Light - Lenses 02

This GCSE Physics quiz on light takes a second look at lenses. When the word lens was first used, it was defined as 'a piece of glass to regulate light rays'. The word comes from the Latin name for the lentil plant as the lentils themselves have a similar shape to the bi-convex pieces of glass that were used at the time for magnifying things. Since that time (the end of the 17th century), many different forms of lens have been devised. They are also made from many different substances, including plastic and there are even gravitational lenses. These are objects in space that have such a large gravitational attraction that they can bend light.

The earliest lenses were convex, so therefore they are classed as being converging lenses - lenses that bring rays of light to a place called the focal point.

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At the focal point, they create an image that can be seen on a screen. This made them useful as they acted as a magnifying glass, but they could also be used to focus the Sun's heat on a small area. The heat was concentrated and they could be used to start fires. A lens that is flat on one side has the prefix plano- so a convex lens that is flat on one side is called a plano-convex lens. Lenses with concave surfaces cause light rays to diverge (spread apart) and are therefore referred to as diverging lenses.

Some lenses are more powerful than others, more curvature means that they will bend light through a larger angle and the focal length will be shorter. The power of a lens is measured in dioptres. A more powerful lens will bring light to a focus closer than a less powerful lens so the dioptre is defined as the reciprocal of the focal length. Theoretically a dioptre has the SI units of m-1 but it is not an officially recognised SI unit and can be written just as the number, without units. It is of particular use to opticians as it is a lot easier to specify a lens of a certain power to correct sight defects.

Lenses refract different wavelengths of light through different angles so white light that passes through a lens is split into its constituent colours. This is called chromatic aberration and was a big problem to makers of early optical instruments. Chromatic aberration makes an image less sharp and you can see a coloured 'halo' around the edges. When you are looking at tiny objects or distant objects like stars and planets, it is unhelpful as they appear to be slightly fuzzy. It was eventually discovered that different types of glass had different chromatic aberration and that putting two types of glass together to form a compound lens solved the problem. A lens system that does not suffer from chromatic aberration is described as being achromatic.

The eye is an optical instrument and often crops up in questions about optics in physics GCSE papers. You need therefore to be familiar with the main parts of the eye and their functions. The part of the eye that focusses the light is the lens. The shape of the lens is controlled by the ciliary muscles. To view distant objects, the shape of the lens needs to be changed to be thinner and vice-versa. The light rays are focussed onto the retina. The changes in the lens that allow it to be focused on near and far objects is called accommodation.

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  1. What is the power of lens which has a focal point of 20 cm?
    Remember that focal length needs to be changed into metres
  2. What part of the eye focuses the light?
    The iris limits the amount of light entering the eye, the suspensory ligament supports the eye and the retina is the 'screen'
  3. The power of a lens (SI system) can be calculated using which of the following formulae?
    Remember to convert focal length to metres
  4. What is chromatic aberration?
    It decreases the quality of the image
  5. A lens which focuses light to a point is called a what?
    Converging means coming together
  6. What is the power of a lens dependent on?
    The focal length depends on the curvature of the lens and the material from which it is made
  7. What is the focal length of a lens which has a power of 0.5?
    This needs you to rearrange the lens power equation
  8. What is the unit of power for a lens?
    The higher the number of dioptres, the more a lens will bend light rays
  9. Calculate the power of a converging lens of focal length 0.05 m.
    The power for a converging lens is given a positive value and for a diverging lens it is given a negative value. A capital letter D is often used to signify dioptres but is not an official SI unit symbol
  10. In the eye, which of the following parts does light NOT travel through?
    The iris is the coloured part of the eye which controls the size of the pupil. So allowing more or less light to enter the eye depending on the conditions

Author: Martin Moore

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