Thread: RELATED QUESTION TO MY LIGHT QUESTION BELOW

Okay, light travels at approx 186,000 miles per second. At this moment in time to our knowledge, there is no evidence that anything travels faster than light. There might be theories that suggest there may well be something's that do travel faster than light but than remains to be proven.

So, I understand that the speed of light is the same regardless of its source. My understanding is that the light from a torch travels the same speed as the light from a distant star, am I correct ?

My other question concerns acceleration of light. Once light leaves its source does it have an acceleration speed ? If so, is there anything that can slow this acceleration down ?

BARCUD

Originally Posted by BARCUD

Okay, light travels at approx 186,000 miles per second. At this moment in time to our knowledge, there is no evidence that anything travels faster than light. There might be theories that suggest there may well be something's that do travel faster than light but than remains to be proven.

Exactly. All such 'theories' are currently, scientifically speaking, more speculations than anything. They aren't even necessarily hypotheses because I do not believe any of them is currently claimed to be testable.

Originally Posted by BARCUD

So, I understand that the speed of light is the same regardless of its source. My understanding is that the light from a torch travels the same speed as the light from a distant star, am I correct ?

Indeed. Light, in terms of wavelength, may bear signatures of its source (a stellar spectrum, for instance, can be interpreted in terms of the composition of that body and even, given Hubble's constant, its distance from us and it speed, relative to us), but an individual photon (you'll have to live with particle-wave duality here) can give no clue as to its origin - it is simply a photon and it travels at a photon's speed.

Bear in mind that, in any given medium, sound waves travel at the same speed, whether they are generated by a child's whisper or a bear's roar. Only their intensity and frequency will vary. This is a standard characteristic of waves in general.

Originally Posted by BARCUD

My other question concerns acceleration of light. Once light leaves its source does it have an acceleration speed ? If so, is there anything that can slow this acceleration down ?

BARCUD

Light does not, strictly speaking, accelerate or decelerate - a photon is created and absorbed/destroyed while travelling at the speed of light, instantaneously. This is possible, of course, because a photon has no rest mass, and would not be possible with any body (particle or otherwise) that had a rest mass.

Great ! Nicely explained. A couple more questions if I may.

Intensity ? Is it the number of photons that determine a lights intensity ? By intensity I mean, a torch light with a low battery compared to a torch light with a full battery. Will the full battery produce more photons ?

Distance ? What determines how far a light can actually travel ? Again using a torch as an example, what determines just how far the light beam travels ?

Energy loss ? I've understood that the speed of light travels at a maximum speed of 186,000 mps, it doesn't accelerate beyond this speed. I've noticed that a squash ball or ping pong ball accelerate after hitting a wall or surface so this collision must create more energy for the balls to speed up BUT what happens to light when it hits a surface ? It doesn't gain speed, so I guess it loses energy ? Or maybe I'm getting the relationship between energy and speed/acceleration wrong ?

Depending on the answer to that last question, what happens to light when it is directed at a mirror ?

BARCUD

Originally Posted by BARCUD

Intensity ? Is it the number of photons that determine a lights intensity ? By intensity I mean, a torch light with a low battery compared to a torch light with a full battery. Will the full battery produce more photons ?

By intensity I mean the energy contained in the 'beam' divided by the area of the beam.

If we are talking about photons, and (to keep things simple) assuming that they are all of the same wavelength (say a specific hue of red), then the intensity is determined by the number of photons in the beam. Each photon (at this particular colour) has exactly the same energy, and this figure is fixed and unchangeable - it is part of the definition of this particular colour.

If you think of it in terms of waves, then the wave frequency is fixed (hence, given that its speed is fixed, so is its wavelength), and the intensity is a function of this frequency and the amplitude of the wave.

Ergo, the amplitude of light as a wave is proportional to the number of photons in the beam under consideration.

Originally Posted by BARCUD

Distance ? What determines how far a light can actually travel ? Again using a torch as an example, what determines just how far the light beam travels ?

There is no theoretical limit. If you consider a vacuum, it is entirely possible to imagine a single photon travelling for all eternity.

In practical terms, however, when talking about your torch example, most of the photons will get absorbed by the atmosphere into which the beam is being shone.

Since a torch beam is not coherent, no matter how well focussed the beam, it will diverge and the photons, if any, that do not interact with the atmosphere and escape into space will in general be so widely spread that they will not be capable of being identified as part of that original beam in general. But again, while you may not see these individual photons going too far, and therefore have an effective limit to the distance of your torch beam (500 metres? 1,000?), there is no reason why some individual photon from your torch, as you wave it at the sky, should not have started on a multiple year journey that will take it past Polaris - to infinity and beyond!

Originally Posted by BARCUD

Energy loss ? I've understood that the speed of light travels at a maximum speed of 186,000 mps, it doesn't accelerate beyond this speed. I've noticed that a squash ball or ping pong ball accelerate after hitting a wall or surface so this collision must create more energy for the balls to speed up BUT what happens to light when it hits a surface ? It doesn't gain speed, so I guess it loses energy ? Or maybe I'm getting the relationship between energy and speed/acceleration wrong ?

