Why isn’t it possible to measure the thermal energy of a substance using a thermometer

This is because a thermometer is only meant for measuring temperature and temperature is not a measure of thermal energy

Explanation:

The reason is because a thermometer is designed and calibrated to measure the temperature of a body.

Although the thermal energy of a body is somehow related to its temperature, the thermal energy of a body is not entirely dependent on the temperature of the body

The thermal energy of a body deals with its heat. While thermal energy basically deals with the amount of heat, temperature deals with the flow from one point to another along the body

So what this means is that since thermometer measures this flow, it cannot be used to ascertain the level of heat ( thermal energy) in the body

54 degrees incident angle = reflected angle

angle is from normal turned to line of ray