# Does perpendicular incident light slow down through glass?

1. Mar 16, 2006

### MindReality

Does perpendicular incident light slow down through glass? When light travels through glass at a perpendicular angle, there is no refraction and hence no bending. If it slows down through glass, does that mean that red will emerge first and violet last?

2. Mar 16, 2006

### KingNothing

Yes, it does slow down through glass, albeit very slightly. In reality, the light is being absorbed and re-emitted.

3. Mar 16, 2006

### Mattara

See this thread in the Physics Forums FAQ:

https://www.physicsforums.com/showpost.php?p=899393&postcount=4 [Broken]

Last edited by a moderator: May 2, 2017
4. Mar 17, 2006

### MindReality

Thanks for the link. But that posts doesn't fully answer my question. When incident light hits glass at a perpendicular angle, it does not refract but it still slows down.

So does red exit first before violet at the other end? How will it look like? Because there is no bending and hence no prism effect, right?

5. Mar 17, 2006

### ZapperZ

Staff Emeritus
When "white" visible light hits a glass at an angle, due to the different index of refraction, the "bending" is different for different frequency. Thus, when they exit, they also come out at different angles and seen to be separated.

But when it enters perpendicularly, while each frequency is still slowed down differently, they all still come out "together" and remix to again form a "white" light. Sure, some parts will come out "first", but unless you are able to look at the front end of the beam, you will not notice such differences.

Zz.

6. Mar 18, 2006

### MindReality

Ok thanx, that reply satisfies my question.

Now I've got a second question,

Does light drop to a certain speed instantly when it hits glass and remains at that speed until it exits, or does it become slower and slower while it is travelling through glass?

7. Mar 18, 2006

### ZapperZ

Staff Emeritus
I thought you said that you read the link?

Zz.

8. Mar 19, 2006

### MindReality

So that means light becomes slower and slower until it stops going any further. That explains why light does not reach the bottom of a very deep sea.

Thanx, I overlooked that part. Now it makes perfect sense to me.

So the first color of light you see when you are ascending from deep scuba diving is red?

9. Mar 19, 2006

### ZapperZ

Staff Emeritus
How did you deduce that from reading the passage that you quoted?

All that passage said was that the absorption and re-emission process is the origin of the apparent slowdown. It said nothing about a gradual slowdown. At the simplest case, the change in the group velocity is immediate as soon as light enters the medium.

The reason why light doesn't read the bottom of the sea has nothing to do with the explanation I gave - this is a completely different mechanism involving not only molecular vibration, but also scattering, something that was not covered in the FAQ.

Zz.

10. Mar 21, 2006

### MindReality

"The emitted photon may encounter other lattice ions as it makes its way through the material and this accumulate the delay."

Doesn't this statement signify that the delay is accumalated over time, which means light slows down gradually through a medium?

In a movie I saw that white light from underwater flashlights look red when seen from a certain distance.

11. Mar 21, 2006

### ZapperZ

Staff Emeritus
Accumulating the delay does not mean it slows down gradually. The longer the distance that light has to pass through, the longer it would be delayed when compared with another light that didn't pass through the medium. But this is nothing more than basic classical physics of something moving with speed v1<v2. If the object moves with v1 over a larger distance, it will be a lot far behind the object moving with velocity v2. This has nothing to do with light slowing down gradually.

Zz.

12. Mar 21, 2006

### DaveC426913

A delay being accumulated over time does not mean that the speed is changing.

Imagine two cars, white and black, travelling at 60mph on the highway. The black car exits the highway at 60mph and gets stuck at a traffic light. It gets stuck at every traffic light for the next 10 miles before getting back on the highway.

Its velocity upon exiting the highway immediately dropped to an average 30mph (or whatever) and remained at that (average) speed the whole time it was on side streets. The delay accumulated (if it had stayed off the highway for 100 miles, the delay would be much greater.)

