Ray Optics and Optical Instruments — Physics STD 12 Science — Question

The plates of a capacitor are charged to a potential difference of 320 volts and are then connected across a resistor. The potential difference across the capacitor decays exponentially with time. After 1 second  the potential difference between the plates of the capacitor is 240 volts, then after 2 and 3 seconds the potential difference between the plates will be

(a) 200 and 180 V

(b) 180 and 135 V

(c) 160 and 80 V

(d) 140 and 20 V

Assertion  :  A double convex lens (m = 1.5) has focal length 10 cm. When the lens is immersed in water (m = 4/3) its focal length becomes 40 cm.

Reason      :  

(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 the assertion and reason both are false.

Energy of the electron in n^th orbit of hydrogen atom is given by . The amount of energy needed to transfer electron from first orbit to third orbit is

(a) 13.6 eV

(b) 3.4 eV

(c) 12.09 eV

(d) 1.51 eV

A freshly prepared radioactive sample of half- life 1 hour emits radiations that are 128 times as intense as the permissible safe limit. The minimum time after which this sample can be safely used is

(a) 14 hours

(b) 7 hours

(c) 128 hours

(d) 256 hours

An electron and a proton with equal momentum enter perpendicularly into a uniform magnetic field, then

(a) The path of proton shall be more curved than that of electron

(b) The path of proton shall be less curved than that of electron

(c) Both are equally curved

(d) Path of both will be straight line

Identify the wrong statement in the following. Coulomb's law correctly describes the electric force that

(a) Binds the electrons of an atom to its nucleus

(b) Binds the protons and neutrons in the nucleus of an atom

(c) Binds atoms together to form molecules

(d) Binds atoms and molecules together to form solids

An EM wave of intensity I falls on a surface kept in vacuum and exerts radiation pressure p on it. Which of the following are true?

The distinction between conductors, insulators and semiconductors is largely concerned with:

The correct graph between the maximum energy of a photoelectron and the inverse of wavelength of the incident radiation is given by the curve

(a) A

(b) B

(c) C

(d) None of the above

An AC source is rated 220V, 50Hz. The average voltage is calculated in a time interval of 0.01s, It: