Light Terms

absolute index of refraction - the ratio of the speed of light in a vacuum (c) to the speed of light in another given medium (v).

angle of deviation - when light passes from air into glass and then back into air, if the refracting surfaces are not parallel (like a prism), then the emerging ray takes a different path than the initial incident ray. The angle between the emerging ray and the incident ray is called the angle of deviation.

angle of incidence - the angle between the incident ray and the normal.

angle of reflection - the angle between the reflected ray and the normal.

angle of refraction - the angle between the refracted ray and the normal.

astronomical unit (A.U.) - the length on the semimajor axis of the Earth's orbit.
1 A.U. = 1.50 x 10¹¹m

beam - a combination of several rays of light.

center of curvature (C) - the center of a spherical reflecting surface.

concave mirror - a mirror with a surface that converges reflected rays.

convex mirror - a mirror with a surface that diverges or spreads reflected rays.

critical angle - a unique angle of incidence that results in a refracted angle of 90°. This only happens when light passes into a more optically dense medium.

diffuse reflection - parallel incident rays that are reflected in various directions from a rough surface.

direct lighting - light travels straight toward an object or surface that produces sharp shadows. Direct lighting light travels straight toward an object or surface that produces sharp shadows.

eclipse - a shadow formed from a moon or planet blocking the light from the sun. A total eclipse is the umbra of a shadow while a partial eclipse would be the penumbra of a shadow.

electromagnetic spectrum - a continuum of electromagnetic waves or energies that include radio waves, microwaves, infrared, visible light, ultraviolet, x-rays, and gamma rays.

focal plane - the distance between the principal focus and the vertex.

illuminance - the amount of light on a surface area measured in lux

incandescent body - a body that emits light after being heated (eg. a light bulb).

incident ray - a ray approaching a surface.

indirect lighting - light that reflects off one or more rough surfaces resulting in few or soft shadows. Indirect lighting is sometimes called soft lighting or diffuse.

lateral displacement - when light passes from air into glass and back into air, if the refracting surfaces are parallel, then the emerging ray is parallel to the incident ray, just shifted over. This shift is called lateral displacement.

laterally inverted - left-right reversal. eg. when looking in the mirror, your right hand appears to be on the left side.

light year - the distance traveled by light in one year.
1 light year = 9.46 x 10¹⁵m.

luminous body - a body that emits light waves (eg. the sun).

luminous flux - the rate of flow of light energy or power of visible light emitted from a light source, measured in lumens

luminous intensity - the amount of light or brightness that is given off by a light source, measured in candelas.

magnification - ratio of the size of an image (optical) to the size of an object.

nonluminous body - a body that reflects light (eg. the moon).

normal - a construction line drawn perpendicular (90°) to the surface at the point of incidence.

opaque - an object or material that does not allow light to pass through.

parsec - the distance from the sun to a star such that it would have a parallax angle of one second of arc as viewed from the Earth.
1 parsec = 3.09 x 10¹⁶m, or 3.26 light years.

partially reflected ray - some of the incident ray refracts and passes through a medium, while some of the light (partially) simply reflects off the boundary surface.

penumbra - a partial shadow

principal axis - a construction line drawn through the vertex and center of curvature.

principal focus (F) - a point on the principal axis where parallel rays reflect and converge or appear to converge (sometimes called focal point).

point of incidence - where an incident ray strikes a surface.

radius of curvature - the distance from the center of curvature to the mirror.

ray - a hypothetical line used to represent a single stream of light. Rays show the path of light but do not exist in nature.

real image - an image that can be focused or projected on a screen.

rectilinear propagation - light travels in straight lines (sometimes just called linear propagation).

reflected ray - a portion of the incident ray that leaves or reflects a surface at the point of incidence.

refraction - the bending of light that takes place at a boundary between two different mediums. Refraction is due to a change in the speed of light as it passes from one medium to another.

specular reflection - parallel incident rays that are reflected parallel from a smooth surface (sometimes called regular reflection).

spherical aberration - an optical defect in curved mirrors that result in reflected rays not all converging at a principal focus.

total internal reflection - results when the angle of incidence is greater than the critical angle of a medium. Light does not passes through and refract, rather it simply reflects off the boundary surface.

translucent - objects that allow some light to pass through but diffuse or scatter in the process. A clear image is not visible through this object.

transparent - objects that allow light to pass through them so that a clear image is visible.

umbra - a total shadow

vertex - the geometric center of a mirror.

virtual image - an image that cannot be projected on a screen. The image appears behind (mirror or lens).

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