Questions on the Physics of the Optics of the Eye 👀Watch
Question 1:The speed of ultrasound in the lens is 1650 ms^-1 and 1520 ms^-1 in the vitreous and aqueous humour, calculate:
a. The thickness of the lens.
Given that distance = speed *time
Convert speed from us to s:
4us=4*10^-6s or 0.000004 s
Therefore, distance = 1650 ms^-1 * 4*10^-6 s
Thickness of the lens = 6.6*10^-3m or 0.0066 m
b. The distance from the back of the lens to the retina.
Time = 35-20=15 us
15us= 1.5*10^-5 s or 0.000015 s
Distance =speed *time
Distance = 1520ms^-1 * 1.5*10^-5 s
Therefore, the distance from the back of the lens to the retina = 2.28 *10^-2 m or 0.0228 m
Question 2: Detailed ultrasound scans called B-scans are used to construct comprehensive images of the entire eye. B-scans are also used to form images of foetuses, as X-rays can be harmful. Explain why B-scans not used for all body imaging?
Ultrasonography is not preferable for imaging bones or any tissues that contain air, like the lungs. The amplitude and intensity of ultrasound waves decrease as they travel through tissue, a phenomenon known as attenuation. Given a fixed distance, attenuation affects high frequency ultrasound waves to a greater degree than lower frequency waves.
Therefore, the attenuation is too high when imaging mature bones meaning the waves are reflected back producing a distorted image.
When assessing a fetus, bones are not as dense as in an adult so the reflection is very low and the image has a higher resolution. Consequently, ultrasound imaging is not as detailed as X-rays, CT or MRI scans as it also is limited to parts of the body as the sound waves cannot travel through air (such as in the lungs) and are blocked by bone.
Might be worth mentioning wavelength as a limit on the minimum feature size that it's possible to image - e.g. I don't think a bone fracture would show up on ultrasound.