Uses of Lenses (Cambridge O Level Physics)

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Magnifying Glasses

  • If the object is placed closer to the lens than the focal length, the emerging rays diverge and a real image is no longer formed
  • When viewed from the right-hand side of the lens, the emerging rays appear to come from a point on the left
    • This point can be found by extending the rays backwards (creating virtual rays)
  • A virtual image will be seen at the point where these virtual rays cross

A Lens Close to an Object Forms a Magnified Image

lens-virtual, IGCSE & GCSE Physics revision notes

A virtual image is formed by the divergence of rays from a point

  • In this case the image is:
    • Virtual
    • Enlarged
    • Upright

  • Using a lens in this way allows it to be used as a magnifying glass
  • When using a magnifying glass, the lens should always be held close to the object

Correcting Sight

  • Converging and diverging lenses are commonly used in glasses to correct defects of sight
    • Converging lenses can be used to correct long-sighted vision
    • Diverging lenses can be used to correct short-sighted vision

Correcting Short-Sightedness

  • People who are short-sighted have eyes that are 'too large'
    • This means they cannot see things that are far away, and only see things that are close to them
  • This is because the eye refracts the light and brings it to a focus before it reaches the retina
    • In other words, the focus point is in front of the retina at the back of the eye
  • This can be corrected by using a concave or a diverging lensĀ in front of the eye
    • This causes the light to focus further back, at the retina

Correcting Long-Sightedness

  • People who are long-sighted have eyes that are 'too small'
    • This means they cannot clearly see things that are close, and can only clearly see things that are far away
  • This is because the eye refracts the light rays and they are brought to a focus beyond the retina
    • In other words, the focus point is behind the retina at the back of the eye
  • This can be corrected by using a convex or converging lens in front of the eye
    • This causes the light to focus further forward, at the retina

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Dan MG

Author: Dan MG

Expertise: Physics

Dan graduated with a First-class Masters degree in Physics at Durham University, specialising in cell membrane biophysics. After being awarded an Institute of Physics Teacher Training Scholarship, Dan taught physics in secondary schools in the North of England before moving to SME. Here, he carries on his passion for writing enjoyable physics questions and helping young people to love physics.