# What happens to the freq of a photon after encountering double slit?

• San K
In summary: Um, I don't think this is right. You are right in pointing out that a mirror should "recoil" when it reflects a photon, but in a Mach-Zehnder interferometer, we treat the mirrors as rigidly fixed with one another so there is net cancellation of the forces acting on the overall mirror apparatus. For example, if we shoot a laser into a half-silvered mirror at the top left corner of a Mach-Zehnder interferometer, the recoil on the top right mirror would be equal and opposite to the recoil on the bottom left mirror, and if we rigidly attach them, then the recoils cancel out.This is a sort of funny way to describe what happens since

#### San K

1. (How) Is the frequency of a photon effected after encountering a double-slit or a half silvered mirror (say in a Mach-zehnder)?

2. Are particles such as photon, electron, assumed to be entangled, at all points in time-space, with something or the other?

1. Photon frequency is not changed in double slit experiment, nor during reflection.

2. Particles are entangled with other particles, if they interacted.

mpv_plate said:
1. Photon frequency is not changed in double slit experiment, nor during reflection.

2. Particles are entangled with other particles, if they interacted.

Thanks mpv.

So, thus the energy of the photon is same throughout? Prior and post double-slit or mach-zehnder

For all practical purposes, yes.

If you want to get really really really really picky about it, when a photon changes direction and therefore changes momentum, the apparatus must recoil in order to conserve momentum. The apparatus therefore gains some kinetic energy, which must come at the expense of some of the photon's energy. Therefore the photon's energy must be very very very very slightly less after reflection. But this is negligible for all practical purposes. If anyone has a reference to experimental results that actually observe this difference, please post it!

I mention this only because based on previous experience here, there is about a 50% probability that someone will bring this up anyway.

jtbell said:
For all practical purposes, yes.

If you want to get really really really really picky about it, when a photon changes direction and therefore changes momentum, the apparatus must recoil in order to conserve momentum. The apparatus therefore gains some kinetic energy, which must come at the expense of some of the photon's energy. Therefore the photon's energy must be very very very very slightly less after reflection. But this is negligible for all practical purposes. If anyone has a reference to experimental results that actually observe this difference, please post it!

I mention this only because based on previous experience here, there is about a 50% probability that someone will bring this up anyway.

Um, I don't think this is right. You are right in pointing out that a mirror should "recoil" when it reflects a photon, but in a Mach-Zehnder interferometer, we treat the mirrors as rigidly fixed with one another so there is net cancellation of the forces acting on the overall mirror apparatus. For example, if we shoot a laser into a half-silvered mirror at the top left corner of a Mach-Zehnder interferometer, the recoil on the top right mirror would be equal and opposite to the recoil on the bottom left mirror, and if we rigidly attach them, then the recoils cancel out.

This is a sort of funny way to describe what happens since the mirrors aren't perfectly rigidly attached (in reality they are attached to a lab bench which is in principle somewhat flexible), but if we use your suggestion, then a mach-zehnder interferometer must continually grow (this is silly) or, if you think the mirrors somehow remain stationary while changing the photons' energy, that would be akin to claiming that a driven oscillator will find an equillibrium oscillation with a frequency that is slightly less than the driving frequency. (Of course this is mathematically impossible. http://en.wikipedia.org/wiki/Damped_harmonic_oscillator#Sinusoidal_driving_force) So the better way to think about an MZ is that the mirrors are rigidly attached to one another.

[If you wanted to go one step further in thinking about this, you could think of the flexible lab bench as some sort of spring joining the two mirrors, and the first few photons hitting the mirrors would cause the lab bench to stretch, and after a long enough stream of photons arrives, the lab bench will reach some equilibrium stretching such that it supplies an equal and opposite restoring force on the mirrors. The equilibrium configuration would keep the mirrors stationary, just as if they were rigidly attached.]

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## 1. What is a photon?

A photon is a fundamental particle of light and electromagnetic radiation. It has no mass and travels at the speed of light.

## 2. What is a double slit experiment?

A double slit experiment is a classic experiment in physics that demonstrates the wave-particle duality of light. It involves shining a beam of light through two parallel slits and observing the interference pattern created on a screen.

## 3. How does a photon behave in a double slit experiment?

In a double slit experiment, a photon behaves like a wave, passing through both slits and creating an interference pattern on the screen. However, when observed, it behaves like a particle and only passes through one of the slits.

## 4. What happens to the frequency of a photon after encountering double slits?

The frequency of a photon remains constant after encountering double slits. This is because the frequency of a photon is determined by its energy, which remains unchanged in the experiment.

## 5. Why does the interference pattern in a double slit experiment occur?

The interference pattern in a double slit experiment occurs because the waves of two photons overlap and interfere with each other. This creates regions of constructive and destructive interference on the screen, resulting in the observed pattern.

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