When light hits a surface the interaction is not like that of a squash ball or a ping-pong ball on a surface.

While are taught to imagine reflection in that way in school, it is as much of a simplification as the planetary model of atomic structure.

We genuinely do not have human-scale analogues for quantum interactions and would be well advised to try as far as possible to forget the human-scale conceptualisation we're doing.

The best description of reflection, refraction etc that I've seen was on this very forum, but some months ago, so I can't be bothered to try to find it - but it was Janus or some such proper scientist who stated it:

Basically, the light photon is absorbed (and therefore instantaneously disappears - no deceleration involved), giving the atom/particle in question a fixed amount of energy that itself, nigh instantaneously, is discharged (with, if you will, perfect energy efficiency - no entropy here) to give off an 'identical' photon (created instantaneously, at the speed of light, so no acceleration involved).

The direction taken by this discharged photon will depend upon the circumstances but in a mirror, as we observe, it will tend to be on the standard angle of reflection that we all studied in physics in school.

And no, I do not know, and could not explain, exactly why this is so.

Originally Posted by BARCUD

Depending on the answer to that last question, what happens to light when it is directed at a mirror ?

BARCUD

See above!

cheer

shanks

Thanks for that Shanks, much appreciated.

With someone like me, outside of physics, some things are just impossible to visualise in the minds eye. Constant questioning does help and I guess the minds eye improves over time.

BARCUD

Originally Posted by BARCUD

My understanding is that the light from a torch travels the same speed as the light from a distant star, am I correct ?

Yes and no. Yes in the sense that light is light whether originating from a star or torch. No in the sense that the speed of light is only constant in a particular medium. Light in one medium however, may be faster or slower than light in another medium.

Originally Posted by BARCUD

My other question concerns acceleration of light. Once light leaves its source does it have an acceleration speed ? If so, is there anything that can slow this acceleration down ?
BARCUD

While light doesn't accelerate, it can change speeds. If it travels through water, it travels slower than it does through a vacuum. This is called the refractive index of a substance. Note, however, that the cosmic speed of light limit is defined as the speed of light in a vacuum, so if your light beam is moving through a substance with crazy high refractive index, you could actually outrun the light beam on foot without any relativity type effects or time travel or anything like that.

So, I understand that the speed of light is the same regardless of its source. My understanding is that the light from a torch travels the same speed as the light from a distant star, am I correct ?

Correct, although I believe it can be slowed down by refraction. The speed is the same only in a vacuum. In denser or rarer mediums,say water, the speed of light does change.

My other question concerns acceleration of light. Once light leaves its source does it have an acceleration speed ? If so, is there anything that can slow this acceleration down ?

Only if you are in a denser medium can you hope to slow incoming light. In a vacum, there is no way to accelerate or decelerate light.

These are intelligent questions, I must say. Most would refer to Einstein's special relativity theory and say that the speed of light is the same, forgetting refraction completely.

Originally Posted by Liongold

So, I understand that the speed of light is the same regardless of its source. My understanding is that the light from a torch travels the same speed as the light from a distant star, am I correct ?

Correct, although I believe it can be slowed down by refraction. The speed is the same only in a vacuum. In denser or rarer mediums,say water, the speed of light does change.

My other question concerns acceleration of light. Once light leaves its source does it have an acceleration speed ? If so, is there anything that can slow this acceleration down ?

Only if you are in a denser medium can you hope to slow incoming light. In a vacum, there is no way to accelerate or decelerate light.

These are intelligent questions, I must say. Most would refer to Einstein's special relativity theory and say that the speed of light is the same, forgetting refraction completely.

Actually photons always travel at c, the speed of light in a vacuum. Even in a medium.

What happens in a medium is that what is perceived as the speed of light is a series of movements by photons, alternately traveling at c, interacting with electrons, being absorbed and being re-emitted. The net effect of those interactions is to reduce the apparent speed of light to something below c, and that results in the refractive index. But "between atoms" the photons travel through the vacuum at c.

You can also explain this in terms of classical electrodynamics, and the effect of materials on the macroscopic electric and magnetic flux density. For a discussion from this perspective you might see Classical Electrodynamics by Jackson.

The question was raised earlier why photons are reflected from a mirror with the angle of reflection being equal to the angle of incidence. That is simply a reflection of the conservation of momentum, in the direction parallel to the plane of the mirror.

Originally Posted by BARCUD

I've noticed that a squash ball or ping pong ball accelerate after hitting a wall or surface so this collision must create more energy for the balls to speed up

While off topic, I wanted to set this straight.
1) The ball cannot accelerate after hitting a wall, it can only accelerate during the collision. (assuming there is no interaction with the air, but in that case the change of motion has little to do with the collision)
2) the collision does not create energy. If the ball speeds up during the collision, either the ball or the surface has to provide this energy. Generally the surface is passive, so it can't provide energy. The ball could be spinning before the collision, and if the ball stops spinning due to the collision this "spinning energy" could result in an acceleration of the ball.

None of this has anything to do with light, though.