Total Internal Reflection The Endoscope

You will need… A semicircular block A power pack A ray box A single slit A protractor A ruler A pencil

Total internal reflection Draw on your normal line half way up the block at 90 o Shine an incident ray into P P You should see a refracted ray i r You should also see a partially reflected ray The angle of reflection should be equal to the angle of incidence

Gradually increase the angle of incidence. The angle of refraction will increase until the refracted ray emerges along the boundary We call the angle of incidence at this position the critical angle We label the critical angle, c c

Remember the law of refraction when the light ray is leaving the glass block The sine of the angle in air = n x the sine of the angle in the block

c The sine of the angle in air = n x the sine of the angle in the block In this case the angle in the air is 90 o r The angle in the block is c sin 90 o = n x sin c Because sin 90 o = 1, the equation becomes: 1 = n x sin c How can this equation be rearranged? n = 1 sin c or sin c = 1 n

What is the refractive index for your block?

Calculate the critical angle for glass with a refractive index of 1.59 sin c = 1/1.59 sin c = c = 39.0 o sin c = 1 n

Total internal reflection If the angle of incidence is increased beyond the critical angle, the light ray is totally internally reflected The angle of reflection, r is equal to the angle of incidence, i i r

Total Internal Reflection allows light to travel around corners Normal i r

The Endoscope

Two bundles of optical fibres next to each other One bundle shines light into the cavity The second bundle of optical fibres has a lens over the end of the fibres allowing an image to be formed using a digital camera

Endoscope laser surgery A laser on the endoscope can carry out surgical procedures It can be used to cauterise blood vessels The colour of the laser can be matched to the tissue to ensure maximum absorption Retinal eye surgery can be carried out by laser by shining a laser through the pupil for a very short time