Reflection of Light
Reflection of light (and other forms of electromagnetic radiation) occurs when the waves encounter a surface or other boundary that does not absorb the energy of the radiation and bounces the waves away from the surface. The simplest example of visible light reflection is the surface of a smooth pool of water, where the light is reflected in an orderly manner to produce a clear image of the scenery surrounding the pool. Throw a rock into the pool, and the water is perturbed to form waves, which disrupt the reflection by scattering the incident and reflected light.
The reflection of visible light is a property of the behavior of light that is fundamental in the function of all modern microscopes. Light is often reflected by one or more plane (or flat) mirrors within the microscope to direct the light path through lenses that form the virtual images we see in the oculars (eyepieces). Microscopes also make use of beamsplitters to allow some light to be reflected while simultaneously transmitting other light to different parts of the optical system. Other optical components in the microscope, such as specially designed prisms, filters, and lens coatings, also carry out their functions in forming the image with a crucial reliance on the phenomenon of light reflection.
Introduction to the Reflection of Light - Because light behaves in some ways as a wave and in other ways as if it were composed of particles, several independent theories of light reflection have emerged. According to wave-based theories, the light waves spread out from the source in all directions, and upon striking a mirror, are reflected at an angle determined by the angle at which the light arrives. The reflection process inverts each wave back-to-front, which is why a reverse image is observed. The shape of light waves depends upon the size of the light source and how far the waves have traveled to meet the mirror. Wavefronts that originate from a source near the mirror will be highly curved, while those emitted by distant light sources will be almost linear, a factor that will affect the angle of reflection.
Augustin-Jean Fresnel (1788-1827) - Augustin-Jean Fresnel, was a nineteenth century French physicist, who is best known for the invention of unique compound lenses designed to produce parallel beams of light, which are still used widely in lighthouses. In the field of optics, Fresnel derived formulas to explain reflection, diffraction, interference, refraction, double refraction, and the polarization of light reflected from a transparent substance.
Christiaan Huygens (1629-1695) - Christiaan Huygens was a brilliant Dutch mathematician, physicist, and astronomer who lived during the seventeenth century, a period sometimes referred to as the Scientific Revolution. Huygens, a particularly gifted scientist, is best known for his work on the theories of centrifugal force, the wave theory of light, and the pendulum clock. His theories neatly explained the laws of refraction, diffraction, interference, and reflection, and Huygens went on to make major advances in the theories concerning the phenomena of double refraction (birefringence) and polarization of light.
Interactive Java Tutorials
Reflection of Light - Reflection of light (and other forms of electromagnetic radiation) occurs when the waves encounter a surface or other boundary that does not absorb the energy of the radiation and bounces the waves away from the surface. This tutorial explores the incident and reflected angles of a single light wave impacting on a smooth surface.
Specular and Diffuse Reflection - The amount of light reflected by an object, and how it is reflected, is very dependent upon the smoothness or texture of the surface. When surface imperfections are smaller than the wavelength of the incident light (as in the case of a mirror), virtually all of the light is reflected equally. However, in the real world most objects have convoluted surfaces that exhibit a diffuse reflection, with the incident light being reflected in all directions. This interactive tutorial explores how light waves are reflected by smooth and rough surfaces.
Concave Spherical Mirrors - Concave mirrors have a curved surface with a center of curvature equidistant from every point on the mirror's surface. An object beyond the center of curvature forms a real and inverted image between the focal point and the center of curvature. This interactive tutorial explores how moving the object farther away from the center of curvature affects the size of the real image formed by the mirror. Also examined in the tutorial are the effects of moving the object closer to the mirror, first between the center of curvature and the focal point, and then between the focal point and the mirror surface (to form a virtual image).
Concave Spherical Mirrors (3-Dimensional Version) - Concave mirrors have a curved surface with a center of curvature equidistant from every point on the mirror's surface. An object beyond the center of curvature forms a real and inverted image between the focal point and the center of curvature. This interactive tutorial explores how moving the object farther away from the center of curvature affects the size of the real image formed by the mirror.
Convex Spherical Mirrors - Regardless of the position of the object reflected by a convex mirror, the image formed is always virtual, upright, and reduced in size. This interactive tutorial explores how moving the object farther away from the mirror's surface affects the size of the virtual image formed behind the mirror.
Convex Spherical Mirrors (3-Dimensional Version) - Regardless of the position of the object reflected by a convex mirror, the image formed is always virtual, upright, and reduced in size. This interactive tutorial explores how moving the object farther away from the mirror's surface affects the size of the virtual image formed behind the mirror. This tutorial utilizes three-dimensional graphics.
Reflection and Refraction with Huygens Wavelets - Near the beginning of the eighteenth century, Dutch physicist Christiaan Huygens proposed that each point in a wave of light can be thought of as an individual source of illumination that produces its own spherical wavelets, which all add together to form an advancing wavefront. This interactive Java tutorial is designed to illustrate the reflection and refraction of light according to the multiple wavelet concept, now known as the Huygens' principle.
Antireflection Surface Coatings - The concept behind antireflection technology is to control the light used in an optical device in such a way that the light rays reflect from surfaces where it is intended and beneficial, and do not reflect from surfaces where this would have a deleterious effect on the image being observed. One of the most significant advances made in modern lens design, whether for microscopes, cameras, or other optical devices, is the significant improvement in antireflection coating technology. This tutorial explores various coatings and their reflectivities as a function of incident angle.
The Critical Angle of Reflection - An important concept in optical microscopy is the critical angle of reflection, which is a necessary factor to consider when choosing whether to use dry or oil immersion objectives to view a specimen at high magnification. Upon passing through a medium of higher refractive index into a medium of lower refractive index, the path taken by light waves is determined by the incident angle with respect to the boundary between the two media. This interactive tutorial explores the transition from refraction to total internal reflection as the angle of the incident wave is increased at constant refractive index.
Common Reflecting Prisms - The angular parameters displayed by various prism designs cover a wide gamut of geometries that dramatically extend the usefulness of prisms as strategic optical components. Reflecting prisms are often designed to be located in specific orientations where the entrance and exit faces are both parallel and perpendicular to the optical axis. This interactive tutorial explores image deviation, rotation, and displacement exhibited by common reflecting prisms.
Right-Angle Prisms - The right-angle prism possesses the simple geometry of a 45-degree right triangle, and is one of the most commonly used prisms for redirecting light and rotating images. This interactive tutorial explores light reflection and image rotation, inversion, and reversion by a right-angle prism as a function of the prism orientation with respect to incident light.
Transmission and Reflection by Beamsplitters - A beamsplitter is a common optical component that partially transmits and partially reflects an incident light beam, usually in unequal proportions. In addition to the task of dividing light, beamsplitters can be employed to recombine two separate light beams or images into a single path. This interactive tutorial explores transmission and reflection of a light beam by three common beamsplitter designs.
Selected Literature References
Reference Listing - The reference materials listed in this section are an excellent source of additional information on the diverse topic of reflection from both specular and diffuse surfaces. Included are references to books, book chapters, and review articles, which discuss the theory and applications of the reflection of electromagnetic radiation and how they relate to the physics of light and color.
Matthew J. Parry-Hill, Robert T. Sutter, and Michael W. Davidson - National High Magnetic Field Laboratory, 1800 East Paul Dirac Dr., The Florida State University, Tallahassee, Florida, 32310.
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