If the wavelength of incident light changes from 400 nm to 300 nm

Subject

Physics

Class

JEE Class 12

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 Multiple Choice QuestionsMultiple Choice Questions

61.

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


62.

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


63.

The rest mass of photon is

  • hvc

  • hvc2

  • hcλ

  • zero


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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


A.

1.0 V greater

E = hcλ   E  1λ  E'E = 400300 = 1.33

But E = eVs, Vs being stopping potential. Thus, stopping potential for photoelectrons from a surface becomes approximately 1.0 V greater. 


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65.

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


66.

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

  • 12

  • 2

  • 14

  • 1


67.

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


68.

Let ne and nh represent the number density of electrons and holes in a semiconductor. Then

  • ne > nh  if the semiconductor is intrinsic 

  • ne < nh  if the semiconductor is intrinsic 

  • ne ≠ nh, if the semiconductor is intrinsic 

  • ne = nh, if the semiconductor is intrinsic 


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69.

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


70.

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.


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