You have cause and effect reversed. The slowing down is the cause of the refraction (well, simplistically, anyway). Thus, there can be slowing without refracting, though there cannot be refracting without slowing.

No, the FIRST colour (after black, that is) you see will be blue. However, underwater is almost entirely blue. As you ascend, you will see more and more red (red is the LAST colour you'll see), which is why it will LOOK very red. Your brain will filter out most of the blue, as will a picture from a camera.

This is a tricky subject, involving subjective observation and other factors. I'll go into it further if you wish.

13. Mar 21, 2006

### Ian

Mind reality,
I think zapper has misled you unintentionally. He/she said "But when it enters perpendicularly, while each frequency is still slowed down differently".
This is not the case, the prism effect is due to the different wavelengths of visible light taking a different track.
Think of it this way; If two cyclists ride with identical speed and one takes take the inside lane on a corner, that rider will exit the corner before the other rider even though their speeds are the same. The same is the case for e-m radiation.
The changing of the velocity of light from air to glass is due to the denser vacuum between the atoms in the glass. It is the state of the vacuum that determines the velocity of light.
Examine a Huygens construction and you will understand better.
By the way, DAVE said in the last post
"No, the FIRST colour (after black, that is) you see will be blue."
Black is not a colour - it is the absence of any colour/light. White is the presence of all visible wavelengths together.

14. Mar 21, 2006

### DaveC426913

You are misreading ZapperZ; his facts are correct.

Light entering a piece of glass normal to the surface will still slow down. Even if the exit angle is also normal, the light will be separated - not by angle but by delay. (The light is still refracted, it just has a refraction angle of zero).

Under idealized conditions, if you were watching a movie through a very thick piece of glass, the bluer elements of the film would lag behind the red elements.

With all due respect: duh. I give MindReality enough credit to know this grade school fact.

The point is, at the very deepest depths, you will see nothing. As you rise, the first colour you will see is blue.

15. Mar 22, 2006

### MindReality

Hi, Thanks for the reply all.

Ian I understand what zapper says.
We are already clear that we are not talking about the prism effect which involves light hitting glass at an angle. You twin cyclist analogy describes the prism effect. But in my case here, the twin car analogy described by DaveC really makes sense of it.

As for what DaveC said:
He means that before I see the light, it will dark and what I see is black. Well black is a color too figuratively. Some people call black light ultraviolet. Well the term black is used for convenience sake. Haha, sometimes it isn't necessary to get so particular about such things.

Ok, and so blue is the color that travels the furthest in a medium. That's interesting. It travels slower but it goes further. It's got the most energy but yet it is that energy that slows it down in the first place.

I checked the dictionary for "refract" and it says to deflect away from a straight path. So if the refraction angle is zero, it means there is no refraction. Just a decrease in speed. Like what you said:
Ahah, so that explains why 3D movie glasses make the blue and red seem separated.

Last edited: Mar 22, 2006
16. Mar 22, 2006

### DaveC426913

What? No!
You'd need lenses ten feet thick to see the effect!

The separation in red-green glasses is merely because it's a lousy (if cheap) way of tricking the brain into seeing stereo - it doesn't work very well.

Last edited: Mar 22, 2006
17. Mar 22, 2006

### Staff: Mentor

Clarification: 3d glasses don't make the red and blue seem separated, the red and blue are separated. Ie, if you take off the glasses and walk up to the screen, you will see two separate images superimposed on each other, but offset slightly. The glasses ensure that only one image is visible to each eye, creating real, but color-skewed stereoscopic 3d.

I don't like that kind of stereoscopic 3d because the color doesn't look right and it hurts my eyes/head after a while.

Last edited: Mar 22, 2006
18. Mar 22, 2006

### DaveC426913

Yeah, I meant the lousy red/green overlapping effect you perceive is because the red/green are separated and it's a lousy technique for...

Oh never mind.

No. Red/green glasses have nothing to do with refraction or ... slowage ... of light.