3. Refraction by prism - Lecture note 3 PDF

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Refraction by prism...

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Refraction by prism Triangular prism -

Two rectangular faces; I (leading face) and E (rear face); form the refracting faces of the prism Light enters prism from I and emerges from E A = apical angle = angle between 2 refracting surfaces Apex = tip of prism where 2 refracting faces meet Applications – dispersion (spectroscopy), beam splitters (interferometry), ophthalmic prisms (for relief and correction of abnormal muscular conditions in the eye (squint eyes))

Ray diagrams and notations

Deviation of light ray Factors affecting angle of deviation: -

Angle of incidence Material Apex angle Wavelengths of light

Thick prism deviation formula (4.9) The angle of deviation produced by the thick prism depends on: -

Apical angle Angel of incidence Angle of emergence

Minimum angle of deviation of prism -

dmin = angle of min deviation at actual min deviation, angle of incidence = angle of emergence occurs only at particular angle of incidence where refracted rays inside prism make equal angles with the prism faces

(refer to lecture slide 4 for min angle of deviation experimental results) Prism spectrometer Used to measure: -

Apex angle Angle of incidence Deviation

Relationship between apical angle, deviation and refractive index (calculating refractive index of prism)

Refractive index of prism

Critical angle & total internal reflection Critical angle conditions: -

a  suffieciently large i1  small i2  large = ic i’2  90°

Total internal reflection conditions: -

i1  very small i2 > ic i’2  non-existent; no emergent ray

Dispersion

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prism disperses light white light = collection of component colours when passes through medium, white light separated into component colours (ROYGBIV) dispersion = separation of visible light into its different colours; dependence of the refractive index on the wavelength of material transparent medium; different colour lights travel with different velocities velocity of blue < velocity of red in glass smaller velocity = higher refractive index = larger degree of refraction

Abbe number (V) -

gives a measure of dispersion n = refractive index of material at mid-wavelength in visible spectrum nF = refractive index for blue, nC = R.I for red greater the difference (nF – nC), more dispersive the material, smaller the Abbe number spectacle lenses; dispersion small, Abbe no large (~50)

Thin prism, deviation prism power Ophthalmic prism -

thin prism with small apex angle (...