A floating plant is a type of plant that grows on the surface of water, with its roots hanging down into the water below. These plants do not need to be rooted in soil and instead obtain nutrients and water directly from the water. Floating plants can be found in various aquatic environments such as ponds, lakes, and slow-moving rivers. Some common examples of floating plants include water lilies, duckweed, and water hyacinth. **

Water lettuce (Pistia stratiotes) is a floating plant that forms only short roots. These short roots help anchor the plant in the water but do not extend deeply into the substrate. Water lettuce is commonly found in ponds and lakes, where its floating rosettes of leaves provide habitat for aquatic organisms. **

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. **

Self-luminous celestial bodies are objects in space that emit their own light and heat. This includes stars, which generate energy through nuclear fusion in their cores, as well as other luminous objects such as brown dwarfs and hot gas giants. These bodies are able to shine and be visible from Earth due to the light they produce, unlike non-luminous celestial bodies such as planets and moons, which reflect light from the Sun. **