Tube lights are also called fluorescent lights. Enclosed in long glass cylinders, light is achieved when electrical current is sent through a gas, which is why it is called a "gas-discharged light." Once the current moves through either neon, argon or a similar gas, it becomes ionized. Ionized gas creates ultraviolet light. A 2nd photon emission process converts invisible ultraviolet light into visible fluorescent light. Fluorescent tube lights have been cost effective light "bulbs" since the late 1930s to early 1940s. By the early 1950s, more light was produced in the United States by these types of bulbs than by incandescent bulbs.
Turn off your light. Use your ladder to examine a flickering or dead tube light. Remove any diffuser or thin plastic cover used to hide your bulbs from view. Your existing lamp will have markings on 1 end, indicating the type of bulb that it is. Measure the bulb for length and select the right length/type replacement bulb.
Twist the bulb gently until the pins on the end of the bulb align with the slot and move through the slot. (Most tube lights are bi-pin or 2 pins on each end.)
Pull down gently and remove the lamp. (Often, lamps are installed in pairs or quads. You may choose to replace all bulbs at once since they have the same relative life.)
Place the discarded lamp/s in a safe place away from children, animals or risk of breakage.
Install your tube light by inserting 1 of the 2 pins on the end of the bulb into the downward facing slot in the round aperture (receiving area) of the light fixture. Insert the 2nd end the same way so that the bulb is even and level with the lamp housing or floor.
Twist the bulb evenly, guiding the 2 pins into place in the round aperture.
Turn the light on. (It will take a moment for the new bulbs to fire up.)
Turn the light off once you are satisfied it is working and reinstall you plastic diffuser.