Waves at interfaces and pulse-echoEdexcel A-Level Physics: Flashcards

What these 14 flashcards ask

What happens to a wave at an interface between two media?
What stays the same when a wave passes into a new medium?
What is ultrasound?
Why is gel applied in an ultrasound scan?
State the pulse-echo depth equation.
Why is there a factor of 2 in d = vt/2?
What gives a stronger echo at a boundary?
Why must pulses be spaced far apart in time?
What limits the smallest detail that can be seen?
How is the length of a pulse in a medium found?
When are echoes from two boundaries separate?
How can resolution along the beam be improved?
What is the disadvantage of using a higher frequency?
What is the wavelength of 5.0 MHz ultrasound in tissue (v = 1540 m s⁻¹)?

Exam questions on Waves at interfaces and pulse-echo

A sonographer is carrying out an ultrasound scan of a patient. She applies a layer of gel to the skin before pressing the transducer against it. The transducer emits ultrasound of frequency 3.5 MHz, which travels through soft tissue at 1540 m s⁻¹.
Calculate the wavelength of the ultrasound in soft tissue.2 marks
In a pulse-echo scan, a transducer sends a short pulse of ultrasound into soft tissue, where the speed of ultrasound is 1540 m s⁻¹. An echo from the boundary of an organ is detected 78 μs after the pulse was transmitted.
The transducer sends pulses at regular intervals. Explain why the interval between pulses must be longer than the time for the echo from the deepest boundary to return.2 marks
A pulse-echo system uses ultrasound of frequency 5.0 MHz. Each pulse lasts 0.40 μs. The speed of ultrasound in the tissue being scanned is 1540 m s⁻¹.
Calculate the wavelength of the ultrasound and the length of one pulse in the tissue.3 marks

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).