Light and the eye
The human eye and the colourful world
Name the parts of the eye and what each does, tell myopia from hypermetropia and pick the lens that corrects each, and explain dispersion, twinkling and the blue of the sky by refraction and scattering.
The eye works rather like a camera: a lens system throwing an image onto a light-sensitive screen. What it has that a camera does not is a lens that changes shape. Ciliary muscles squeeze the jelly-like lens thicker to look at something close and let it thin out again for something far away, and the focal length changes with it. That trick is called accommodation, and most of the chapter's defects of vision are that trick going wrong.
Where the image lands
In a myopic eye, the image of a distant object falls short and lands in front of the retina; a concave lens spreads the rays a little first, and the image drops back onto the retina. In a hypermetropic eye, light from a close object is heading for a point behind the retina; a convex lens brings it forward. Read the defect off where the image falls, and the lens follows.
Worked example
Why does a star twinkle while a planet does not?
Starlight is refracted continuously on its way down through the atmosphere, whose refractive index changes gradually with height.
So the star's apparent position is not quite its real one — and near the horizon it appears higher than it is.
Try it together
Two people come to an optician. One cannot read the board from the back of the class; the other cannot read a book held at a comfortable distance.
Work out what each has and what will correct it.
1.The first sees nearby things clearly but not distant ones. Which defect of vision is that?
Have a go
Have a go on your own. An older person finds that the near point has crept away from them over the years, so close work has become difficult. What is this defect called?
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.