What does a photometer measure?

• A. The speed of light
• B. The frequency of light
• C. The intensity of light
• D. The wavelength of light

Answer: (C)

A photometer is an instrument specifically designed to measure the intensity of light. It assesses how much light passes through a certain area and provides a quantitative measure of its brightness. In practical applications, photometers can be used to determine the light output of various sources or the illuminance at a specific point in a space.

The other options refer to aspects of light that are not measured by a photometer. For instance, while the speed of light is a fundamental constant known in physics, it doesn't change based on environmental conditions in a way that would require a photometer to measure it. The frequency of light relates to its color and energy, and though frequency is an important characteristic, it is typically determined through other means, such as spectroscopy. Similarly, while wavelength is a crucial property of light, it is usually measured using devices designed for that purpose rather than a photometer. Thus, the focus of a photometer on light intensity makes it the appropriate choice for this question.