# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: nudephil on 23/10/2020 18:36:12

Title: How do we know the speed of light changes when refracted through a lens?
Post by: nudephil on 23/10/2020 18:36:12
Here's a question sent in by listener Andrew:

Do we have proof that the speed of light changes when light is refracted through a lens?

Title: Re: How do we know the speed of light changes when refracted through a lens?
Post by: Bored chemist on 23/10/2020 20:08:15
It will have been measured but,if you have one of these
https://www.toolstation.com/hand-tools/distance-measures/c991
you can sort of do the experiment yourself.
You can measure the height of a bottle with one of those laser distance measuring devices.
If you measure it full of water you get a different answer to measuring it full of air.

Strictly speaking they don't measure the time it takes the beam to reach the bottom and get back- they do some complicated autocorrelation thing- but the outcomes is the same. They think the bottle is deeper when it is full.
Title: Re: How do we know the speed of light changes when refracted through a lens?
Post by: evan_au on 23/10/2020 22:53:27
Quote from: OP
Do we have proof that the speed of light changes when light is refracted through a lens?
Light acts as a wave.

If you look at other types of waves, you can see the same effect.
- For example, ocean waves slow down when the water depth gets shallower. You can see the waves get refracted towards the beach and around headlands because the velocity is reduced in shallower water.
- Just like light is refracted when it hits a denser medium (eg glass or water)
Title: Re: How do we know the speed of light changes when refracted through a lens?
Post by: hamdani yusuf on 24/10/2020 16:38:54
You can use optical fiber to measure speed of light through materials, using the time required for light to pass through an optical fiber with known length.
just google "measuring speed of light through fiber optic", this is one of the result:
https://i-fiberoptics.com/pdf/if-sl_man.pdf
Title: Re: How do we know the speed of light changes when refracted through a lens?
Post by: Janus on 24/10/2020 17:07:19
Here's a question sent in by listener Andrew:

Do we have proof that the speed of light changes when light is refracted through a lens?

Experiments that measured a difference in the speed of light passing through a medium date back to the mid 1800's.
Title: Re: How do we know the speed of light changes when refracted through a lens?
Post by: Bored chemist on 24/10/2020 17:15:11
I just tried the experiment with a laser distance measuring thingy and a pipe full of water (with an bung in the end)

Apparent length  with air 711 mm
Apparent length with water 923 mm
Actual length 628 mm

The actual length is different from the real length in air- because the gadget adds about 3 inches because it's about 3 inches long.

So, the offset is 711-628 = 83mm
So the "true" measured length in water is  923-83 = 840mm
For a true  length of 628
So the apparent ratio of the velocity of light in air compared to water is 840/628 =1.335
The textbooks tell me the refractive index of water is 1.333

I'd say that's close enough.
Title: Re: How do we know the speed of light changes when refracted through a lens?
Post by: Petrochemicals on 26/10/2020 14:42:15
The speed through materials has been measured with some history, slow enough to run along side of.

https://en.m.wikipedia.org/wiki/Slow_light#:~:text=In%201998%2C%20Danish%20physicist%20Lene,semiconductor%20to%209.7%20kilometers%20per

Quote from: OP
Do we have proof that the speed of light changes when light is refracted through a lens?
Light acts as a wave.
I think you will find it acts as a particle
Title: Re: How do we know the speed of light changes when refracted through a lens?
Post by: Bored chemist on 26/10/2020 14:45:34
I think you will find it acts as a particle
I think you will find that it acts as light; neither a particle nor a wave, but something which approximates to those in some circumstances.
Title: Re: How do we know the speed of light changes when refracted through a lens?
Post by: alancalverd on 26/10/2020 16:04:32
I think you will find it acts as a particle
So why does it speed up on leaving the medium? Every other particle loses energy quasi continuously and exits at the last speed it had in the  medium
Title: Re: How do we know the speed of light changes when refracted through a lens?
Post by: Bored chemist on 26/10/2020 17:40:02
It is remarkable that light has been behaving in the same way for a good 400 years or so (that we know of) and, for all that time it has steadfastly refused to be pigeonholed as a particle or as a wave, yet people are still having arguments about which one it is.
If it was a wave, it wouldn't be quantised but it is quantised therefore it is not a wave.
If it was a particle, irt would not diffract, but it diffracts so it is not a particle.
So, it is not a particle, and it is not a wave.
It's something else.
Specifically, it's light.
Title: Re: How do we know the speed of light changes when refracted through a lens?
Post by: Petrochemicals on 26/10/2020 19:00:01
I think you will find it acts as a particle
So why does it speed up on leaving the medium? Every other particle loses energy quasi continuously and exits at the last speed it had in the  medium
Due to the photons being descrete as seen in the photo electric effect.
Title: Re: How do we know the speed of light changes when refracted through a lens?
Post by: Bored chemist on 26/10/2020 20:20:29
I think you will find it acts as a particle
So why does it speed up on leaving the medium? Every other particle loses energy quasi continuously and exits at the last speed it had in the  medium
Due to the photons being descrete as seen in the photo electric effect.
How do they speed up?
Title: Re: How do we know the speed of light changes when refracted through a lens?
Post by: hamdani yusuf on 03/11/2020 10:08:53
So the apparent ratio of the velocity of light in air compared to water is 840/628 =1.335
The textbooks tell me the refractive index of water is 1.333

I'd say that's close enough.
That's very close indeed, considering that refractive index is affected by temperature and impurities. Did you use pure water? What was the water temperature during the experiment?
Title: Re: How do we know the speed of light changes when refracted through a lens?
Post by: Bored chemist on 03/11/2020 10:21:51
It was tap water and I'm afraid I didn't measure the temperature- probably near 20C
But the variation with temperature is roughly the same as the variation of volume  with temperature- a couple of hundred pppm/ K.
Water is one of the less "expansive" materials. Alcohol, for example, is about 5 times higher.

|I probably "got lucky" with the numbers to some extent but that measuring device is good to a few parts in ten thousand (for measuring actual distances)