Question

the themometer is made of glass . glass is a bad conductor of heat . but how does the heat reach to the mercury in the glass bulb.

Answer 1

Bad conductors are those materials which do not allow heat to flow from one point to another point.
Glass is a bad conductor of heat. But if we keep on supplying heat (energy) to the glass the molecular vibration of the molecules in the glass increases and glass is heated up.

In case of thermometer Glass thermometer is protected by metal sheaths. These sheaths may conduct heat to or from the neighborhood of the thermometer bulb and cause to read either high or low.