Ray Optics and Optical Instruments

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. 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. The two types of spherical mirrors are concave mirror and convex mirror. Sign convention is convention 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 relation between focal length, image distance and object distance. Refraction of Light is the phenomenon in which light passes from one transparent medium to another transparent medium and changes its speed when bends at the interface of the two media (if it is incident obliquely). 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. Total internal reflection is the phenomenon of reflection of light into a denser medium from an interface of denser medium and a rarer medium. Multiple internals reflections in diamond, totally reflecting prism, mirage, optical fibres are some of the examples of total internal reflection. Refraction of light also takes place at spherical surfaces and lens. The two types of refracting surfaces are convex refracting surface and concave refracting surface. Lens is a transparent optical medium bounded by two surfaces; at least one of which should be spherical. A relation connecting the focal length of a lens to the radii of curvature and refractive index of the lens is called Lens Maker’s Formula. 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. Refraction of light also takes place through a Prism. A ray of light suffers two refractions on passing through a prism and hence deviates through a certain angle from its original path. Dispersion is the splitting of light into its constituent colours. In dispersion of light through prism,a narrow beam of white light or sunlight passes through a prism and splits into its constituent colours. Rainbow, blue colour of the sky, reddish colour of sun during sunrise and sunset, etc. are few interesting natural phenomena due to sunlight which can be well explained by Ray Optics. Human eye is like a camera and has a convex lens of few centimetres. Few man made optical instruments which are used for magnification and to observe distant objects are microscopes and telescopes. The two types of microscopes are simple microscope and compound microscope. Simple microscope is an optical instrument used for observing magnified images of small objects. Compound microscope is an optical instrument used for observing highly magnified images of tiny objects. Telescopes are optical instruments used for viewing distant objects like heavenly bodies.

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