Light energy

Wave-particle duality

The principle stating that radiant energy behaves simultaneously as an electromagnetic wave and as a stream of discrete energy packets called photons.

Speed of light in a vacuum

Approximately $3 \times 10^8\,\text{m/s}$ (equivalent to $300{,}000\,\text{km/s}$).

Parallel beam

A collection of light rays that travel parallel to one another without converging or spreading outward.

Convergent beam

A collection of light rays that travel inward toward and meet at a single common focal point.

Divergent beam

A collection of light rays that spread outwards in multiple directions from a single point source.

Transparent material

A substance that transmits almost all incident light without scattering, allowing objects behind it to be seen distinctly.

Translucent material

A substance that allows partial transmission of light while scattering it, preventing objects behind it from being seen distinctly.

Opaque material

A substance that blocks all incident light from passing through, casting a shadow behind it.

Difference between incandescence and luminescence

The former produces light due to high thermal temperatures, whereas the latter emits light without requiring high heat.

Chemiluminescence

The emission of visible light produced by a chemical reaction without generating significant heat.

Bioluminescence

The emission of light produced through biochemical reactions occurring within living organisms.

Phosphorescence

The delayed emission of light where an illuminated substance absorbs radiant energy and releases it slowly over an extended duration.

Triboluminescence

The emission of light produced when certain materials are subjected to friction, scratching, crushing, or mechanical breakage.

LASER (acronym meaning)

Light Amplification by Stimulated Emission of Radiation.

Total internal reflection (TIR)

The complete reflection of a light ray back into an optically denser medium when striking the boundary with a less dense medium at an incident angle greater than the critical angle.

Two necessary conditions for total internal reflection

1. Light must travel from an optically denser medium toward an optically less dense medium. 2. The angle of incidence must exceed the critical angle.

Role of the cladding in an optical fibre

To provide an outer boundary layer with a lower refractive index than the core, enabling total internal reflection to confine light within the core.

First law of reflection

The principle stating that the incident ray, the reflected ray, and the normal at the point of incidence all lie within the same plane.

Second law of reflection

The principle stating that the angle of incidence is equal to the angle of reflection ($i = r$), with both angles measured relative to the normal line.

Regular (specular) reflection

Reflection from a smooth, polished boundary where parallel incident rays reflect uniformly parallel, producing clear images.

Diffuse (irregular) reflection

Reflection from a rough or uneven surface that scatters light rays in multiple directions, preventing the formation of a distinct image.

Image characteristics of a plane mirror

Virtual, erect (upright), laterally inverted, equal in size to the object, and located at an image distance equal to the object distance.

Image characteristics of a convex mirror

Always virtual, upright, diminished in size, and covering a wide field of view.

Image characteristics of a concave mirror with object within focal length

Virtual, upright, and magnified.

Vision defect corrected by a convex lens

Long-sightedness (hypermetropia), corrected by converging incoming light rays before they enter the eye.

Vision defect corrected by a concave lens

Short-sightedness (myopia), corrected by diverging incoming light rays before they enter the eye.

Real image vs. Virtual image

The former is formed by the actual convergence of light rays and can be projected onto a screen, whereas the latter is formed by rays that only appear to diverge from a point and cannot be projected.

Mirage

An optical illusion caused by the atmospheric refraction of light through air layers of differing temperatures and refractive indices.

Mirage

An optical illusion caused by the atmospheric refraction of light through air layers of differing temperatures and refractive indices.

Rectilinear propagation of light

The fundamental optical property stating that light travels in straight lines through a uniform, homogeneous medium.

Image characteristics produced by a pinhole camera

Real, inverted, and typically diminished, formed due to straight-line ray travel through a tiny aperture.

Umbra vs. Penumbra

The former is the totally dark, central shadow receiving zero direct light from the source, while the latter is the partially illuminated outer shadow region.

Antumbra

The lighter shadow zone beyond the umbra where the blocking body appears completely enclosed by the light source, producing an annular eclipse.

Celestial alignment during a solar eclipse

$\text{Sun} \rightarrow \text{Moon} \rightarrow \text{Earth}$ (the Moon passes directly between the Sun and Earth, casting its shadow onto Earth).

Celestial alignment during a lunar eclipse

$\text{Sun} \rightarrow \text{Earth} \rightarrow \text{Moon}$ (Earth passes directly between the Sun and Moon, casting its shadow onto the Moon).

Dispersion of light

The splitting of white light into its component spectrum of colours when passing through a prism due to different wavelengths bending by varying amounts.