Infrared Radiation: Understanding Its Properties and Impact

Infrared radiation (IR) is a type of electromagnetic radiation with wavelengths longer than those of visible light. It is often associated with heat, as it is emitted by anything that has temperature. Understanding its properties is crucial in various fields, from climate science to technology.

Infrared Radiation and Earth's Climate System

Let's analyze statement (i): "Infrared radiation plays an important role in keeping the earth cool."

• The Earth absorbs energy from the Sun, primarily in the form of visible light. To maintain a temperature balance, the Earth re-emits this absorbed energy back into space as infrared radiation.
• However, certain gases in the Earth's atmosphere, known as greenhouse gases (like carbon dioxide, methane, and water vapor), absorb some of this outgoing infrared radiation.
• This absorption and subsequent re-emission of infrared radiation by greenhouse gases traps heat within the atmosphere, leading to a warming effect known as the greenhouse effect. This process is essential for keeping the Earth warm enough to support life, but an excess can lead to global warming.
• Therefore, infrared radiation, particularly when trapped by greenhouse gases, contributes to warming the Earth, not keeping it cool. The statement suggesting it keeps the Earth cool is incorrect.

Infrared Rays Emission by Semiconductors

Now, let's examine statement (ii): "Infrared rays are emitted by certain semiconductor light-emitting diodes."

• This statement is correct. Many semiconductor devices, specifically infrared light-emitting diodes (IREDs), are designed to emit infrared radiation.
• These devices are commonly used in various applications such as remote controls for televisions, security cameras (for night vision), fiber optic communication systems, and optical sensors.
• The emission of infrared radiation from these diodes is a fundamental aspect of their operation, converting electrical energy into electromagnetic radiation in the infrared spectrum.

Water Vapour as an Infrared Trapper

Consider statement (iii): "Water vapour is an excellent infrared ray trapper."

• This statement is correct. Water vapor ($\text{H}_2\text{O}$) is one of the most potent and abundant greenhouse gases in Earth's atmosphere.
• It effectively absorbs and re-emits infrared radiation, significantly contributing to the natural greenhouse effect.
• The presence of water vapor in the atmosphere plays a crucial role in regulating Earth's temperature by trapping outgoing longwave (infrared) radiation.

Thermal Motion and Infrared Ray Absorption

Finally, let's evaluate statement (iv): "The thermal motion of the materials decreases after they absorb infrared rays."

• When materials absorb infrared radiation, they absorb energy. This energy increases the internal energy of the atoms and molecules within the material.
• Increased internal energy manifests as increased kinetic energy of the constituent particles. For solids and liquids, this means increased vibrational and rotational motion of molecules and atoms. For gases, it means increased translational motion as well.
• Thermal motion refers to the random movement of atoms and molecules within a substance, which is directly related to its temperature. Therefore, when a material absorbs infrared rays, its temperature generally increases, leading to an increase in the thermal motion of its constituent particles, not a decrease.
• Thus, the statement that thermal motion decreases after absorbing infrared rays is incorrect.

Conclusion on Incorrect Statements

Based on the analysis:

• Statement (i) is incorrect because infrared radiation primarily warms the Earth through the greenhouse effect, rather than cooling it.
• Statement (iv) is incorrect because absorbing infrared radiation increases the internal energy and thus the thermal motion of materials, rather than decreasing it.

Therefore, the statements that are NOT correct are (i) and (iv).

The final answer is Both (i) and (iv).