## Core Concept
Infrared (IR) rays are a form of electromagnetic radiation with wavelengths longer than visible light but shorter than microwaves. They are primarily associated with heat transfer and are used in various medical and industrial applications. The biological effects of infrared radiation are mainly related to its thermal energy.

## Why the Correct Answer is Right
The correct answer, , indicates that infrared rays cause heating of tissues. When infrared radiation is absorbed by tissues, it increases the kinetic energy of the molecules, leading to a rise in temperature. This thermal effect is utilized in physical therapy for pain relief, muscle relaxation, and increasing blood flow to specific areas. The heating effect is due to the absorption of infrared photons by water and other molecules in the tissue, which then undergo rotational and vibrational movements, generating heat.

## Why Each Wrong Option is Incorrect
- **Option A:**
- **Option B:**
- **Option C:**

## Clinical Pearl / High-Yield Fact
A key clinical application of infrared radiation is in ophthalmology, where infrared radiation can cause cataracts and retinal damage, but therapeutic uses include promoting healing and reducing pain and inflammation. A memorable point is that prolonged exposure to high-intensity infrared radiation can lead to thermal burns and eye damage.

**Correct Answer: D. Heating of tissues.**