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angle of incidence angle of refraction beam of light beam of parallel called candle centre of curvature colours concave lens concave mirror conjugate focus converge convex lens convex mirror critical angle deviation direction disc distance edge equal equation Expt falls Find the position focal length front gas-flame grease-spot Hence hole illuminating power image and object image is formed image produced inches incident ray index of refraction lamp lantern lenses light diverge light is reflected light travels luminous point magnified measured moves normal object is placed observer looking optical centre paper path pencil penumbra plane mirror plate point of incidence principal axis principal focus prism radius of curvature ray of light real image reflected back refracted ray relative sizes screen secondary axis secondary image seen shadow sizes of image slit source of light spectrum standard candle straight line suppose surface total reflection vertical virtual image