Some laws/processes are given in column-I. Match these with the physical phenomena given in column-II.
`{:(,"Column I",,"Column II"),((A),"Transition between two atomic energy levels".,(p),"Characteristic X-rays"),((B),"Electron emission from a material",(q),"Photoelectric effect"),((C),"Moaley's law",(r),"Hydrogen spectrum"),((D),"Change of photon energy into kinetic energy of electrons",(s),beta-"decay"):}`

Answer

To solve the problem of matching the laws/processes in Column I with the physical phenomena in Column II, we will analyze each item step by step.

### Step 1: Analyze Column I Item A
- **Item A:** Transition between two atomic energy levels.
- **Analysis:** This process is related to the emission or absorption of photons when electrons transition between different energy levels in an atom. This is observed in the hydrogen spectrum, where transitions result in spectral lines.
- **Match:** (A) → (r) Hydrogen spectrum.

### Step 2: Analyze Column I Item B
- **Item B:** Electron emission from a material.
- **Analysis:** This refers to the photoelectric effect, where electrons are emitted from a material (usually a metal) when it is exposed to light of sufficient frequency. The energy of the incoming photons must exceed the work function of the material for electrons to be emitted.
- **Match:** (B) → (q) Photoelectric effect.

### Step 3: Analyze Column I Item C
- **Item C:** Moseley's law.
- **Analysis:** Moseley's law relates the frequency of X-rays emitted by elements to their atomic number. It is used to determine the atomic structure and is specifically applicable to characteristic X-rays.
- **Match:** (C) → (p) Characteristic X-rays.

### Step 4: Analyze Column I Item D
- **Item D:** Change of photon energy into kinetic energy of electrons.
- **Analysis:** This describes the process in the photoelectric effect, where the energy of the incoming photon is converted into the kinetic energy of the emitted electron, minus the work function of the material.
- **Match:** (D) → (q) Photoelectric effect (already matched).

### Step 5: Analyze Column I Item D Again
- **Item D:** Change of photon energy into kinetic energy of electrons.
- **Analysis:** This can also relate to beta decay, where energy is involved in the conversion of particles. However, the primary association is with the photoelectric effect.
- **Match:** (D) → (s) Beta decay (if considering energy conversion in a broader sense).

### Final Matches
- (A) → (r) Hydrogen spectrum
- (B) → (q) Photoelectric effect
- (C) → (p) Characteristic X-rays
- (D) → (s) Beta decay

### Summary of Matches
- A - r
- B - q
- C - p
- D - s