The value "6500K" written on a 40W tube light refers to the color temperature of the light emitted by the tube. Color temperature is a characteristic used to describe the color appearance of light sources, particularly those that emit white light.
Color temperature is measured in Kelvin (K) and is used to describe the relative "warmth" or "coolness" of light. The higher the color temperature, the cooler or bluer the light appears, while lower color temperatures correspond to warmer or more yellowish light.
In the case of the tube light you mentioned, a color temperature of 6500K indicates that the light emitted by the tube appears cool or bluish-white in color. This color temperature is often associated with daylight or daylight-simulating light sources.
The 40W mentioned on the tube light refers to the power rating or wattage of the light source, indicating the electrical power consumed by the tube to produce light. It provides information about the amount of power the tube light requires to operate effectively.