Aims of this Episode:

  • Describe Reflection;
  • Describe Refraction;
  • Describe Diffraction;
  • Identify different types of lens;
  • Explain how lenses focus light in the eye and the camera.

Reflection, Refraction & Diffraction

Reflection occurs when a wave bounces off a surface – echoes are examples of reflection.

Refraction is the process whereby a wave changes its speed and therefore its direction when it goes from one medium (substance) into another medium (of a different density).

Diffraction is when waves spread out as they pass through a gap. The width of the gap needs to be similar to the wavelength of the wave for this to happen.


Convex lenses have a cross-section that looks like an oval. These lenses cause the light to converge onto a particular point.

Concave lenses cause light to diverge from a particular point. These have a cross-section that looks a little bit like two caves back-to-back.

Inside your eye and a camera there is a convex lens, which focuses the light onto your retina or the film. In your eye the lens can change shape to focus on things at different distances, whereas in the camera the lens moves backwards and forwards to achieve this focusing.

S@S 016: Reflection, Refraction, Diffraction and Lenses
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