51.

If  K₁ and K₂ are maximum kinetic energies of photoelectrons emitted when lights of wavelengths  λ₁ and λ₂ respectively incident on a metallic surface. If  λ₁ = 3λ₂, then

• K₁ > ( K₂ / 3)

• K₁ < ( K₂/3)

• K₁ = 2 K₂

• K₂ = 2K₁

Two radioactive substances A and 8 have decay constants 5A and A respectively. At I =0, they have the same number of nuclei. The ratio of number of nuclei of A to those of 8 will be (1/e² ) after a time

• 4λ

• 2λ

• $\frac{1}{2\mathrm{\lambda }}$

• $\frac{1}{4\mathrm{\lambda }}$

The intensity of gamma radiation from a given source is  I.  On passing through 36 mm of lead, it is reduced to l/8. The thickness of lead which will reduce the intensity to l/2  will be

• 18 mm

• 12 mm

• 6 mm

• 9 mm

If the energy, E = G^p h^q c^r, where G is the universal gravitational constant, h is the Planck's constant, h is the Planck's constant and c is the velocity of light, then the values of p, q and r respectively

• -1/2, 1/2 and 5/2

• 1/2, -1/2 and -5/2

• -1/2, 1/2 and 3/2

• 1/2, -/2 and -3/2

The wavelength of Lyrnen series for first number
is

• $\frac{4 \times 1.097 \times {10}^7}{3}$ m

• $\frac{3}{4 \times 1.097 \times {10}^7}$ m

• $\frac{4}{3 \times 1.097 \times {10}^7}$ m

• $\frac{3}{4}$× 1.097 × 10⁷ m 

Assertion:  A pure semiconductor has negative temperature coefficient of resistance.

Reason:  On raising the temperature, more charge carriers are released, conductance increases and resistance decreases.

• If both assertion and reason are true and reason is the correct explanation of assertion

• If both assertion and reason are true but reason is not the correct explanation of assertion

• If assertion is true but reason is false

• If both assertion and reason are false.

Assertion:  At a fixed temperature, silicon will have a minimum conductivity when it has a smaller acceptor doping.

Reason:  The conductivity of an intrinisic semiconductor is slightly higher than that of a lightly doped p-type

• If both assertion and reason are true and reason is the correct explanation of assertion

• If both assertion and reason are true but reason is not the correct explanation of assertion

• If assertion is true but reason is false

• If both assertion and reason are false.

Assertion:  Communication in UHF/VHF regions can be established by space wave or tropospheric wave. 

Reason:  Communication in UHF/VHF regions is limited to line of sight distance

• If both assertion and reason are true and reason is the correct explanation of assertion

• If both assertion and reason are true but reason is not the correct explanation of assertion

• If assertion is true but reason is false

• If both assertion and reason are false.

Assertion:  If objective and eye lenses of a microscope are interchanged then it can work as telescope. 

Reason:  The objective lens of telescope has  small focal length.

• If both assertion and reason are true and reason is the correct explanation of assertion

• If both assertion and reason are true but reason is not the correct explanation of assertion

• If assertion is true but reason is false

• If both assertion and reason are false.

Assertion:  The de Broglie equation has significance for any microscopic or submicroscopic particles. 

Reason:  The de Broglie wavelength inversely proportional to the mass of the object if velocity is constant.

• If both assertion and reason are true and reason is the correct explanation of assertion.

• If both assertion and reason are true but reason is not the correct explanation of assertion

• If assertion is true but reason is false.

• If both assertion and reason are false.