# Photons - Trying to find distance

• Nitric
In summary, to detect a 60-W lightbulb, the night-adapted eye would need to be at a distance where it can receive at least 5 photons per second. This can be calculated using the equation for the number of photons hitting the eye, which includes the distance. By choosing a frequency near the low end of the visible range, the energy efficiency of the bulb and the energy of the photon can be used to determine the number of photons emitted per second. Then, by considering how the photons spread out in space and the number of photons hitting the eye, the distance to the lightbulb can be solved for. This problem-solving approach, where an unknown quantity is used to solve for other related quantities, is a
Nitric
1. The night-adapted eye can detect as few as several photons per second -- say five to be definite. Assuming that the pupil of the eye is 0.6 cm in diameter, from what distance would it be possible for the naked eye to detect a 60-W lightbulb?

2. I'm not sure which equation to use. In the section where the problem is from, I get teh follow equations (which don't help me)
E=pc, E=hf, h=6.63x10^-34, p=h/lamda

3. Not sure which formula to use

Well, you will need to know how many photons the light emits per second. You can do this by picking a frequency near the low end of the visible range (to end up with the minimum), and using the energy of the bulb and the energy of the photon. Guess or look up the energy efficiency of a light bulb.

The next thing is how the photons from the bulb spread out in space. They go out in all directions equally, so light up an imaginary sphere at the distance where the eye is. The question is what portion of that lit up area is caught by the eye.

It says 5 photons/sec in the question.

Yes, you know the 5 per sec answer so you can use the expression for the # of photons hitting the eye - which has a d in it - to solve for the distance.

Physics often works that way. You pretend you know something like the distance to the source, work out other quantities based on that unknown, and when you find one you know the answer for, you can solve for the unknown. Very useful trick!

## 1. What is a photon?

A photon is a fundamental particle of light that carries energy and has no mass. It behaves both as a wave and a particle.

## 2. How do we measure the distance of a photon?

The distance of a photon can be measured using the speed of light and the time it takes for the photon to travel from one point to another. The formula for distance is d = ct, where c is the speed of light and t is the time it takes for the photon to travel.

## 3. Can photons travel an infinite distance?

Yes, photons have been observed to travel extremely long distances in space. However, they can be absorbed or scattered by particles in their path, which affects their distance traveled.

## 4. How do we use photons to measure distances in space?

Scientists use a method called parallax, which involves measuring the slight shift in the position of a distant object when viewed from two different positions. The distance can then be calculated using trigonometry and the known distance between the two viewing points.

## 5. Can we use photons to measure distances on Earth?

Yes, photons can be used for distance measurements on Earth using a variety of methods such as radar, lidar, and GPS. These methods use the time it takes for photons to travel and bounce back to determine the distance to an object.

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