2.2: Refraction, Snell's law.
- Page ID
- 128447
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Slower wave speed implies shorter wavelength. From this and the geometry of wave fronts and rays, it follows that rays are refracted at interfaces, and that this is given by Snell’s law of refraction: , where n, the refractive index, is the ratio of c to the speed of light in the medium. n varies with wavelength and causes dispersion, as we’ll see soon. The reflected ray becomes stronger with increasing angle of incidence, and the refracted ray weaker. Refraction explains lenses, as we’ll see later, mirages and much else.
- Refraction follows Snell’s law.
- Refraction depends on wavelength, hence dispersion.
- Refraction explains lenses, mirages and much else.
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Links to related material |
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| Snells law and refraction Refraction at a plane interface. Derivation of Snell's law. Experimental determination of refractive index. Examples. |
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| Newton's prisms Dispersing white light into component colours, then recombinging them. |
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| Mirages and the Green Flash Refraction from air with a thermally induced density gradient. Inferior mirages. 'Floating islands. The green flash. |
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| Dispersion and chromatic aberration The refractive index varies with wavelength, therefore so does the focal length of a simple lens, giving chromatic dispersion in the image. |
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| Total internal reflection No refraction if the angle of incidence exceeds the critical angle. |
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| Optical fibres and cladding Total internal reflection in an optical fiber. Adding cladding reduces pulse broadening. |
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| Chromatic dispersion, rainbows and Alexander's dark band Dispersion on refraction produces rainbows. One internal reflection gives the primary (40°) rainbow, two gives the secondary (50°) bow. |
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| Colour of the sky Scattering varies strongly with wavelength. Scattered light gives the blue of the sky, and the unscattered light gives the red or orange of a sunset. |
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| Mirrors and images Plane mirrors and virtual images. Inverted images. Real images in a concave mirror. Parabolic reflectors. Fermat's principle. Focusing with mirrors. Aberration. The mirror equation. Convex mirrors. Distortion. |
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| Lenses and images Focussing and forming images with lenses. How to draw ray diagrams. Relating object and image distance to focal length (theory and experiment). |
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| Microscopes and magnifiers The magnifier or simple microscope. The compound microscope and ray diagrams. |
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| Reflecting Newtonian telescope Parabolic reflector and the Newtonian or reflecting telescope. |
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| Refracting telescope Refracting telescope: two converging lenses produces a virtual, inverted image. |
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| Acoustic telescope Parabolic reflector for sound. Compare with Newtonian telescope. Wavelength limitations. |
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Geometrical Optics Experiments |
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