2.  MRI is non-invasive, but it is expensive.  We can use different non-invasive techniques instead.  Non-invasive techniques do not involve ionising radiation.  Methods we will look at:  Endoscopy  Ultrasound

3.  These use optical fibres  These can be inserted into body openings.  Light is passed down one set of optic fibres.  These are arranged so that they have the exact same arrangement in the bundle at the bottom as at the top.  This allows the doctor to see what is being examined through an eye piece.

4.  These are waves that are above the audible sound frequency range (20 Hz to 20,000 Hz)  In medical ultrasounds the frequencies are in the megahertz range.  These cause no ionisation.  Ultrasound can show both muscle and blood.  A low frequency is used as often as possible as high frequency ultrasounds can be destructive to tissues.  Will be examined in greater detail later on

6.  If you move towards a stationary source of sound you will not hear the same frequency as when you were stationary.  Extra waves will have passed into your ears.  This change in frequency due to the persons movement is called the doppler effect.

7.  Stationary source  200 Hz  c = 340 m/s  Wavefronts = 1.7 m apart  If person is stationary, wavefronts go past at 200/s  Imagine you are travelling towards the source at 30 m/s.  You will have passed an extra (30/1.7) = (17.6 wavefronts.  Altogether, 217.6 wavefronts will have passed.  This means the frequency will be 217.6 Hz. You will hear a higher pitch.

8.  When the source of the sound is moving, it creates a different effect.  Wavelengths behind the source are larger.  Wavelengths in from are shorter.  This is the principle of ultrasound.

13.  Can you think of 2 examples where the wave source is travelling faster than the wave itself?

14.  Calculate the frequency heard when a train, travelling towards you with a speed of 60 m/s, sounds its whistle of frequency 400 Hz.  Velocity of sound is 340 m/s.