Rules of refraction for converging (convex) lenses (a) Rays parallel to the principal axis are refracted through the focus.1 (b) Rays passing through the focus are refracted parallel to the principal axis. (c) Rays passing through the optic centre are undeviated.
2F
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C (a)
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C (b)
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C (c) Fig 13.16: Rules of refraction for converging lenses Rules of refraction for diverging (concave) lenses
(a) Rays parallel to the principal axis are refracted so as to appear to be coming from the focus.
(b) Rays passing through the optic centre are undeviated.
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C (a) Fig 13.17: Rules of refraction for diverging lenses Finding the Position and Nature
of Image by ‘Ray-tracing’ The position and nature of an image in a spherical lens can be found by ‘ray-tracing’. This means drawing rays of light from the object to the lens and applying the rules outlined above.
1In reality there would be refraction twice, once as the light enters and again as it leaves the lens, as shown in Fig. 13.18. It is acceptable to draw the ray of light bending once as it passes through the midline of the lens: as this is easier to draw, this is the approach taken in this book.
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C (b)
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2F Fig 13.18: Ray diagram showing double refraction of light, as it enters and leaves the lens REFRACTION OF LIGHT 221
