The waves emitted when a metal target is bombarded with high energy electrons are

Solution

Physics sir · 4 steps

Fast electrons slamming into metal give up keV-sized energy packets, and photons that big are X-rays.

1. What happens. Electrons accelerated through tens of kilovolts hit the target and are stopped suddenly. They also knock out inner-shell electrons of the metal atoms. Both processes release photons.
2. Size of the photon. An electron accelerated through V volts carries energy eV, so the most energetic photon it can produce has λₘᵢₙ = hc/eV.

   λₘᵢₙ = 12400/V Å ⇒ V = 30 kV gives λₘᵢₙ ≈ 0.41 Å

3. Place it. Wavelengths around 0.1 to 10 Å lie in the X-ray band. Infrared, microwaves and radio waves have wavelengths thousands of times longer, so their photons carry far too little energy to come from this process.
4. So. The waves emitted are X-rays, option (d). This is exactly how an X-ray tube (Coolidge tube) works.

The idea to keep. Match the photon energy to the source. Electrons with keV energies give X-rays, while slow molecular vibrations and rotations give infrared and microwaves.

Answer from the NTA final answer key. Solution by PrepWiser.