The power of the combination of a convex lens of focal length 50 cm and concave lens of focal length 40 cm is

• + 1 D

• − 1 D

• − 0.5 D

• + 0.5 D

Image formed by a convex lens is virtual and erect when the object is placed

• at F

• between F and the lens

• at 2 F

• beyond 2 F

The rest mass of photon is

• $\frac{\mathrm{hv}}{\mathrm{c}}$

• $\frac{\mathrm{hv}}{{\mathrm{c}}^2}$

• $\frac{\mathrm{hc}}{\mathrm{\lambda }}$

• zero

64.

If the wavelength of incident light changes from 400 nm to 300 nm, the stopping potential for photoelectrons emitted from a surface becomes approximately

• 1.0 V greater

• 1.0 V smaller

• 0.5 V greater

• 0.5 V smaller

Let the potential energy of hydrogen atom in the ground state be regarded as zero. Then its potential energy in the first excited state will be

• 20.4 eV

• 13.6 eV 

• 10.2 eV

• 6.8 eV

Two radioactive nuclides x and y have half-lives 1 h and 2 h respectively. Initially the samples have equal number of nuclei. After 4 h the ratio of the numbers of x and y is

• $\frac{1}{2}$

• 2

• $\frac{1}{4}$

• 1

₉₂U²³⁸ decays successively to form ₉₀Th²³⁴, ₉₁Pa²³⁴, ₉₂U²³⁴, ₉₀Th²³⁰, ₈₈Ra²²⁶ then during the reaction the number of α-particles emitted is

• 4

• 3

• 5

• 2

Let n_e and n_h represent the number density of electrons and holes in a semiconductor. Then

• n_e > n_h  if the semiconductor is intrinsic 

• n_e < n_h  if the semiconductor is intrinsic 

• n_e ≠ n_h, if the semiconductor is intrinsic 

• n_e = n_h, if the semiconductor is intrinsic 

In a n-p-n transistor amplifier, the collector current is 9 mA. If 90% of the electrons from the emitter reach the collector, then

• α = 0.9, β = 9.0

• the base current is 10 mA

• the emitter current is 1 mA

• α = 9.0, β = 0.9

In a properly biased transistor

• both depletion layers are equally large

• both depletion layers are equally small

• emitter-base depletion layer is large but base-collector depletion layer is small

• emitter-base depletion layer is small but base-collector depletion layer is large.