Light and Sound
4 marks · EasyOptical fibres use total internal reflection to transmit light signals, and sound waves can also be used in medicine.
Explain how total internal reflection allows light to travel through an optical fibre without escaping.
Doctors use optical fibres in endoscopes to see inside the body. State one reason why this is better than traditional surgery. Give one way sound is used in medicine.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Light hits the boundary inside the fibre at an angle greater than the critical angle, so it is totally internally reflected and stays inside the fibre. · must show Comparison to critical angle, Light does not escape; remains trapped
(b) Keyhole (less invasive) surgery is less stressful for patients or helps quicker recovery. Sound: ultrasound scans (imaging), or using sound to break up kidney stones. · must show Less invasive operations or quick recovery, Any valid medical use of sound waves
Also accept: Any other medical use of sound
Common mistakes: Not describing total internal reflection; Mixing up endoscope with ultrasound
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States total internal reflection occurs at the boundary inside the fibre.
M2 (a) · 1 mark · Notes angle is greater than the critical angle or mentions trapping of light.
B1 (b) · 1 mark · States a correct reason why endoscope is better (e.g. less invasive, quick recovery, less scarring).
B2 (b) · 1 mark · States one valid medical use of sound (e.g., ultrasound scan, breaking kidney stones).
Worked solution
a) In an optical fibre, the core has a higher refractive index than the outside. Light entering the fibre strikes the edge at an angle greater than the critical angle, resulting in total internal reflection. The light is reflected many times along the fibre without escaping.
b) Using endoscopes (with optical fibres) for keyhole surgery causes smaller wounds, reducing stress for patients and allowing quicker recovery. Sound waves are used for ultrasound scans to produce images inside the body, or to break up kidney stones.