Nuclei — Physics STD 12 Science — Question

Myopia is due to

(a) Elongation of eye ball	(b) Irregular change in focal length
(c) Shortening of eye ball	(d) Older age

A compound lens has been manufactured by combining lenses of power +2.50 D and -3.75 D . The value of its focal length in cm will be :

The contribution in the total current flowing through a semiconductor due to electrons and holes are   and   respectively. If the drift velocity of electrons is   times that of holes at this temperature, then the ratio of concentration of electrons and holes is

(a) 6 : 5

(b) 5 : 6

(c) 3 : 2

(d) 2 : 3

The energy required to charge a capacitor of 5μF  by connecting a d.c. source of 20 kV is

(a) 10 kJ

(b) 5 kJ

(c) 2 kJ

(d) 1 kJ

The power rating of an electric motor which draws a current  of 3.75 amperes when operated at 200 V is about

(a) 1 H.P.

(b) 500 W

(c) 54 W

(d) 750 H.P.

When a thin metal plate is placed in the path of one of the interfering beams of light

(a) Fringe width increases	(b) Fringes disappear
(c) Fringes become brighter	(d) Fringes becomes blurred

Assertion  :   Electric lines of force cross each other.

Reason      : Electric field at a point superimpose to give one resultant electric field.

(a) If both assertion and reason are true and the reason is the correct explanation of the assertion.

(b) If both assertion and reason are true but reason is not the correct explanation of the assertion.

(c) If assertion is true but reason is false.

(d) If assertion is false but reason is true.