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Light and Sight

Without light it would be impossible for us to see anything, so it seems important to find out why it is necessary, and what happens when things go wrong?

Vision basics:

We can see two types of object. If the light has been reflected off the object and into our eyes, these are called illuminated objects, whereas if the object produces this light itself, it’s a luminous object.

The pupil controls how much light enters the eye, which depends on how far away the object being viewed is. The cornea (the front lens of the eye) and the lens inside the eye then focus this light onto the retina. The cornea focuses 60-70% of the light whereas the lens inside only focuses 30-40% which comes to a focal point on the retina. The retina is made up of rods and cones which send information through the optic nerve to the brain which processes this image into an image.

The brain can do this in about 13 milliseconds and this process happens constantly throughout the day.

eye_cross_section

Colour coding:

What we call “light” is actually part of a spectrum of wavelengths. Creatively named the ‘Visible Light Spectrum’, these wavelengths are actually the colour spectrum, with red being the most high frequency and violet being the lowest. White light is a combination of all the colours and black is the absence of them.

The reason we can see different colours is because of the cones in the retina are sensitive to red, green and blue light but, because of the variations in wavelengths, they can detect colours such as orange and yellow as well. When light hits these cones, a chemical reaction occurs which triggers an electrical impulse through the nerves to the brain which translates into the colour you see.

wavelength-nm

When things go wrong: Why some people need glasses:

Very few of us have exactly perfect vision. Perfect vision is called 20/20 vision because when you are having an eye test, you are able to read the line labelled 20 on the letter board from 20 feet away. But what is the cause of bad vision? There are four main categories:

Near-sighted people find it difficult to see objects far away from them. This is because the eye is longer than normal and so the focus point falls in front of the retina, rather than on it. How far away the focus point is from the retina determines how bad the eye sight is.

Far-sighted people have the opposite problem. They have trouble seeing things close to them because their eyes are smaller than normal, meaning the light is focused behind the retina.

Astigmatism is where the cornea is odd-shaped, meaning the light is focussed in two places on the retina. Whereas normal cornea’s are spherical, irregular ones are more rugby-ball shaped.

Presbyopia affects nearly everyone at some point in their life, usually when they are over 40. This makes things difficult to see when close-up and is caused by the natural lens losing elasticity, as this is necessary to change focus between objects close up and those far away.

home_glasses

These conditions are only aggravated when the lighting is not correct. As people grow older they need more light in order to see as the muscles that control pupil size weaken and cannot adjust to changing light as easily. This is why brightness is highly important when considering new light bulbs and why dimmable lighting may be a good option for home or office as it means you can adjust the lighting to the people in the room.

Further Resources:

More explanation about how light and sight relate to each other here: http://www.physicsclassroom.com/class/refln/Lesson-1/The-Role-of-Light-to-Sight

More about lights and colours here: http://sciencelearn.org.nz/Contexts/Light-and-Sight/Science-Ideas-and-Concepts/Colours-of-light

Find out about, and see if you qualify for, a NHS free eye-tests here: http://www.specsavers.co.uk/glasses/nhs-eye-test

Abigail Houseman

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Published 2015/07/20