F
 Friction: Friction is tangential force between the contact surfaces of two bodies. Forced vibration: This is vibration of the body which exists because of exertion of external periodic force constantly on body.  Fictious force (Pseudo force): It is force which does not act on object but appears due to acceleration of non inertial frame of reference. Focaults Pendulum: It is a device to demonstrate the rotation of earth and the fact that each is not an inertial frame of reference. Fermions: They are the particles which obey Fermi-Dirac statistics. Fermi Level: It determines probability of occupancy of allowed energy states. Forward biased diode: The ‘P’ junction of PN diode is kept at high potential and ‘N’ junction at low potential. Feedback: The process of injecting a portion of output energy of some device to the input is known as feedback. Franck & Hertz experiment: It is the experiment which revealed the existence of discrete stationary states of electrons. Fraunhofer lines: When sun’s light is allowed to fall on slit of a spectrometer, its spectrum is found to consist of many dark lines. The lines are called fraunhofer lines. The number of lines is approximately 700. These lines are due to absorption of certain wavelengths in sunlight by gases in sun’s outer sphere.     Fluid: Substances capable of flowing are fluids. They don’t have any fixed shape of their own. They take shape of containing vessel. Face centered cubic crystal structure: The crystal structure found for many metals has a unit cell of cubic geometry, with atoms located at each of the corners and the centers of all the cube faces. Fluid pressure: Normal force per unit area of surface. Surface may be a body immersed in fluid or any imaginary surface with in fluid. Frank- Condon Principle: The principle states that an electronic transition takes place so rapidly that a vibrating molecule does not change its inter nuclear distance appreciably during the transition. That is during an electronic transition, the inter nuclear distance remains the same means straight line representing a transition between electronic states will be vertical.