• Melissa Hay

Five Fascinating Facts About Your Eyes

How do we see colour?

The human eye sees basically three colours: red, green and blue as we have specific retinal cells recognizing these colours. These are pretty basic colours, but can be mixed together to create other hues. We see white as a combination of the three, the black as the negative of these three. Yellow, purple or mauve form through the combination of two basic colours; these are called secondary colours.

Why do we blink?

The front surface of the eyeball and the inner eyelids are covered by a transparent layer called conjunctiva.  This layer protects the eye against the environment, like our skin does to our body.  While blinking, the conjunctiva is moisturised and cleaned of dirt and dust by the tears.  We blink once or twice every 10 seconds, and a blink lasts 0.3 seconds.  So that’s a lot of blinks each day! Infants start blinking at the age of 6 months.  Tears drain into the nasal cavity and are more abundant if there is dust or dirt, you know that feeling of irritation when you have something in your eye – you eyes over-compensate and produce lots of tears to try and flush it away.  We also blink in bright lights to protect our eye from the damaging light rays.

What is the macula?

We hear about the macula, as it is common in older people to develop macula degeneration.  It always breaks our hearts when an older person starts struggling to read or do their hobbies.  The macula is a region on the retina where we have a dense concentration of cells to help us see.  This central spot on the retina gives us the fine focus vision, which allows us to see in detail.  That's why to clearly see an object, we have to move the eyes, so that the projection is formed on macula.  Macula degeneration is where the cells in this area start to break down or die, limiting our clarity and vision.

What is tridimensional vision?

Our nose is between our eyes for a reason, it allows us to get visual information from two slightly different angles.  Because the eyes watch the environment from separate angles, they send different information to the brain. The brain "learns" from our first days to assemble the two images, so that we do not see a double image. But the difference between the two images helps the brain detect the location of the objects in the space and distance. This is the tridimensional vision, provided by binocular sight.  Tridimensional vision is commonly referred to as 3-D vision.  It is something predatory animals need for moving on the branches or hunting.  Prey animals usually have lateral positioned eyes. Their visual fields do not interpose, so this we call monocular vision. They cannot assess distances well, but they have larger visual fields, overlooking the environment for predators.

Why is vision blurry underwater?

This has to do with the refraction indexes. Air has the refraction index of 1, the cornea and water about 1.33.  The cornea is the clear window on the front of our eyes  The human eye cannot focus properly the light when experiencing the refraction index of the water, as we have been designed to focus in the open air.  That's why we see underwater things as being blurry.  Using goggles or a snorkel mask can mitigate this as it gives us enough air in front of our eyes to focus correctly.

Melissa Hay, Visique Optometrists, 155 Kitchener Road, Milford, 09 489 4797, milford@visique.co.nz