Final answer:
A photo(option b) tachometer utilizes flashing light to measure the rotational speed of an object. It operates on a non-contact basis and is similar to a car's speedometer in that it measures speed, but instead, it uses light reflections to gauge rotational velocity.
Step-by-step explanation:
A tachometer is a device that is designed to measure the speed of a rotating object, such as a wheel or an engine. In this context, the student is asking about a specific type of tachometer that uses a flashing light to measure the speed of a moving object. The correct answer to the student's question is: A photo tachometer is a device that uses flashing light to measure the speed of a moving object. Similar to the way that Fizeau used a rapidly rotating wheel to measure the speed of light, as depicted in Figure 1.3, a photo tachometer uses light to calculate the rotational speed of an object without the need for physical contact.
For instance, similar to the way a car's speedometer measures its instantaneous speed, a photo tachometer can measure the rotational speed of say, an airplane propeller. It emits a beam of light that reflects off the object in motion. By counting the number of reflected flashes of light per unit time, it can determine the speed of the object's rotation. It is important to note that this device is different from an analog meter, which provides readings as a needle movement over a marked gauge, or a bridge device that is utilized for making null measurements in circuits.
To restate, a photo tachometer is a non-contact device that uses the frequency of light reflected from a rotating surface to measure its speed, which makes it indispensable in various applications, including machinery maintenance and automotive diagnostics. It is the mention of using a flash of light that clues us into the fact that photo is the correct option from the list provided in the student's question.