Light

The One Thing

Light changes direction when its speed changes between transparent media, and this principle explains refraction, total internal reflection, lens images, prism dispersion and several effects produced by scattering.

Reactions, Processes and Experiments

Key Terms

• Refraction: The bending of light as it enters a different transparent medium because its speed changes.
• Optically denser medium: A medium in which light travels more slowly and generally has a higher refractive index.
• Optically rarer medium: A medium in which light travels faster and generally has a lower refractive index.
• Normal: An imaginary line perpendicular to the surface at the point where light strikes it.
• Angle of incidence: The angle between the incident ray and the normal.
• Angle of refraction: The angle between the refracted ray and the normal.
• Laws of refraction: The incident ray, refracted ray and normal lie in one plane; for a given pair of media, sin i divided by sin r is constant.
• Refractive index: A measure of how much a medium slows and bends light; n = c/v.
• Snell's law: For two given media, n1 sin i = n2 sin r.
• Critical angle: The angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90 degrees.
• Total internal reflection: Complete reflection of light back into the denser medium when it travels from a denser to a rarer medium and the angle of incidence exceeds the critical angle.
• Conditions for total internal reflection: Light must travel from an optically denser to an optically rarer medium, and the angle of incidence must be greater than the critical angle.
• Optical fibre: A thin transparent fibre that transmits light through repeated total internal reflection.
• Lens: A transparent optical device bounded by two spherical surfaces, or by one spherical and one plane surface, that refracts light.
• Convex lens: A converging lens thicker at the centre that can form real or virtual images.
• Concave lens: A diverging lens thinner at the centre that always forms a virtual, erect and diminished image for a real object.
• Principal axis: The straight line passing through the optical centre and the centres of curvature of a lens.
• Optical centre: The point near the centre of a thin lens through which a ray passes approximately without deviation.
• Principal focus: The point on the principal axis where rays parallel to the axis meet, or appear to come from, after refraction.
• Focal length: The distance between the optical centre and the principal focus.
• Lens formula: For a thin lens, 1/f = 1/v - 1/u using the Cartesian sign convention.
• Magnification: The ratio of image height to object height; for a lens, m = v/u.
• Power of a lens: The ability of a lens to converge or diverge light; P = 1/f when f is measured in metres, with the unit dioptre.
• Prism: A transparent refracting medium bounded by two plane surfaces inclined at an angle.
• Angle of prism: The angle between the two refracting faces of a prism.
• Angle of deviation: The angle between the original direction of the incident ray and the emergent ray.
• Minimum deviation: The least angle through which a prism deviates a ray; at this position, the path is symmetrical and the angles of incidence and emergence are equal.
• Dispersion: The splitting of white light into its constituent colours when it passes through a prism.
• Spectrum: The band of colours obtained when white light is dispersed, commonly remembered as VIBGYOR.
• Scattering: The redirection of light in different directions by particles in a medium.
• Tyndall effect: The visible scattering of light by colloidal or very fine particles, making the path of a beam visible.
• Rayleigh scattering: Scattering by particles much smaller than the wavelength of light; shorter wavelengths scatter more strongly.

Easily Confused

• Optically denser and optically rarer media: Light travels more slowly in an optically denser medium and faster in an optically rarer medium.
• Refraction and total internal reflection: Refraction occurs when light enters another medium; total internal reflection returns all the light to the denser medium under the required conditions.
• Critical angle and total internal reflection: At the critical angle the refracted ray travels along the boundary; total internal reflection occurs only when the angle of incidence is greater than the critical angle.
• Convex and concave lenses: A convex lens converges light and has positive power; a concave lens diverges light and has negative power.
• Real and virtual images formed by a convex lens: A real image is formed for object positions beyond the focus, whereas an object between the optical centre and focus produces a virtual, erect and enlarged image.
• Angle of prism and angle of deviation: The angle of prism is between the refracting faces; the angle of deviation is between the original incident direction and the emergent ray.
• Dispersion and scattering: Dispersion separates white light into colours through a prism; scattering redirects light in different directions through interaction with particles.
• Tyndall effect and Rayleigh scattering: The Tyndall effect is the visible scattering by colloidal or very fine particles; Rayleigh scattering specifically concerns particles much smaller than the wavelength of light.
• Red and violet light in dispersion: Violet has the shortest wavelength and greatest deviation; red has the longest wavelength, lowest visible frequency and least deviation.
• Lens sign convention and ordinary distance measurement: In the Cartesian convention used in school problems, distances measured in the direction of incident light are usually negative, while distances measured opposite to it are positive.

What Gets Asked

• Applying refraction rules: Questions may ask whether a ray bends towards or away from the normal when moving between rarer and denser media. The mark-losing error is reversing the direction of bending.
• Using refractive-index equations: Problems may require $n=c/v$, relative refractive index, or Snell's law $n1\sin i=n2\sin r$. A common slip is treating the frequency as changed during refraction; the frequency remains unchanged.
• Identifying total internal reflection: Students must state both conditions: travel from a denser to a rarer medium and incidence at an angle greater than the critical angle. At $i=C$, the ray is refracted along the boundary rather than totally internally reflected.
• Determining lens images: Questions may specify an object's position relative to $F$ and $2F$. Marks are lost by confusing the image descriptions for positions beyond $2F$, at $2F$, between $F$ and $2F$, at $F$, and between the optical centre and $F$.
• Calculating lens quantities: Questions may use $1/f=1/v-1/u$, $m=v/u$, $P=1/f$, or $P=P1+P2$. The specific risk is using an incorrect sign convention or forgetting that focal length must be in metres for power in dioptres.
• Explaining prisms and scattering: Questions may ask why violet deviates most, why the sky is blue, why the Sun appears reddish at sunrise and sunset, why clouds appear white, or why stars twinkle. The required distinctions are wavelength-dependent scattering, nearly equal scattering by large cloud droplets, and repeated atmospheric refraction.