Chapter 4 – Refraction of Light at Plane Surfaces

Vocabulary flashcards summarising key terms, definitions, and influencing factors related to the refraction of light at plane surfaces.

Refraction of Light

The bending of an oblique ray when it passes from one transparent medium to another of different optical density because its speed changes.

Optical Density

A property of a medium that indicates how much it slows light; greater optical density means lower light speed and higher refractive index.

Rarer Medium

The medium in which light travels faster and whose refractive index is lower than that of the adjoining medium.

Denser Medium

The medium in which light travels more slowly and whose refractive index is higher than that of the adjoining medium.

Normal (in optics)

An imaginary line drawn perpendicular to the boundary surface at the point where a light ray strikes.

Angle of Incidence (i)

The angle between the incident ray and the normal at the point of incidence.

Angle of Refraction (r)

The angle between the refracted ray and the normal in the second medium.

Snell's Law

The law stating that the ratio sin i / sin r is constant for a given colour and pair of media; also written as μ.

Refractive Index (μ)

A unit-less constant equal to sin i / sin r or to the ratio of light speed in vacuum to that in the medium.

Absolute Refractive Index

The ratio c / v, where c is the speed of light in vacuum and v is its speed in the medium.

Conditions for No Refraction

Refraction is absent when the incident ray hits the boundary normally (i = 0°) or when the two media have identical refractive indices.

Lateral Displacement

The parallel shift of a ray passing through a parallel-sided slab, given by t sin(i − r)/cos r.

Prism

A transparent refracting medium bounded by two plane surfaces that meet at an angle (the refracting edge).

Principal Section (of a prism)

The cross-section of a prism taken perpendicular to its refracting edge.

Angle of Deviation (d)

The angle between the directions of the incident and emergent rays after passage through a prism.

Angle of Minimum Deviation (Dm)

The smallest possible deviation produced by a prism, occurring when the incident and emergent angles are equal.

Factors Affecting Angle of Deviation

Depends on the angle of incidence, material (refractive index) of the prism, prism angle, and wavelength of light.

Apparent Depth

The depth at which an object under a transparent medium appears to lie; always less than the real depth due to refraction.

Shift or Rise

The difference between real and apparent depth (Shift = Real depth – Apparent depth).

Critical Angle (ic)

The angle of incidence in the denser medium for which the angle of refraction in the rarer medium is 90°; ic = sin⁻¹(1/μ).

Total Internal Reflection (TIR)

Complete reflection of light back into a denser medium when it strikes the boundary with a rarer medium at an angle greater than ic.

Total Reflecting Prism

A right-angled isosceles glass prism that uses TIR to deviate a ray by 90°, 180°, or erect an inverted image without loss of brightness.

Mirage

An optical illusion on hot surfaces produced by TIR in air layers of different temperatures, making distant objects appear reflected.

Optical Fibre

A thin flexible strand of glass or plastic that transmits light signals via repeated total internal reflections with negligible energy loss.

Factors Affecting Refractive Index

Determined by the nature (optical density) of the medium, temperature (inverse relation), and wavelength or colour of light (inverse relation).

Factors Affecting Critical Angle

Critical angle increases with longer wavelength (red > violet) and with higher temperature (due to lower refractive index).

Speed-Wavelength-Frequency Relation

Given by v = f λ; during refraction, speed and wavelength change but frequency remains constant.