Refraction- The bending of light as it crosses the boundary between two materials in which it travels at different speeds. Reflection Image- The visual counterpart of an object formed by refraction or reflection of light that came from the object. Focal Point- The point at which light from a very distant object converges after being refracted or reflected. Focal Length- The distance from the center of the lens or a mirror to its focal point. Objective- The main light gathering element—lens or mirror – of a telescope. It is also called a primary lens. Eyepiece- The magnifying lenses (or combination of lens) used to view the image formed by the objective of a telescope. Dispersion- The separation of light into its various wavelengths upon refraction. Chromatic aberration- The defect of optical systems that results in light of different colors being focused at different places. Achromatic lens- An optical element that has been corrected so that it is free of chromatic aberration. Angular size- The angle between two lines drawn from the viewer to the opposite side of the object. Magnifying power/Magnification- The ratio of angular size of an object when it is seen through the instrument to its angular size when seen with the naked eye. Light gathering power- A measure of the amount of light collected by an optical instrument. Aperture size- The diameter of the lense Resolving power- The smallest angular separation detectable with an instrument. Thus it is a measure of an instrument’s ability to see detail. Field of View- The actual angular width of the scene viewed by an optical instrument. F-ratio- Length compared to diameter