CHAPTER 19 Lenses

• Describe the action of lenses of different shapes.
• Define the 'focal length' and the 'power' of a lens.
• Explain how the focal length of a convex lens is determined.
• Know uses of convex and concave lens.
• Solve problems involving lenses. Use the lens equation to find the position of images.

SCIENTIFIC PROCESSES

• Draw diagrams showing the effects of convex and concave lenses on a beam of parallel rays.
• Draw ray diagrams to show images formation by convex and concave lenses.

COMPLEX REASONING PROCESSES

• Solve complex problems on lenses and lens combinations.

CHAPTER 19 SUMMARY

• Lenses come in many shapes and sizes but the most commonly used are convex (converging) or concave (diverging) lenses.
• Lenses refract light towards the thickest part of the lens.
• Convex lenses converge light rays which are parallel to the principal axis to the focal point.
• Concave lenses diverge light rays which are parallel to the principal axis so as they appear to come from the principal focal on the same side of the lens as the object.
• When drawing ray diagrams the three rays that can be used are:
  (a) The ray parallel to the principal axis refracts through the principal focus for a convex lens, and appears to come from the focus for the concave lens.
  (b) A ray through the focus refracts parallel to the principal axis for the convex lens. For a concave lens the ray that is lined up with the focus on the opposite side refracts parallel to the principal axis.
  (c) The ray through the centre of the lens continues unchanged in direction.
• Images in lenses can be described using terms: real or virtual, upright or inverted, and smaller or larger.
• Convex lenses produce real, inverted images when the object is outside the principal focus, and virtual, upright images when the object is inside the principal focus.
• Concave lenses always produce virtual, upright and smaller images.
• The lens formula used for both convex and concave lenses is:
  1/f = 1/v + 1/u Where: u is the object distance from the centre of the lens,
  v is the image distance from the centre of the lens, and f is the focal length.
• The magnification formula used with lenses is:
  M = v/u = H_i /H_o
  Where: H_o is the object height,and H_i is the image height.
• The focal length of concave lenses is negative and virtual image distances are negative.
• Spherical aberration is the inability of a convex lens to refract rays of light to a precise point. It can be overcome by only using the middle section of a lens.
• Chromatic aberration is the production of a coloured haze around images due to the fact that all colours of light are not refracted the same amount. It can be overcome by using an achromatic lens.
• The focal length of convex lenses can be found by focusing parallel light rays either from a light box or from a distant object onto a screen.
• Lenses have many uses; being part of the construction of the eye, spectacles, telescopes, microscopes, overhead projectors, and cameras

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