Scattering Of Light

Step 1: Understanding the Question:
We need to evaluate the truthfulness of an Assertion and a Reason related to the scattering of light, and then determine if the Reason correctly explains the Assertion.
Step 2: Detailed Explanation:
Analysis of Assertion (A):
"Tiny air molecules scatter blue light more than red light." This phenomenon is known as Rayleigh scattering. According to Rayleigh's law of scattering, the intensity of scattered light ( ) is inversely proportional to the fourth power of the wavelength ( ) of the light, provided the scatterer's size is much smaller than the wavelength.
Since blue light has a shorter wavelength ( ), it is scattered much more strongly than red light by the air molecules. Thus, Assertion (A) is true.
Analysis of Reason (R):
"The refractive index of a medium is greater for blue light than red light." This phenomenon is called dispersion. According to Cauchy's relation, the refractive index (n) of a transparent medium depends on the wavelength ( ), approximately as , where A and B are constants. Since , the refractive index for blue light ( ) is greater than for red light ( ). Thus, Reason (R) is true.
Relating Reason and Assertion:
The scattering of light by molecules is due to the absorption and re-radiation of light by the electrons in the molecules. The molecules behave like oscillating dipoles under the influence of the light's electric field. The polarizability of the molecules, which determines how strongly they interact with light, is related to the refractive index of the medium. A higher refractive index (as for blue light) implies a stronger interaction between light and the medium, leading to more scattering. Therefore, the fact that the refractive index is wavelength-dependent is the underlying physical cause for Rayleigh scattering. Hence, (R) is the correct explanation for (A).
Step 3: Final Answer:
Both Assertion (A) and Reason (R) are true statements, and the Reason provides the correct physical explanation for the Assertion.

Step 1: Understanding the Question:
The question describes an experiment where a thermometer shows a temperature increase when placed in the region just beyond the red end of the visible light spectrum. We need to identify the type of electromagnetic radiation responsible for this heating effect.
Step 2: Detailed Explanation:
The visible spectrum of light consists of colors arranged from violet to red (VIBGYOR).
The region of the electromagnetic spectrum just beyond the red light has wavelengths longer than red light. This region is called the infrared region.
Infrared (IR) radiations are primarily responsible for the heating effect of sunlight. They are readily absorbed by most materials, causing an increase in the internal energy and temperature of the material.
A blackened bulb is used because black surfaces are good absorbers of all types of radiation, making the temperature change more noticeable.
Ultraviolet (UV) radiation is found beyond the violet end of the spectrum and has more energy per photon, but infrared radiation carries more of the sun's heat energy. X-rays and Radio waves are other parts of the electromagnetic spectrum, but they are not located adjacent to the red region and are not the primary cause of heating in this experiment.
Step 3: Final Answer:
The rapid rise in temperature beyond the red region is caused by the absorption of Infrared radiations.