The Physics
Hypertextbook
Opus in profectus

# Reflection

## Summary

• Optics
• Optics is the study of the nature and behavior of light
• It can be divided into subdisciplines based on the type of model used to describe light.
• In physical optics, light is assumed to behave like a classical wave.
• In quantum optics, light is assumed to have both wave and particle properties.
• Particles of light are called photons.
• In geometric optics, light is assumed to travel in a definite direction with relatively little diffraction.
• This behavior is known as rectilinear propagation.
• The path of propagation of a light wave is a geometric ray.
• The rays of geometric optics…
• are perpendicular to the wave fronts of physical optics.
• indicate the most probable path of the photons of quantum optics.
• A ray is the path of least action connecting two points in space and is also…
• the path of least time (the quickest path)
• unique and therefore reversible
• The principle of reversibility states that light will follow the exact same path if its direction of travel is reversed.
• Rays are…
• The eye can see something only if a ray of light from the object reaches the eye. • Interface
• An interface is the boundary between…
• two different media.
• two regions of a medium with different characteristics such as…
• density (which is often related to temperature)
• concentration of solute (salinity, for example)
• mechanical stress
• When an incident ray meets an interface it will be partially
• reflected
• Reflected rays obey the law of reflection described in this section of this book.
• transmitted
• Transmitted rays obey Snell's law, which is described in the next section of this book.
• absorbed
• Absorbed rays obey the law of conservation of energy. (The energy of the ray is not destroyed, but changes form.)
• Angles in geometric optics are measured with respect to a line normal to the interface.
• The angle of incidence is the angle between the incident ray and the normal.
• The angle of reflection is the angle between the reflected ray and the normal.
• The angle of refraction is the angle between the transmitted ray and the normal.
• • Reflection
• Law of reflection: The angle of incidence equals the angle of reflection (θi = θr).
• The law of reflection can be derived from the principle of least action.
• Types of reflection
• Regular reflection or spectral reflection
• Incident rays of light are reflected in one direction according to the law of reflection.
• Regular reflection occurs on smooth surfaces.
• polished surfaces (metal, glass, etc.)
• naturally flat surfaces (still water, black ice, etc.)
• Parallel incident rays remain parallel after regular reflection (from a flat surface).
• Regular reflection results in image formation. • Diffuse reflection
• Incident rays of light are reflected in many directions with a statistically random distribution.
• Diffuse reflection occurs on rough surfaces…
• textured surfaces (brushed metal, crumpled aluminum foil, frosted glass, etc.)
• naturally rough surfaces (paper, cloth, etc.)
• multiple randomly oriented surfaces (foam, snow, etc)
• subsurface irregularities (polished marble and a smooth coat of paint still reflect diffusely)
• Parallel incident rays are not parallel after diffuse reflection.
• Diffuse reflection is what makes most nonluminous objects visible.
• • Scattering
• THIS NEEDS TO BE FIXED
• Light is diffracted off of small particles so that the reflected waves are nearly spherical.
• raindrops and smoke?
• Light is absorbed by small particles and then emitted (effectively instantaneously) with a change in direction that is statistically random.
• nitrogen molecules in the air?
• Optical Images
• Every point on an object emits a set of diverging rays (called a ray bundle) in all possible directions.
• If any of the rays in a bundle…
• converge somewhere else, an in-focus real image of the original point will form at that location.
• Real images can be captured on a screen.
• can be traced backward to converge somewhere else, an in-focus virtual image of the original point will form at that location.
• Virtual images cannot be captured on a screen.
• Plane Mirrors
• The image formed in a plane mirror is…
• the same distance behind the mirror as the object is in front (dido)
• the same size as the object; that is, neither magnified nor reduced (siso)
• upright or erect; that is, not upside down (as long as the mirror is vertical)
• laterally inverted or perverted; that is, right handed objects produce left handed images (chirality is reversed)
• virtual; that is, located at the apperent intersection point of the reflected rays