Physics for Life Sciences II: Wave and Ray Models of Light

Practice vocabulary flashcards covering the wave model, ray model, diffraction, and reflection as presented in the Section 17.6 and 18.1-18.2 lecture notes.

Wave Model

A model that considers light to be a wave propagating through space, appropriate when light interacts with objects whose size is roughly less than about $0.1\,mm$.

Huygens’ principle

The principle stating that each point on a wave front is the source of a spherical wavelet, and the wave front at a later time is tangent to all the wavelets.

Diffraction

The spreading of a wave after it passes through an opening.

Constructive and destructive interference

The overlap of two or more waves as they spread behind openings.

Circular-Aperture Diffraction

A phenomenon where light waves passing through a circular opening spread out to generate a circular diffraction pattern consisting of a central maximum and secondary bright fringes.

$\theta_{1}$ (for circular aperture)

The angle that locates the first minimum in the intensity for a circular aperture of diameter $D$, defined as $\theta_{1} = 1.22 \frac{\lambda}{D}$.

$w$ (circular aperture central maximum)

The width of the central maximum on a screen a distance $L$ from the aperture, calculated as $w = \frac{2.44 L \lambda}{D}$.

Ray Model of Light

A model of light that ignores diffraction and is valid as long as any apertures through which the light passes are larger than about $1\,mm$.

Light ray

A line in the direction along which the energy of light is flowing.

Index of refraction ($n$)

A value used to determine the speed of light in a material using the formula $v = \frac{c}{n}$.

Self-luminous objects

Objects that directly create light rays, such as lightbulbs or the sun.

Reflective objects

Objects that reflect rays originating from self-luminous objects, such as a piece of paper or a tree.

Point source

An idealized, infinitely small source of light that emits light rays in every direction.

Extended source

A light source where the entire surface is luminous and every point acts as a point source; examples include flames and the sun.

Parallel-ray source

A source that produces a bundle of parallel rays, such as a flashlight, movie projector, or a very distant star.

Ray diagram

A simplified diagram that shows a few light rays to represent the movement of light in a given situation.

Diffuse reflection

The process of reflecting incident light in all directions due to the microscopic roughness of a surface.

Scattering

A process in which single rays are broken into many weaker rays that leave in all directions, often by small particles like dust or air molecules.

Specular reflection

Reflection from a smooth, shiny surface such as a mirror or polished metal, where incident and reflected rays are in a plane normal to the surface.

Law of reflection

The rule stating that the incident and reflected rays are in the same plane perpendicular to the surface, and the angle of reflection equals the angle of incidence ($\theta_{r} = \theta_{i}$).

Virtual image

A point from which reflected rays appear to diverge, though no rays actually leave that point.

Plane mirror

A flat mirror that forms a virtual image at an image distance $s^{\prime}$ equal to the object distance $s$.