# refraction help needed

Discussion in 'Science' started by sunseeker2834, Apr 5, 2011.

1. ### sunseeker2834

when teaching refraction and that light slows down when going from air through a solid. is it correct to use the gas liquid solid particle diagram to explain this.This is for a year 8 lesson

thanks very much

2. ### blazerStar commenter

I have never used the gas/liquid/solid diagrams for this. Does your school have a ripple tank?

3. ### jdanyl

there are also simple demos on refraction like the ''coin in the mug ''experiment or thsse pencil in an empty glass/water in glass.
The kids will be able to appreciate that these happen because light ''bends'' from one medium to another.

4. ### sunseeker2834

Thanks not sure if there is a ripple tank as its an interview i have always used the particle model to explain light slowing down from air to a solid which pupils grasp easily however i am questioning whether technically if this is correct. i plan to use the pencil in a glass of water as a starter but was wondering how to explain in year 8 terms

You could get them to think about the difference between walking through 'air' and then walking waist high in water. It is more difficult to wade through water since it is denser than air - so you slow down. Likewise, light finds glass & water 'denser' than air & slows down when it passes through them.
If it a bright Y8 group, you can use the words 'optically dense'. If they have understood the idea & they know what a vacuum is, they could try to predict the conditions in which light woudl travel the fastest (no air ==> vacuum)

6. ### aldebaran

what I find more troublesome is explaining why an emergent beam speeds up again! Where does the energy come from?

7. ### physics_suits_you

Whilst it would be wrong to say this has NOTHING to do with solid/liquid/gas, this has nothing to do with solid/liquid/gas!
If you think the material is harder to go through, so it slows down, then YES you will have problems with why it speeds up. For Y8, just ACCEPT that light goes slower in glass than in air and don't try to explain it (especially if you don't understand it - degree level physics required).
Also, please try not to say light "bends". Light travels in straight lines (assuming homogeneous media) but MAY "change direction" as it goes from one medium to another.

I approach it in a similar way and get pupils to walk around the room at steady speed. Then place two stools a few metres apart and tell them that there is a pool of treacle between these two stools. How is that going to affect your progress?
Naturally they understand that they will slow down and speed back up when they have passed through it. Not too big a leap then for them to understand that this is what happens to rays of light as they change between thicker and thinner (or more/less dense) mediums.

The pupils will need to understand the slowing down & speeding up BEFORE they can consider the changing direction. In addition, they will also need to be comfortable with the different representations of light (wavefronts, rays or beams) and key concepts such as wavelength & speed.
There are some good animations around on the interweb.

10. ### wire247New commenter

<font size="2" face="Tahoma" color="#000000">There is no need for extra energy as light has no mass. Objects with mass would require more kinetic energy if they wish to speed up, this is not the case with light. </font><font size="2" face="Tahoma"></font><font size="2" face="Tahoma" color="#000000"></font><font size="2" face="Tahoma" color="#000000">The speed of light is determined by the ease with which the electric and magnetic disturbances created by the passing of the light are able to move through the medium (namely the permittivity and permeability) . The extent to which the field is 'resisted' by the different media dictates the speed with which the light will propagate.</font><font size="2" face="Tahoma"></font>
When on holiday try running to the beach, then on the beach, then back to hotel. You will find your speed changes even though there is no change in the effort you exert.

<font size="2" face="tahoma">Don't know why I thought of that analogy..... Oh yes I do! Tenerife here I come!</font>

11. ### wire247New commenter

How much do I hate Vista?
The post was in reply to aldebaran.

12. ### bogstandardcompNew commenter

returning to the original poster, you may find some of these useful

http://www.freezeray.com/physics.htm

13. ### aldebaran

thanks wire247! Pleased to read a reply which contains some science rather than suggesting all is explained if I simply run through treacle. <<Light has no mass...>> but what about the wave/particle duality problem? Electrons have wavelengths much shorter than visible light yet can be demonstrated to display mass...

14. ### physics_suits_you

That's because electrons are NOT part of the electromagnetic spectrum. If you must extend a topic (it was about Y8 and the refraction of light) please try to understand a little bit of the science you desire to know more about.
Wave/particle duality is important at A level in the discussion of diffraction etc & the photo-electric effect. Gamma "rays" are easily detected in lumps (photons) but no-one suggests they have mass. Electrons can be demonstrated to diffract (the basic tube costs about &pound;300, and most schools have an appropriate stand and power supply) but I've never heard anyone linking them to refraction, polarisation or any of the other phenomena which we think about when dealing with transverse waves.

15. ### blazerStar commenter

There are lots of waves that are not part of the EMS. Sound and water to name 2.

16. ### doublehelix

Thanks Blazer. Does this mean there is also a change of wavelength when a light wave slows down due to refraction? Wouldn't the wavelength reduce? If so, is there a way of demonstrating the effect?

17. ### doublehelix

And, following on, the fact that white light is dispersed by a prism must mean that different wavelengths of light have their speed reduced by different amounts. Is this correct?

18. ### blazerStar commenter

Yes, use a ripple tank.

19. ### physics_suits_you

Thanks for the compliment - it's nice to have people say good things about us teachers (shall we join the "happiness" group?)
Electrons are basic building blocks of matter. They do possess mass (and negative charge) so their momentum can be demonstrated as you recall, with a paddle wheel in an evacuated tube.
But they are not, basically, waves. However, they can exhibit wave behaviour, notably diffraction by layers of atoms. This is used in electron microscopes, enabling researchers to investigate much smaller objects than can be "seen" using visible light, or any other member of the electro-magnetic spectrum eg X-rays. Why a particle can behave like a wave gets you into the strange domain of quantum mechanics - common sense goes out of the window BUT it gives answers which work (hence most of our modern electronic technology).
The easiest waves to consider are transverse waves on strings or springs. Transverse waves are what you see on water; they are also what light is like. Light is a combination of an electric field and a magnetic field, both at right angles to each other and to the direction of travel. Slinkies are useful for showing wave motion because the wave travels reasonably slowly. They also allow you to see longitudinal waves, the same sort as you get with sound.

thanks