised. Multiple polarisation filters can be used together, and they can have different effects on the resulting light beam depending on how the filters are rotated.
Loudspeakers use the motor effect to convert electrical signals into sound waves. Variations in the electrical signals create variations in the magnetic field created by the electromagnet in the loudspeaker. This causes the speakers cone to move in and out which creates pressure variations in the air around the speaker, creating sound waves.
What is the range of frequencies for ultrasound waves?
Over 20000 Hz
The diagram shows the size of the vibrations of a loudspeaker for a range of frequencies emitted. What is the frequency of the loudest sound that the speaker is producing?
An ultrasound wave which is emitted by a source takes 0.9 × 10⁻⁵ seconds to return to a detector after it has been reflected by a material boundary in the body. Calculate the depth of the material boundary if the speed of the ultrasound is 1250 m/s in the material.
d = v × t. d = 1250 × 0.9 × 10⁻⁵ = 0.011 m there and back. 0.011 ÷ 2 = 0.0056 m = 5.6 mm
The depth of water being measured using echo sounding is 550 m and the speed of ultrasound in water is 1600 m/s. Calculate the amount of time taken for the ultrasound pulse to be received after it was emitted.
time = distance ÷ speed
time = 550 ÷ 1600 = 0.34 s
0.34 × 2 = 0.69 s
For echo sounding in deep water a wave with a frequency of 50 kHz is used. Suggest a reason why this frequency is ideal for measuring the depths of water.
This frequency is absorbed by the water at a slower rate than for lower frequencies, meaning the signal can travel further before becoming too weak to use.
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