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Light - The Eye

This GCSE Physics quiz about light will test your knowledge of the eye. The eye gives us one of our main senses - sight. The eye can be thought of in some respects as a lens and a camera - the only difference being that we can't download our images to our computers...yet! There are several key parts of the eye which are essential learning for the GCSE syllabus, and knowing them could get you a lot of easy marks.

The inside of the back of the eyeball contains the retina. This is the equivalent of the CCD (or film) in a camera as it is where the image is formed. This contains sensors that transfer light energy into electrical energy that is transmitted to the brain. The brain interprets these signals, creating the images of the world around you.

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The front of the eyeball contains the lens. This is a flexible sac filled with a jelly-like substance. The walls of the sac are flexible and bulge outwards at the centre so it is a bi-convex lens. This focuses the light entering the eye onto the retina. If the eyeball is too long or too short for the power of the lens, short sightedness or long sightedness is the result. If the lens cannot focus light on the retina, or is too fat or too thin, that can also cause short or long sight. These two common eye defects are easily corrected by using glasses which hold lenses of the correct strength in the correct position in front of the eyes. The lens of the eye is held in place by the suspensory ligament and its shape is controlled by the ciliary muscle. The lens becomes less flexible with age.

Covering the lens and protecting it is the transparent membrane that we call the cornea. This is also involved in focusing the light entering the eye but does not change shape. It is a meniscus lens.

The iris is the equivalent of the diaphragm of a camera lens, it controls the amount of light that enters the eye. It is the coloured part of the eye. It is situated between the cornea and lens, close to the lens. The hole through the middle of the iris is called the pupil. The size of the pupil is determined by the iris. When the light is bright, the muscles of the iris relax, reducing the size of the pupil. When the pupil is very small, it protects the retina from being damaged by high light intensities. In low light, the muscles of the iris contract so the pupil becomes dilated, allowing more light into the eye when it gets dark.

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  1. What is/are a cause(s) of long sightedness?
    If the lens is too thin for the length of the eyeball, light entering the eye will be focused behind the retina
  2. What is the formula for the power of a lens?
    P is power and is measured in dioptres, D. The focal length, f, is measured in metres, m
  3. What is the power of a lens whose focal length is 0.1 m?
    D stands for dioptre
  4. The focal length of a lens is determined by what?
    Materials with higher refractive indices and lenses with greater curvature bend light the most
  5. A CCD in a digital camera is the equivalent of what in the eye?
    CCD stands for charge coupled device and is the electronic equivalent of a film
  6. A manufacturer wants to make a flatter (less curved) lens for a given focal length. How would the manufacturer make this?
    Normally, a flatter lens will focus light further away, however, if the material has a higher refractive index, it will bend the light through a greater angle. The new flatter lens therefore can be made to focus light at the same point as the original, more curved lens
  7. What is/are a cause(s) of short sightedness?
    If the lens is too fat, it will bend the light too much and the focal point will be in front of the retina
  8. Which answer is not a part of the structure of the eye?
    The malleus is part of the ear. The structure of the eye is made up of the retina, lens, iris, cornea, pupil, ciliary muscle and suspensory ligaments
  9. What types of lenses are used to correct long and short sightedness?
    These are designed to refract the light just enough to enable the lens to focus it directly onto the retina
  10. The eye can focus on objects ranging between the near point and the far point. In humans with perfect vision, what is approximately the distance of the near point and the far point?
    You can test the near focal length yourself by bringing your finger close to your eye. You will notice that as you bring your finger closer than around 25 cm, your finger will start to become slightly blurred

Author: Martin Moore

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