Photolithotrophs are a group of organisms that obtain energy from radiations and derive their hydrogen source from inorganic compounds. This means that they are capable of using light as an energy source and inorganic compounds as a source of hydrogen.

**Explanation:**

1. **Definition of photolithotrophs:**
- Photolithotrophs are a specific type of autotrophic organisms that use light energy to drive their metabolic processes.
- These organisms are capable of converting inorganic substances into organic compounds through the process of photosynthesis.

2. **Obtain energy from radiations:**
- Photolithotrophs are unique in their ability to use light as an energy source.
- They possess pigments, such as chlorophyll, that can absorb light energy and convert it into chemical energy.

3. **Hydrogen source from inorganic compounds:**
- In addition to obtaining energy from light, photolithotrophs also require a source of hydrogen for their metabolic processes.
- Unlike other types of autotrophs (such as chemoautotrophs), photolithotrophs do not obtain hydrogen from organic compounds.
- Instead, they utilize inorganic compounds, such as water (H2O), as a source of hydrogen.

4. **Process of photosynthesis:**
- Photolithotrophs perform photosynthesis, a process that allows them to convert light energy into chemical energy.
- During photosynthesis, light energy is absorbed by pigments (such as chlorophyll) in the organism's cells.
- This energy is then used to split water molecules into hydrogen (H2) and oxygen (O2).
- The hydrogen is used to reduce carbon dioxide (CO2) to produce organic compounds, while oxygen is released as a byproduct.

5. **Examples of photolithotrophs:**
- Photolithotrophs include plants, algae, and some bacteria.
- Plants and algae are able to photosynthesize using chlorophyll, while some bacteria utilize different pigments like bacteriochlorophyll or rhodopsin.

In conclusion, photolithotrophs are organisms that obtain energy from radiations (light) and derive their hydrogen source from inorganic compounds. They are capable of using light energy to drive photosynthesis, converting inorganic substances (such as water) into organic compounds. This ability distinguishes them from other types of autotrophs that obtain energy from different sources, such as organic compounds or inorganic chemicals.

Photolithotrophs used light as energy and inorganic electron donor (like H2, H2S) as hydrogen source. Purple and green sulphur bacteria are examples of photolithotrophs