 Harry Potter has a code of invisibility, so can scientists really make such a clock? Now before I tell the story, I want to emphasise that invisibility is not the same as stealth. Stealth technology has been around for quite some time. A stealth object either absorbs or deflects radar away from the sensor so the sensor cannot detect the object by radar. So a stealth object is in fact a dark object, it is not invisible. By invisibility what we mean is perfect transparency as we have seen in many movies. Now I can assure you that perfect transparency is much more difficult to realise than stealth. Now in the last century, science writers have the vision that maybe one day a man can drink some chemicals and transform them to become invisible. Now such chemicals doesn't exist. Even if they do, I would suggest that you don't try that because modifying your body chemically is very dangerous and by drinking something you cannot make your clothes go away. Now in more modern concepts there is a lady, a superhero called the invisible woman. She is way smarter than the invisible man. She doesn't try to drink something. What she tries to do is to create a force field to guide light around herself so that you cannot see her. The question is, can scientists really make something that can make light go around the body so that you cannot see that body? So can light travel in a curve in the first place? Now the answer is yes it is. There is a natural phenomenon called mirage which indeed light do travel in a curve due to thermal gradient of air and that creates an optical illusion. In the year 2006 British scientists invented a new method called transformation optics. And with that method they designed invisibility clocks which really can guide light around an object so that you do not see that object. Now technically speaking you can say that that is in fact a mirage effect or you can say that they have achieved what the invisible woman wanted to do. Now this is regarded as a very major big through in science. However there is no free lunch. It turns out that nothing in nature can do that job. You have to make those material by yourself using really advanced technology. But if you can do that the same idea can actually allow us to design not only clocks for light but also clocks for other kinds of waves such as sound wave, heat wave and even seismic wave. Now these are some examples that made in Europe and United States that can cloak sound waves. But that is not the end of the story. Now it turns out that for the invisibility clock that was designed what it works is by excluding light from a region. So that the object or the body being clocked inside the clock is totally blinded by the clock. What it means is that if a reporter has this kind of clock then you cannot see him but he sees nothing. Now in order to get around that problem we in Hong Kong invented another way. What we do is that we design what we call an anti-object and we use that anti-object to cancel the light scattered by an object. It is like 1 plus minus 1 equal to 0 so that an outside viewer cannot see the object and you cannot see the anti-object. So the object becomes nothing. But you see that you have done that but invisibility is just one form of illusion. Can we go one step further? For example can I make an apple look like a banana? Well the answer is yes. Let me show you an example that we have done. So in the next slide you are going to see that on the left hand side I am going to show you the scattering pattern of a spoon. And on the right hand side you see the scattering pattern of a cup. Now in the middle I designed something which is a material where it put next to the spoon. Actually it makes the spoon look like a cup for someone far away so that you do not see that clock. You do not see the spoon but instead you see a cup instead. Now but up to now all the design we have we use for passive materials. If you allow me to use power and energy I can also design active sources which can emit waves. They can transform one object and make it look like another. Say for example I can make the example so I can make an apple look like a banana. So in some way I will say in the past it is chemists who made most of the materials. Right now physicists can also use computers to design new materials according to laws of nature. And some of these materials may help us to go one step closer to the technologies that you use to see in science fiction. Thank you.