# Ray Optics and Optical Instruments

## •    Geometrical optics or Ray optics is the branch of Physics that describes many aspects of light such as reflection, refraction and the formation of images by mirrors and lenses. •    The phenomenon of reflection of light takes place through plane mirrors and spherical mirrors. •    Spherical mirror is a part of hollow sphere whose one side is reflecting and other is opaque. •    Concave mirror and convex mirror are the two types of spherical mirrors. •    Sign conventions are adopted to derive the formulae for reflection of light by spherical mirrors and refraction of light by spherical lenses. •    In a spherical mirror, mirror equation gives the relationship between focal length, image distance and object distance. •    The phenomenon in which light passes from one transparent medium to another transparent medium, changes its speed and bends at the interface of the two media (if it is incident obliquely) is known as refraction of Light. •    Refraction of light is governed by certain laws. The refraction of light through the earth’s atmosphere is responsible for many interesting phenomena like advance sunrise or delayed sunset. •    The phenomenon of reflection of light into a denser medium from an interface of denser medium and a rarer medium is known as total internal reflection. •    Multiple internal reflections in diamond, totally reflecting prism, mirage, optical fibres are some of the examples of total internal reflection.•    Spherical surfaces and lenses also offer the phenomenon of refraction. •    The two types of refracting surfaces are convex refracting surface and concave refracting surface. •     A lens is a transparent optical medium bounded by two surfaces; at least one of which should be spherical. •    Lens Maker’s Formula gives the relationship between the focal length of a lens to the radii of curvature and refractive index of the lens. •    Two or more lenses are combined to increase the magnification of the image, to make the final image erect with respect to the object and to reduce certain aberrations. •    The phenomenon of refraction of light can also takes place through a prism. On passing through prism a ray of light suffers refraction twice and hence deviates through a certain angle from its original path. •    The splitting of white light into its seven constituent colours is known as dispersion of light. •    Some interesting natural phenomena due to sunlight include Rainbow formation, blue colour of the sky, reddish colour of sun during sunrise and sunset, etc. can be explained by Ray Optics. •    The best camera on the earth is a human eye. •    Man-made optical instruments which are used for magnification and to observe distant objects are microscopes and telescopes etc. •    The two types of microscopes are simple microscope and compound microscope. An optical instrument used for observing magnified images of small objects is known as simple microscope. Compound microscope is an optical instrument used for observing highly magnified images of tiny objects. For viewing distant objects and heavenly bodies like moon, planets, etc. we use an optical instrument known as telescope.

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