Optics Sign Convention and Lens Basics

Question-and-answer flashcards covering sign convention rules, vergence, fundamental optical definitions, and basic lens properties.

In the standard sign convention, light is assumed to travel in which direction along the optical axis?

From left to right.

According to the sign convention, a distance measured in the same direction as light travels (left → right) is assigned what sign?

Positive (+).

How is a distance measured opposite to the direction of light travel (right → left) treated in sign convention?

It is given a negative (–) sign.

When measuring vertical distances from the optic axis, how are points above and below the axis signed?

Above the axis is positive; below the axis is negative.

From where is the angle between a ray and the optic axis measured?

From the ray to the optic axis.

Angles of incidence, refraction, and reflection are measured from what reference line?

From the normal to the ray.

In the sign convention, an angle measured counter-clockwise is considered , while clockwise is .

Counter-clockwise is positive; clockwise is negative.

What does an arrowhead placed on a line or curve indicate in optical diagrams?

The direction in which the distance or angle is being measured.

Define vergence of a wavefront.

The curvature of a wavefront at a given distance from its origin or focus, expressed in diopters.

What sign is assigned to vergence for converging (toward focus) and diverging (away from focus) wavefronts?

Converging vergence is positive; diverging vergence is negative.

Write the formula that relates vergence (D) to focal length (f).

D = 1 / f, where f is in meters.

What is the definition of a ray in geometrical optics?

A hypothetical line perpendicular to wavefronts that indicates the direction of wave propagation.

What is a pencil of rays?

A bundle of rays emanating from a point source after passing through a limiting aperture.

Which ray in a pencil passes through the center of the limiting aperture?

The chief ray.

What is an object in optical terminology?

The physical source of light (or absence of light) existing in object space, which can be real or virtual.

When is an object considered virtual?

When light is converging toward it before being interrupted by a subsequent surface of the optical system.

Define a real image produced by an optical system.

An image actually formed by converging rays that can be projected onto a screen placed at the image plane.

Why can a virtual image still be seen by an observer, even though it cannot be projected on a screen?

Because the human eye is a converging optical system that can focus the diverging rays to form an image on the retina.

Differentiate object space from image space.

Object space is where light travels before encountering the optical system; image space is where light travels after exiting the system.

Give the standard symbols for (a) index of refraction, (b) focal length, and (c) vergence/diopters.

(a) n, (b) f, (c) D.

Name two common optical aberrations related to lenses and describe them briefly.

Spherical aberration and chromatic aberration—both are failures to focus all rays or colors to a single point.

What material property measures the bending of light when passing between media?

Refractive index.

What is the Abbe number and how does it relate to dispersion?

A measure of how much a material disperses light; a lower Abbe number means higher dispersion.

State the lens type, shape, and basic image characteristics of a convex lens.

Convex lens: converging type, thicker in the center, magnifies objects and forms real, inverted images.

State the lens type, shape, and basic image characteristics of a concave lens.

Concave lens: diverging type, thicker at edges, minifies objects and forms virtual, erect images.