Explain the following :
(a) Role of `Na^(+)` in the gneration of action potential.
(b) Mechanism of generation of light- induced inpulse in the retina .
(c ) Mechanism through which a sound produces a nerve impulse in the inner ear .

The light stricking the retina generates nerve impulse.Which of the following options correctly describes the path of light ?

Following is a list of the events (in a random order) that lead to the formation of an auditory impulse. (i)Vibration is transferred from the malleus to the incus to the stapes. (ii)Basilar membrane moves up and down. (iii)Nerve impulse is transmitted in cochlear nerve to auditory cortex of brain for impulse analysis and recognitions. (iv)Sound waves pass through ear canal. (v)Stereocilia of hair cells of organ of Corti rub against tectorical membrane. (vi)Sound waves cause ear drum to vibrate. (vii)Nerve impulse is generated (viii)Vibrations move from fluid of vestibular canal to the fluid of tympanic canal. (ix)Membrane at oval window vibrates. Which of the following options represents these events in a correct order ?

Mark the correct choice as : (a) Both assertion and reason are true and reason is the correct explanation of assertion. (b) Both assertion and reason are true but reason is not the correct explanation of assertion. (c) Assertion is true but reason is false. (d) Assertion is false but reason is true. (e) Both Assertion and reason are false.Reason : Sound is mechanical wave, which require medium to travel. Assertion : We cannot hear the sound produced by a vibrating pendulum. Reason : The frequency of the pendulum is very less.

A list of steps involved in mechanism of vision is given below in a random order. (i)Neural impulses are analysed and image formed on retina is recognised by visual cortex. (ii)Membrane permeability changes (iii)Ganglion cells are excited (iv)Bipolar cells are depolarised (v)Action potentials (impulse) are transmitted by optic nerves to visual cortex. (vi)Potential differences are generated in the photoreceptor cells. (vii)Light energy causes a change in shape of rhodopsin, leading to the dissociation of retina (an aldehyde of vitamin A ) from opsin (a protein) (viii)Structure of opsin is changed. Which of the following options represents these events in a correct order ?

Which of the following facts explain how shadows are formed?(a)light travel in straight lines (b).Opaque bodies do not allow light to pass through them.(c)Reflection occurs at a smooth surface like mirrors.(d).Lateral inversion happens