You are here:

Physics/Regarding light's speed



What is the speed of light that is emitted by a torch light. I don't see the light emitted by a torch light after a few meters. Does the speed of light depend on the luminosity of the object? Scientists say that the speed of light is close to 3,00,000m/s, what are the conditions to have that speed? I mean, what medium and what object would give light matching to that speed?

Also why the speed of light is close to a whole number(3,00,000), is it accidental and co-incidence?


ANSWER: Q1: Speed of light coming from a torch is the same as all other light in air, about 299 million meters/second.

Q2: Generally the speed of light does not depend on the luminosity of the source.  The exception is for low intensity sources (single photon sources) where quantum effects change things a little and very bright sources (powerful lasers, high energy physics) where the light distorts the medium it is passing through.

Q3: The speed of light is dependent on the media you are passing through.  Light is fastest in a vacuum and slower through more dense media.  For example, light moves about 300 million meters/second in a vacuum and closer to 299 million meters/second in air. In glass it travels about half that speed (150 million meters/second).  There is a particular property which refers to the speed at which light moves through an object: the refractive index.  You can read more on this at

Q4: There is not a reason for the speed of light to be a whole number.  Basically, the meter is an arbitrary unit of distance and the second is an arbitrary unit of time.  This means that the number we arrive at for any speed is just a description of what we observe in arbitrarily selected units.  

I hope this helps!

Take care.

---------- FOLLOW-UP ----------

QUESTION: Thanks for your reply. If speed of light from a torch light is 299 million meters/second, why do we not see the light travelling that much distance. If I light a torch in a dark area, I see that it lights up only few meters and beyond a particular distance, I see the area to be dark.

I read that even if an object carrying light moves, the speed of the light will still remain as c. Suppose if the object moves at the speed of 0.5c, does the relative speed with respect to the stationary observer is still c? Is this scientifically measured and proved? If it is true that the speed of the light is constant even if the object moves, does science have explanation why this phenomenon is unique to light.


ANSWER: The concept that is important is to understand that having a speed does not tell us how far the light goes, just how fast.  Also, don't trust your eyes here - only the light that bounces off of something AND makes it back to your eyes can be seen.

An analogy would be firing a bullet at a metal wall - the bullet may bounce off (reflection), glace off at an odd angle (scattering), pass into the metal (at a slower speed) and become part of the wall (absorption).  Reflection, scattering, refraction and absorption all reduce how far the light will go - descriptions on wikipedia of these effects on light should be sufficient.

For your second set of questions things are trickier: in short, yes, the outside observer sees light travelling at C.. though at a different wavelength (this effect is called red or blue shifting and is a common method in astronomy to detect the speed of very fast interplanetary objects). Yes, it is proven and used in applications like radar and satellite/global positioning systems every day.  For your final question - basically, everything in our universe is either an electromagnetic photon (light) or made out of subatomic particles (matter).  There really isn't anything else.  Matter follows one set of rules and light follows another based on their intrinsic nature.
However, as a side note, the rules for light and matter look very similar at extremes: very dense light acts like matter somewhat and very small pieces of matter (single atoms and electrons) act quite a bit like light.  Note that the term light does not just apply to what you can see - light includes gamma rays, x-rays, ultraviolet light, visible light, infrared light, microwave radiation and radio waves.  

---------- FOLLOW-UP ----------

QUESTION: Thanks for your time and reply. Human body is composed of cells. Are the cells composed of atoms? For my final question, you wrote that light follows its intrinsic nature. I was asking why the speed of the light is constant even when the object moves. Is it because of light's intrinsic nature or any other explanation available in science? Also light is an electromagnetic waves, hence aren't the electrons more basic than light?

Also I read that quantum process slows down when the object travels. Does science have any explanation for this?


Q1: Yes, humans (and cells) are entirely made of atoms.

An aside on 'scientific explanation': You continue to reference an concept of 'scientific explanation'.  Science, in isolation, explains nothing.  It is merely a philosophy for observing the universe, testing it and predicting what it will do.  Any explanation that occurs as a result of the application of the philosophy (science) is the result of interpretation (human element) of the findings which emerge.  There is nothing in philosophy of science or the application of the scientific method that explains anything.  The distinction may be subtle, but it is important to note before I continue.

Q2: The speed of light is constant due to the universe existing in both time and space - one of the most common models for the universe include the universe acting as a medium (of very very small discrete units - google: Planck scale).  In this medium, there is a maximum rate that an object can relocate between units.  Photons (light) are defined as the particles that move at this maximum rate. So, by this description, a particle being called 'light' is being called such due to an emergent property - light does not have a particular speed, rather we define something which can move at the maximum speed that the universe allows as light.

Q3: If you google Standard Model of particle physics, you will quickly see that photons and electrons are on similar footing. Oddly, protons and neutrons are more complex and comprised of collections of other particles.   Of note: current estimations indicate that there are 1,000,000,000 times more photons in the universe all other particles combined.

Q4: All material appears to function like both a particle and wave (eg quantum effect) the wavelike oscillations becoming trivial/unobservable in large objects.  However, this effect can be seen and understood in smaller objects (like electrons). Without a very strong math background, there is not an easy way to discuss the effect (special relativity) in a holistic manner - though an analogy may help. In short, if you think of the energy for particle like movement being one pot and the energy of wave like oscillation being another, then as you go faster the particle like movement requires more energy,  reducing the energy for oscillation and in turn reducing the rate of the oscillation.

Best regards!


All Answers

Answers by Expert:

Ask Experts


Dr. Jeffery Raymond


Materials chemistry. Materials science. Spectroscopy. Polymer science. Physical Chemistry. General Physics. Technical writing. General Applied Mathematics. Nanomaterials. Optoelectronic Behavior. Science Policy.


Teaching: General Inorganic Chemistry I & II, Organic Chemistry I & II, Physical Chemistry I, Polymeric Materials, General Physics I, Calculus I & II
My prior experience includes the United States Army and three years as a development chemist in industry. Currently I am the Assistant Director of the Laboratory for Synthetic Biological Interactions. All told, 13 years of experience in research, development and science education.

Texas A&M University, American Chemical Society, POLY-ACS, SPIE

Journal of the American Chemical Society, Nanoletters, Journal of Physical Chemistry C, Journal of Physical Chemistry Letters, Ultramicroscopy Proceedings of SPIE, Proceedings of MRS, Polymer News, Chemical and Engineering News, Nano Letters, Small,, Angewandte

PhD Macromolecular Science and Engineering (Photophysics/Nanomaterials Concentration), MS Materials Science, BS Chemistry and Physics, Graduate Certificate in Science Policy, AAS Chemical Technology, AAS Engineering Technology

©2016 All rights reserved.