Lenses & Their Uses

Converging lenses are used in magnifying glasses and in cameras - all lenses change the direction of light rays by refraction

• What are the two main types of lens? [2]

  • Converging
    • Is a converging lense concave (caves inwards) or convex (bulges outward)? [1]
      • Convex
        • What effect foes it have on parallel rays of light? [1]
          • They converge (move together) to a focus
            • What is the focal point? [1]
              • Where the rays all meet
            • What type of image does an object make if placed at exactly twice the distance of the focal point? [3]
              • Real
              • Upside down
              • Same size as the object
            • How will the image change if the object is moved further than twice the distance of the focal length? [1]
              • It gets smaller than the size of the object
            • How will the image change if the object is brought closer than twice the distance of the focal point (but not passed the focal point)? [1]
              • It gets Bigger than the object
            • How does the image change if brought even closer so that it is closer than the focal point? [3]
              • Virtual
              • Right way up
              • Bigger than the object
            • How do you draw a ray diagram for an image through a converging lens? [6]
              • 1 - Pick a point on top of the object. Draw a ray going from the object to the lens parallel to the axis of the lens
              • 2 - Draw another ray from the top of the object going right through the middle of the lens
              • 3 - The incident ray that's parallel to the axis is refracted through the focal point. Draw a refracted ray passing through the focal point.
              • 4 - The ray passing through the middle of the lens doesn't bend
              • 5 - Mark where the rays meet. That's the top of the image
              • 6 - Repeat the process for a point on the bottom of the object. When the bottom of the object is on the axis, the bottom of the image is also on the axis.
  • Diverging
    • Is a diverging lense concave (caves inwards) or convex (bulges outward)? [1]
      • Concave
        • What effect does it have on parallel rays of light? [1]
          • It causes them to diverge (spread out)
            • What is the focal point of a diverging lens? [1]
              • The point where rays appear to all come from (you can trace them back until they all appear to meet up at a point behind the lens)
            • What type of image does a diverging lens produce - no matter where the object is? [4]
              • Virtual
              • Right way up
              • Smaller than the object
              • On the same side of the lens as the object
            • How do you draw a ray diagram for an image through a diverging lens? [6]
              • 1 - Pick a point on the top of the object. Draw a ray going from the object to the lens parallel to the axis of the lens
              • 2 - Draw another ray from the top of the object going right through the middle of the lens
              • 3 - The incidence ray that's parallel to the axis is refracted so it appears to have come from the focal point. Draw a ray from the focal point. Make it dotted before it reaches the lens.
              • 4 - The ray passing through the middle o thelens doesn't bend
              • 5 - Mark where the refracted ray meet. That's the top of the image
              • 6 - Repeat the process for a point on the bottom of the object. When the bottom of the object is on the axis, the bottom of the image is also on the axis.

• What type of lenses do magnifying glasses use? [1]

  • Convex Converging Lenses - they work by creating a magnified virtual image
    • Where is the ideal location for the object being magnified to be placed? [1]
      • Closer to the lens than the focal length
    • What is the magnification formula to work out the magnification produced by a lens at a given distance? [1]
      • Magnification = Image height / Object height
        • E.g. A coin with a diammeter 14mm is palced a certain distane behind a magnifying lens. The cirtual image produced has a diammeter of 35mm. What is the magnification of the lens at this distance?
          • formula:  Magnification = image height / object height
            • substitute:  Magnification = (35 mm) / (14 mm)
              • solve:  Magnification = 2.5

• What type of lens do Cameras and Eyes use? [1]

  • Convex converging lenses - they work by flipping a real image of the object onto a screen / retina
    • The image is real, smaller than the object and inverted (upside down).  Where must the object be placed in relation to the lens? [1]
      • Further than twice the distance of the focal length

• Classwork

  • 1) Lenses can be either converging or diverging
    • Which type of lens has a virtual focus?
  • 2) In the following sentences the words in brackets have been replaced by the letters W, X, Y, Z.  Write down which words are represented by W, X, Y and Z.
    • (parallel, converging, focal point, incident)
    • "An W ray passing through the centre of a X lens from any angle carries on in the same direction."
    • "A X lens causes all W rays Y to the axis to meet at the Z."
    • "A X lens causes all W rays passing through the Z to emerge Y to the axis"
  • 3) Which of the following incident rays do not have their direction changed by either type of lens? (choose all that apply)
    • Any ray parallel to the axis
    • Any ray passing along the axis
    • Any ray passing through the centre of the lens
    • Any ray passing through the focal point
  • 4) An object has a heiht of 1 cm. It stands on the axis of a converging lens, 5 cm away from it. The focal length of the lens (distance from the lens to the focal point) is 2.5 cm.
    • What size will the image be?
    • Where will the image be formed, relative to the lens and the object?
  • 5) An aubergine is placed 6.1 cm away from a converging lens with a focal length of 7 cm.  Will the image formed by the lens be...
    • ...upright or inverted?
    • ...on the same side of the lens or on the opposite side?
    • ...real or virtual?
  • 6) The aubergine is now placed at a distance X from the lens. The image is now bigger than the object and inverted. Which of the options below could be distance X?
    • 3.9 cm
    • 7.0 cm
    • 10.2 cm
    • 14.0 cm
    • 15.3 cm
  • 7) The diagram below shows an object placed next to a diverging lens. The focal points are marked.
    • On the diagram, draw the path of a ray coming from the top of the object and travelling in the direction of the centre of the lens
    • Draw the path of a ray coming from the top of the object and going towards the focal point on the far side of the lens
    • Draw the image formed by the lens
  • 8) Magnifying lenses use convex lenses to produce images which are larger than the object.
    • What is the furthest distance the object can be from the lens when a covnex lens is used as a magnifier?
    • Will the image be a real image or a virtual one?
  • 9) The magnification of a lens system can be worked out using the heights of the object and the image
    • Write down the formula relating magnification, object height and image height
    • A 1.5 cm stamp was observed through a magnifying glass. The cirtual image it produced was 6 cm high. What was the magnification?
    • A camera was used to take the picture of a tree. if the magnification was 0.002 and the image of the tree was 2 cm high, what was the actual height of the tree?
  • 10) Lenses are used in cameras.  Choose the correct words in these statements about a camera being used to take a normal family photo.
    • The magnification is ........... (less than 1 / exactly 1 / more than 1)
    • The image is ........... (real / virtual)
    • The image is ............ (Upside down / the right way up)

• Homework

  • 1) Lenses are also used in film projectors.
    • Choose the correct words in these statements abou lenes in film projectors
      • The magnification is ........... (less than 1 / exactly 1 / more than 1)
      • The image is ........... (real / virtual)
      • The image is ............ (upside down / the right way up)
    • Which way up must the film (the object) go in order to produce the correct image on the cinema screen?
    • A film projector was moved from one cinema to another cinema where the screen was further away from the projector. How should the distance between the film and the lens be changed in order to focus the picture correctly on the cinema screen?
  • 2) Write a mini newspaper article (or youtube clip) for a technology magazine about lenses in magnifying glasses, cameras, eyes, etc.  (Include pictures, ray diagrams and the magnifying formula)