Assertion: If the accelerating potential in an X-ray tube is increased, the wavelengths of the characteristic X-ray do not decrease.
Reason: When an electron beam strikes the target in an X-ray tube, part of Kinetic energy is converted to X-ray energy.
A) Both Assertion and Reason are correct and Reason is the correct explanation of Assertion.
B) Both Assertion and Reason are correct and Reason is not the correct explanation of Assertion.
C) Assertion is correct but the reason is incorrect.
D) Assertion is incorrect but the reason is correct.

Hint: We must know that to accelerate means to speed up. An accelerating potential is basically a type of energy potential imparted to the beam of electrons that gives additional speed to the electrons.

Complete step by step solution:
The wavelength of the X-ray depends on the material of the target properties and it has nothing to do with accelerating potential.
Precisely, the atomic number of the target element depends on the characteristic X-ray wavelength.
Some part of the kinetic energy is changed to X-ray energy when the X-ray tube is struck upon by an electron beam.
Therefore, both the statement and Reason are individually true but Reason is not the explanation of Assertion.

Hence, the correct option is (B).

Additional Information:
Photons are emitted only when an electron moves from one quantum state to another quantum state and energy that is supplied or released is equal to the difference in the energy levels between two quantum shells and that energy is constant and since the energy is constant so the wavelength is also constant. We get electrons by heating metal and a proton by stealing an electron from hydrogen. Accelerating charges develops electromagnetic radiation because changing electric fields produce magnetic fields and changing magnetic fields produce electric fields. And finally, the result leads to electromagnetic radiation .i.e. it means that where changing fields travel.

Note:
The wavelength of the characteristic X-ray does not depend on the accelerating potential. An accelerator gives the velocity to the particle by creating large electric fields that attract or repel the particles.