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Grades 8–10 Advanced About 25 min Version 1 Awaiting review Premium

Lenses on the optical bench

A convex (converging) lens bends parallel rays inwards so they meet at its principal focus F; the distance from the lens to F is its focal length f. A concave (diverging) lens spreads parallel rays out as if they came from a focus on the same side. To find an image, draw a ray parallel to the axis that then passes through F, and a ray through the centre of the lens that does not bend. Where they meet is the image. The lens formula 1/f = 1/u + 1/v links the object distance u and image distance v (real-is-positive). When the object is at 2F, the image is also at 2F, the same size, so f = u ÷ 2.

What you will learn

  • Draw ray diagrams for convex and concave lenses.
  • Describe how the image changes as the object moves towards a convex lens.
  • Find the focal length of a convex lens with an optical bench.

Before you start

  • Know that light refracts when it passes through glass.

Materials

  • A lit object (an arrow cut in card in front of a bulb)

Equipment

  • Optical bench with convex lenses (f = 10 cm and 15 cm), a concave lens (f = −10 cm) and a screen

Procedure

  1. With the 10 cm convex lens, place the object 50 cm away and record the image.
  2. Move the object to 20 cm (2F), then 15 cm, recording each image.
  3. Move the object to 5 cm, inside F, and look through the lens.
  4. Change to the concave lens and record an image.

How this simulation is simplified

  • Thin lenses; distances are measured from the centre of the lens. Real distances are positive, virtual ones negative.
  • The object is 3 cm tall.