GCSE Physics (AQA)

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Principal & Virtual Focus

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Principal & Virtual Focus

6:13 Lenses and Ray Diagrams
Spec 4.6.2.5
Previous Colour Filters Next Converging Ray Diagrams
  • Light entering a lens at a specific angle does not bend.
  • Lenses use refraction to change light paths and form images.
  • Convex lenses cause light rays to converge; concave lenses cause them to diverge.
  • Rays entering and exiting a lens along their normals do not refract.
  • The principal axis of a lens is where light rays do not change path.
  • Convex lenses converge parallel light rays to a point called the principal focus.
  • The principal focus is where rays parallel to the principal axis converge after refraction.
  • Concave lenses have a virtual focus on the same side as incoming rays.
  • Virtual focus is where rays appear to converge after refraction.
  • Focal length is the distance between the lens center and the principal or virtual focus.
  • Focal length is determined by the lens material and curvature.
  • Lenses are symmetrical, having focuses on both sides.
  • Convex lenses have two principal focuses; concave lenses have two virtual focuses.
  • Understanding focus and focal length is important for exams.

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