Stone can undergo a process called sublimation, where it transitions directly from a solid to a gas without passing through a liquid state. However, this process is typically only observed under extreme conditions such as high temperatures and low pressures. On the other hand, plastic does not typically sublime as it tends to melt and degrade when exposed to high temperatures. **

Iodine sublimes at room temperature because its vapor pressure is high enough to allow it to transition directly from a solid to a gas without passing through the liquid phase. This is due to the weak intermolecular forces between iodine molecules, which allows them to easily break free from the solid lattice and transition into the gas phase. As a result, iodine can sublime at room temperature without needing to be heated to its melting point. **

Self-luminous bodies are objects that emit their own light, as opposed to reflecting light from another source. Examples of self-luminous bodies include stars, the sun, and certain types of light bulbs. These objects generate their own energy through processes like nuclear fusion or electrical current, which produces light as a byproduct. **

No, the luminous clock is not radioactive. The luminescence in the clock is typically achieved using a phosphorescent material that absorbs light and then slowly releases it over time. This process does not involve any radioactive materials. The glow from the clock is simply a result of this light-absorption and emission process. **

A luminous point is a theoretical concept used in physics and optics to represent a source of light that emits light in all directions. It is often used as a simplification in the study of light and its behavior, allowing for easier calculations and analysis. In reality, a luminous point does not exist as a physical object, but it is a useful tool for understanding the behavior of light in various systems. **

The luminous intensity is specified for reflector lamps because it measures the amount of light emitted in a specific direction, which is important for directional lighting applications such as spotlights and floodlights. Reflectors are designed to control and direct light in a specific direction, so specifying luminous intensity helps to ensure that the lamp is suitable for its intended purpose. Luminous flux, on the other hand, measures the total amount of visible light emitted by a source in all directions, which may not be as relevant for reflector lamps where the focus is on directing light in a specific direction. **

In physics, a self-luminous object is one that produces its own light, such as the sun or a light bulb. These objects emit light due to internal processes, such as nuclear fusion or electrical excitation. On the other hand, an externally luminous object is one that reflects light from another source, such as the moon or a mirror. These objects do not produce their own light but appear illuminated due to the light they reflect from another source. **

Luminous flux is calculated by multiplying the illuminance (in lux) by the area over which the light is distributed. The formula for calculating luminous flux is: Luminous flux (in lumens) = illuminance (in lux) × area (in square meters) This formula takes into account the intensity of the light (illuminance) and the size of the area over which the light is spread to determine the total luminous flux emitted by the light source. **