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Light - Lenses 01

This GCSE Physics quiz about light looks at lenses. You will no doubt know someone who wears lenses, or you may even wear them yourself if you own a pair of spectacles. However, lenses are used in other ways too. Lenses are a crucial part of our modern lives, they allow us to correct people's vision, view the stars and examine bacteria.

Lenses are not new, the oldest lens known dates back almost 3000 years. The first written evidence from history of the use of lenses comes from Greek and Roman times. They were used by craftspeople to carry out fine work, for checking that official seals were authentic and occasionally to help with vision. They were also used as 'burning glasses' to concentrate the Sun's heat onto wood or other inflammable items to light fires. These early lenses were made from a naturally transparent material known as rock crystal which is a type of quartz.

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Eventually, people learnt how to make good quality glass and in the 11th and 12th centuries, monks cut glass spheres in half to form plano-convex (flat on one side, curved on the other) lenses to help them with producing their beautifully-illustrated illuminated manuscripts. Some of these were as good quality as lenses made nearly 800 years later in the middle of the 20th century. Glasses were invented in the 11th century but it wasn't until the end of the sixteenth century that people started to combine lenses to make optical instruments like the microscope and telescope.

Lenses have one or two curved surfaces and work because they refract (bend) light. Convex lenses bring parallel rays of light to a focus, known as the principal focus. The distance from the lens to this focal point is called the focal length of the lens. The focal length is much longer for thinner lenses - they bend light a lot less than thicker lenses. Convex lenses magnify but concave lenses make things look smaller. A convex lens can be used to produce an upside-down image on a screen; we say this is a real inverted image. It is impossible to produce an image on a screen using a concave lens so we say they produce a virtual image.

Light that hits the surface of the lens at right angles is not bent, however, if it hits at an angle, it bends towards the normal (an imaginary reference line at a right angle to the surface). The amount of bending is predictable and depends on two things: the angle at which the light ray hits the surface (angle of incidence); and the refractive index of the glass. The refractive index is measured by measuring several angles of incidence and the corresponding angle of refraction. Dividing sin i by sin r gives a number - the refractive index. This is simply a ratio and has no units.

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  1. What is refraction?
    As light travels from one medium to another, it either speeds up or slows down. This change in speed means that any light entering at an angle will bend due to one side of the beam hitting the medium before the other
  2. How does a lens work?
    Because it has a curved surface, most of the light entering the lens hits at an angle and is therefore refracted
  3. Consider a convex lens - parallel rays of light are brought to a focus at which point?
    The principal focus or focal point is the point at which a lens focusses the light which is incident upon it
  4. What is the distance from the lens to the principal focus called?
    Less curved lenses have longer focal lengths
  5. Which formula correctly states how refractive index is related to the angles of incidence and refraction?
    It is a ratio and therefore has no units
  6. What does i represent in the above equation?
    In optics, the incoming light hitting any surface is called the incident light
  7. What does r represent in the above equation?
    In addition, on a ray diagram, the normal line is a line drawn at the point at which the light beam enters the material and is drawn perpendicular to the surface of the material
  8. The nature of an image produced by a lens is defined by which attributes?
    Make sure that you know how to describe images from lenses and other optical devices
  9. Which type of lens would be a good choice to use for the purposes of magnification?
    Concave lenses cause rays of light passing through them to diverge. The image is always virtual and diminished
  10. What is the refractive index of a medium when light is incident upon it at 45o to the normal and the angle of the refraction is 30o from the normal?
    You need to find the sine of each angle and then you can substitute the values into the refractive index equation

Author: Martin Moore

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