The regions of spectrum which do not excite the retina and hence are not visible, are collectively called ………... spectrum.

Options

A. Impure
B. Pure
C. Invisible
D. Sensitive

Explanation

To solve the question, we need to identify the term that describes the regions of the electromagnetic spectrum that do not excite the retina and are therefore not visible to the human eye.

### Step-by-Step Solution:

1. **Understanding the Electromagnetic Spectrum**: The electromagnetic spectrum consists of various types of electromagnetic radiation, which are categorized based on their wavelengths. These include cosmic rays, gamma rays, X-rays, ultraviolet rays, visible light, infrared rays, microwaves, and radio waves.

2. **Identifying the Visible Region**: Among these types, only the visible light region is capable of exciting the retina in our eyes, allowing us to see. The visible spectrum typically ranges from approximately 400 nm (violet) to 700 nm (red).

3. **Recognizing the Non-visible Regions**: The regions of the spectrum that lie outside the visible range (either shorter wavelengths like ultraviolet, X-rays, gamma rays, or longer wavelengths like infrared, microwaves, and radio waves) do not stimulate the retina.

4. **Collective Term for Non-visible Spectrum**: The question asks for a collective term for these non-visible regions. The appropriate term used in physics for these regions is "invisible spectrum."

5. **Final Answer**: Therefore, the regions of the spectrum which do not excite the retina and hence are not visible are collectively called the **invisible spectrum**.