IGCSE 0625 · LIGHT · 3.2

Light

Reflection, refraction, dispersion, and lenses in one view, with the key relationships and the Extended ideas of total internal reflection and optical fibres.

TOPIC 3.2: LIGHT CORE CORE EXTENDED EXTENDED LIGHT & OPTICS REFLECTION The angle of incidence equals the angle of reflection. A plane mirror gives a virtual, upright image, same size, as far behind as the object is in front, and laterally inverted (left-right swapped). i r normal REFRACTION Light bends when its speed changes at a boundary. Entering a denser medium it slows and bends toward the normal; leaving, it bends away. Frequency stays the same; wavelength changes. glass (denser) i r DISPERSION OF WHITE LIGHT A prism splits white light into a spectrum. Each colour refracts by a different amount. Red bends least, violet bends most. Order: red, orange, yellow, green, blue, violet. white red violet KEY RELATIONSHIPS n = sin i / sin r n = refractive index n = 1 / sin c c = critical angle Refractive index also links to speed: n = c / v (Extended) LENSES A converging (convex) lens bends parallel rays to a principal focus F. Focal length f = lens to F. 2F F F 2F object real image An object beyond 2F gives a smaller, inverted, real image. Inside F the lens acts as a magnifying glass (virtual). TOTAL INTERNAL REFLECTION EXTENDED Inside a denser medium, if the angle of incidence exceeds the critical angle c, all light reflects. Needs light going from dense to less dense. At exactly c the ray skims along the boundary. Refractive index and c are linked by n = 1 / sin c. dense medium below the boundary i > c OPTICAL FIBRES EXTENDED Light is guided along a thin glass fibre by repeated total internal reflection, even around bends. Telephone and internet data over long distances with little signal loss. Medical endoscopes to see inside the body.
TOPIC 3.2: LIGHT CORE CORE EXTENDED EXTENDED LIGHT & OPTICS REFLECTION The angle of incidence equals the angle of reflection. A plane mirror gives a virtual, upright image, same size, as far behind as the object is in front, and laterally inverted (left-right swapped). i r normal REFRACTION Light bends when its speed changes at a boundary. Entering a denser medium it slows and bends toward the normal; leaving, it bends away. Frequency stays the same; wavelength changes. glass (denser) i r DISPERSION OF WHITE LIGHT A prism splits white light into a spectrum. Each colour refracts by a different amount. Red bends least, violet bends most. Order: red, orange, yellow, green, blue, violet. white red violet KEY RELATIONSHIPS n = sin i / sin r n = refractive index n = 1 / sin c c = critical angle Refractive index also links to speed: n = c / v (Extended) LENSES A converging (convex) lens bends parallel rays to a principal focus F. Focal length f = lens to F. 2F F F 2F object real image An object beyond 2F gives a smaller, inverted, real image. Inside F the lens acts as a magnifying glass (virtual). TOTAL INTERNAL REFLECTION EXTENDED Inside a denser medium, if the angle of incidence exceeds the critical angle c, all light reflects. Needs light going from dense to less dense. At exactly c the ray skims along the boundary. Refractive index and c are linked by n = 1 / sin c. dense medium below the boundary i > c OPTICAL FIBRES EXTENDED Light is guided along a thin glass fibre by repeated total internal reflection, even around bends. Telephone and internet data over long distances with little signal loss. Medical endoscopes to see inside the body.
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