# B Why do diamonds lose their sparkle in water?

1. Jul 30, 2016

### Angela Liang

I'm thinking whether it's got to do with the refractive index of water? Because previously when I was searching there were many different answers. Some say it's because there is light reflected off the surface of water...but I think the amount of light reflected off should be very little so it shouldn't affect the amount of light reaching the diamond. And some say because water causes light to refract but I don't see how it affects much since light can shine from all directions(?)

2. Jul 30, 2016

### tech99

The refractive index of diamond is high, about 2.4. Glass is about 1.5 and water is 1.3.
The sparkle of diamond happens because light can easily bounce around inside it. Once light enters the crystal, it reflects easily from the internal surfaces because the critical angle is small. The critical angle is the angle from the normal above which total internal reflection takes place, so a small critical angle allows many light rays to be internally reflected.
If the diamond is placed in water, the critical angle is increased. This is because it depends on the ration of the indices of refraction across the boundary. So whilst in air a diamond has a ratio across the diamond/air boundary of 1/2.4=0.42, in water it is 1.3/2.4=0.52. The critical angle for the two cases is calculated using Snell's Law: Angle = inv sin N1/N2.
So for diamond in air we get 24.6 degrees, for glass in air we get 41 degrees and diamond in water we get 32 degrees. The smaller angles allow more light rays to bounce around inside the crystal, so diamond in water is more sparkly than glass in air but less than diamond in air.

3. Jul 30, 2016

### Angela Liang

Thanks. I think this means that less light rays strike at an angle greater than critical angle, but it doesn't mean no light strikes at a greater angle than critical angle. So that means there are still sparkles, but just not that much. So the sparkle will not be 'lost', right?( my textbook says it will lose its sparkle...)

4. Jul 30, 2016

### Angela Liang

And also is there another reason which is: when there is water, the light rays are refracted so the light strikes surface of diamond at a smaller angle, so the light enters the diamond at a even smaller angle. This means less light able to have an angle larger than diamond's critical angle and so more rays are refracted out?

5. Jul 30, 2016

### sophiecentaur

Yes. Critical angle is the thing that counts here. The critical angle for diamond into water is a lot less than for diamond into air so all those lovely internal reflections at the big range of angles that you get with a diamond in air are just not there and you do not get so many multiple reflections that will produce increased 'colour' dispersion and for which ever direction you look. Also, the effect of the dispersion will be less.
You should clean diamonds (the back facets, too) regularly with washing up liquid and a soft toothbrush to remove dirt and oils from the skin for the same reason.

6. Jul 30, 2016

### pixel

There is less of an index mismatch in water compared to air. Using n(diamond) = 2.4, n(air) = 1.0 and n(water) = 1.33, the reflection coefficient for normal incidence of light is 0.17 for diamond/air and 0.08 for diamond/water, a factor of 2 decrease. The numbers for non-normal incidence are different but still less for diamond/water. So even for angles smaller than the critical angle, there is less amount of light bouncing around in the diamond and returning to your eye.

Last edited: Jul 30, 2016
7. Jul 30, 2016

### and7barton

What would be the result if the diamond were viewed in a vacuum ?

8. Jul 30, 2016

### sophiecentaur

No easily detectable difference.

9. Jul 30, 2016

To quantify, the index of refraction of vacuum is 1.0000 and the index of refraction of air is approximately 1.0003. That is being contrasted with the diamond whose index is 2.4.

10. Jul 30, 2016

### tech99

Are you saying that more light enters the diamond when in water? That seems counter to the observation that the sparkle is less in water. These reflection coefficients are small, so the eye may not notice the effect when compared to the change in critical angle.

11. Jul 30, 2016

### sophiecentaur

He's not saying that. He's saying that there are fewer directions of incident light that will produce an output in your particular direction. With diamond, there is such a wide cone of spectrum that you have a better chance of seeing some colour from each light source. The multiple reflections (as well as the high dispersion in each passage) will widen the resulting band of colour. You can see just one colour from your particular direction.

12. Jul 30, 2016

Depending on how the diamonds are immersed in the water and the position of the light source and position of the observer, a fair amount of light could be lost at the air- water interface, both before the light is incident on the diamond and after it exits the diamond. Because the index of the diamond is so high (2.4), the difference between n=1.33 and n=1.0 is going to no doubt have some effect, but not an overwhelming one. If the diamond was immersed in a clear liquid with index 2.4, the diamond would be invisible/transparent when immersed in the liquid. And likewise, a gem made of (clear) glass with index of 1.33 is transparent when immersed in water.

Last edited: Jul 30, 2016
13. Jul 30, 2016

### pixel

Yes, but the front surface reflection at the air/diamond or water/diamond interface is of secondary importance. The main effect is due to the light bouncing around inside the diamond and returning upward toward your eye (assuming you are looking down at the diamond). In water, due to the smaller index difference more light is lost to refraction at each bounce. In addition, as mentioned above, the critical angle is increased, so less of the light is totally reflected at each bounce.

14. Jul 31, 2016

### Angela Liang

Thanks:)Is there another reason which is: when there is water, the light rays are refracted so the light strikes surface of diamond at a smaller angle, so the light enters the diamond at a even smaller angle. This means less light able to have an angle larger than diamond's critical angle and so more rays are refracted out?

15. Jul 31, 2016

### sophiecentaur

That's re-stating the comments about reflection but the other way round. If the light isn't totally reflected then it will exit and change direction (refraction).

16. Jul 31, 2016

### pixel

Not sure what you mean here - are you referring to the refraction of light at the surface of the water? If so, then it could strike the surface of the diamond at a smaller or larger angle depending on the surface's orientation.

17. Aug 1, 2016

### Angela Liang

yes

18. Aug 1, 2016

### Staff: Mentor

I would expect that one could cut a diamond to sparkle underwater by cutting it with angles between the faces different than the angles one uses to make it sparkle in air.

Someone better than I at optics could probably tell us what angles to cut it for underwater sparkes.

19. Aug 1, 2016

### sophiecentaur

That's a good thought. It would probably consist of a very shallow disc shape and the circumference would need to have many facets to redirect rays back through the disc. I'd bet that de Beers will have a computer program that could be doctored to do just what you suggest.
(Aimed at wealthy swimmers, I guess)