Light behaves as both all the time. It doesn't change when it discovers your experiment. It is detected in one place, like a particle, by detectors that measure which place it is in, and it is detected as spread out like a wave by experiments that measure to see if it is spread out (interference experiments). This is shown by experiments like Young's slits. This shows that even one photon is spread out and 'sees' both slits, but the photon is detected by only one detector, at one place. However, if lots of photons are sent (one at a time or all at once) then detectors will detect none, one, or more photons each, at lots of positions, but in an interference pattern that shows that each photon 'saw' both slots and behaved as a wave. If one slot is closed, then the interference pattern goes away.

For most people and most uses of light, only its wave nature is important and observed. We know light has a wave personality because it produces interference. It is only when you may be studying atomic scale interactions where the particle nature reveals itself significantly. We know light has a particle personality because it carries momentum (it can slam into things and move them) and there is a minimum amount of light possible (i.e. a photon).

In photoelectric cells (solar panels) the particle nature of light is very important, because it sets a cut-off frequency (colour) for each type of construction below which (at redder colours than) there is no contribution to the photoelectricity. This was what Einstein got his Nobel prize for explaining. The particle nature of light is also important in biology, chemistry, physics and astronomy because the energy being in lumps is what explains atomic and molecular spectra, and is used in photosynthesis. It is important in the operation of lasers and LEDs

Dear Satish,

Light is nothing but a wave, no particle. A tsunami is nothing but a wave: it transports momentum and energy from the medium (water molecules) from which the wave is fabricated. If the water is flat and calm, there is no wave, and no momentum or energy is transported.

Remark: also particles are nothing but waves: they are "self-trapped" waves into self-closing orbits by a yet undisclosed mechanism, and therefore they can behave as independent local entities. However, even electrons are cut by the two-slit experiment, so that the self-trapped wave-entity is freed and transforms again into 'running' waves that are going through the slits.

Electron-positron pairs do the same: they interfere inside the system so that spontaneous transformation into gamma rays occur.

With best regards,

Thierry De Mees

I have asked about the shape of electron if you coild kindly comment on it.

Dear Satish,

Electron behaves as a point particle in scattering experiments. This has been confirmed many times in many scattering experiments.. Here is a one link ... https://www.physi.uni-heidelberg.de/~reygers/seminars/2015/nobel_prizes_in_particle_physics/talks/schweiger_structure_of_nuclei.pdf

Dear Satish,

Electron is a point particle... Here is a recent proof ... https://www.sciencenews.org/article/electron-shape-round-standard-model-physics?utm_source=quora&utm_medium=referral

https://www.nature.com/articles/s41586-018-0599-8.epdf?referrer_access_token=DsuGdCFY9LUNfk0Z-6IaZNRgN0jAjWel9jnR3ZoTv0PfAsJPWCr3ZaUcxY33i4Ut_fU7xhmKu7oeslGzYzVDTT8da13IcRaySq4SXrdLcVFa5HIwBNoOWEmuUEdQFonhHaS3B1huHRs8W_p2wGadL3yEzzJXS_l6AeNZw8V9IVRsxa3oVw8nb4uLK2xE_RfDE6TOqQxwc3HP5f2wLuTeyPwspkCdUhv04uapExL_XKx-nDe2gEH1XSW9gr68zesx7PsAdku3FqBO5RJ9LIiIv8uKAbfVOkIlSjqGiDFGARMd87eia45DOJ1nho0KYsVj&tracking_referrer=www.sciencenews.org

Electron's mass is known and behaving as particle in cathode ray tube

Dear Gokaran,

I don't agree that an electron is a point particle, and that this would have been proven by measurements. You should provide an official paper that states how the measurements were done and what were the results.

On the contrary, the double slit experiment suggests that electrons are split by the slit into ordinary (electromagnetisc) waves.

Best regards,

Thierry De Mees

Electron and photons both behave in the same way. They behave as particles when detected by point-like detectors, but as waves (more or less localised wave packets, but containing the mass-energy of one particle) when they travel. This is true for every fundamental particle, and some small non-fundamental particles (probably for everything, but undetectable for most large objects).

Don't look for an either/or answer. All fundamental particles are spread out in space-time with a wave nature and can be shown to diffract, so behave like waves. Each particle can only be detected or not detected at any one place, so is either detected to be there, or not, so is detected as if it were a point particle. This is after the wave-function has 'condensed' - before that it can exist in several places at once (it is wave), but hasn't actually been detected.