Science & Technology in Action

5th Edition

Improving Human Vision

Health Products Regulatory Authority

The light-focusing structures in the eye are the cornea and the lens. This lesson describes how some eye defects can be corrected by using spectacles or contact lenses.

Contains the full lesson along with a supporting toolkit, including teachers’ notes.

Lesson excerpt

What is a Medical Device?
Any product, except medicines, used in healthcare for the diagnosis, prevention, monitoring or treatment of an illness or disability is called a medical device. There are many medical devices on the market including heart valves, hospital beds, pacemakers, resuscitators, surgical instruments, syringes and wheelchairs. It is very important that medical devices are safe and effective for people to use. In Ireland, the Irish Medicines Board (IMB) oversees and regulates the safety, quality and efficacy of all medicines and medical devices. Many people are surprised to learn that contact lenses are classified as medical devices. In this lesson we examine the operation of the human eye and function of contact lenses in correcting defective vision.

The Eye is an Optical Marvel
The resolution of a digital camera is often expressed as the number of picture elements, or pixels, it contains. For example an array with 3,000 rows of 4,000 individual sensors could be described as a 12 megapixel sensor. The human eye is much better, having typically 100 million light sensitive cells. These are of two types, called rods and cones, and are densely packed on the retina, at about 40,000 per square millimetre near the periphery and about 160,000 per square millimetre at the fovea. The rod cells are more numerous and more sensitive but are not colour sensitive. At the fovea there are no rod cells but only cone cells packed tightly. The part of the image that is projected onto thefovea is seen most clearly. For example, when reading, our eyes must follow the text because only a few letters at a time can be focussed onto the fovea.

Many cameras have autofocus capability; the distance between the lens and the sensor array is adjusted automatically so that the subject of interest is sharply focussed. The human eye can also focus automatically but in a slightly different way. The shape of the lens in the eye is adjusted by the ciliary muscles, thereby changing its focal length. This happens so quickly that we are generally unaware of it.

Quiz questions

  1. The lens does all the focusing of light onto the retina. false
  2. The iris controls the size of the pupil. true
  3. The fovea contains only rod cells. false
  4. The ‘blind spot’ is where the optic nerve passes through the retina. true
  5. The aqueous humour fills the back of the eye behind the lens. false
  6. The cones of the retina do not operate in dim light. true
  7. The rods are responsible for colour vision. false
  8. The overlapping visual fields of our eyes give us stereoscopic vision. true
  9. Big pupils make you appear more attractive than small pupils. true
  10. A short sighted person cannot see close objects clearly. false