Study Guide

# Optics In the Real World

### Invisibility Cloaks: What would it take to be invisible?

Everyone from Harry Potter, Jean-Luc Picard, and the United States Army, to desperate test takers and people on awkward dates all over the world, is interested in being invisible. What's the hold up? Why do we still not have invisibility cloaks of our own? The simple answer is because light is just too darn good at interacting with things.

We see things because light bounces off of them, then goes into our eyes . The simplest way to make something invisible is to eliminate reflection. That's much easier said than done. Not only is no reflection allowed, the light would have to bend around an object or be able to pass through it without changing the original course of the light. Based on what we learned about light, under what circumstances does light do that?

If two materials have the same index of refraction the path of light doesn't change. Knowing that, we can design experiments to make a glass rod disappear. Vegetable oil has about the same index of refraction as glass. If we place a glass rod into a beaker full of vegetable oil, it disappears.

That's a fun little magic trick, but not very practical. We still see the oil. We might be able to mix up some solution that has the same refractive index as a car, but we don't just want cars to be invisible when we're all submerged in a pool full of invisibility-goo. We want an invisibility cloak that works no matter where we are.

To achieve invisibility scientists decided to try to make "cloaks" that change the path of light, They essentially bend light around the objects we are trying to hide, like water in a stream flowing around a boulder. This requires designing materials with a negative index of refraction.

Scientists at Imperial College in London have built special materials called metamaterials, which have properties that not found in nature. The Imperial College metamaterials take light and bend it around whatever is "wearing" these materials. If we were wearing this type of a material, photons would never hit us, it would bend around us. Neither does light reflect off of these cloaks, leading to invisibility of the cloak as well. This device looks a lot like a honeycomb.

These metamaterials are expensive to make and at this point, we are far from a Harry Potter-esque invisibility cloak. However, the London scientists have some competition.

Dr. Baile Zhang's invisibility cloak uses calcite, a cheap mineral. Two crystals of calcite glued together refracts light in such a way that whatever is placed between the two crystals is invisible.

Although cheap, Dr. Zhang's invisibility suit is still a long way from becoming a practical cloak. On its own, the calcite doesn't bend the light waves well enough to render large objects (us) invisible. It needs help. The crystal, and therefore whatever it is cloaking, has to be submerged in laser oil. Why, we may ask?

Remember how we learned that how much light is refracted depends on the difference in refractive index of the two materials that it is traveling between? Well, when light travels from air into the calcite crystal, the calcite is visible by reflected light when in air. Most materials reflect and transmit (refract) light simultaneously.the refractive index of air is too low in comparison to the index of refraction of calcite. To avoid that, we need something that has the refractive index of laser oil. See, physicists building invisibility cloaks think about what we learned about in this Chapter.

But although Dr. Zhang's invisibility cloak is not going to be coming to a Target near you any time soon, it's definitely worth checking out.

Scientists at Duke University are taking yet another approach. Their invisibility cloak consists of thousands of tiny, microscopic mirrors that reflect light around an object. This cloak can renders objects invisible to microwaves. This is light of longer wavelengths than visible light. To adapt this cloak to visible light, we'd need to manufacture smaller mirrors, something we don't know how to do quite yet.

A sleight-of-hand invisibility cloak was used in a recent Mercedes Benz commercial. They covered one side of a car in a mat of LEDs. On the other side of the car they mounted a camera. Whatever the camera recorded was projected onto the LEDs. To someone facing the car on the LED side, the car was nearly invisible. The images taken by the camera that were projected on the LED mats, essentially allowing people to see straight through the car.

None of these scientists have perfected the invisibility cloak quite yet, but in trying to make things invisible, they are thinking about all of the same principles that we learn while studying optics. That'd be a career worth pursuing.

### Improving Solar Cells

How do we catch photons, and lots of them? That's one of the big problems to solve when building a solar cell.

Photovoltaic cells absorbs the energy of light, creating an electric current in a solar cell. These are not cells like the cells in our bodies. They are essentially tiny power stations made of semiconductor material like silicone. When a photon hits the silicone in a photovoltaic cell, the energy of the photon knocks loose electrons in the silicone and allows them to become part of an electric current, or in other words, we get electricity.

And guess what's most expensive in building a solar cell? Yup. The photovoltaic cell. It's a cruel, cruel world.

One way to make solar cells more affordable, is to reduce the amount of photovoltaic material needed, but how could we do that? How could we concentrate the light onto a photovoltaic cell?

None other than lenses and curved mirrors do the job.

By using conversing lenses or mirrors, we can collect light over the relatively large surface of the lens or mirror, and then focus all of that light on a very small surface, the photovoltaic material.

There we have it. Using converging lenses and mirrors improves efficiency while lowering the cost of solar cells. Optics for the win!

### To Reflect or Refract: Which Telescope is Better?

We want to buy a telescope. If not today, then some day. There are two types of telescopes: refractor and reflector telescopes. Before learning about optics, those names probably wouldn't have told us all that much, but now we can make an educated guess about how each of these two telescopes work.

The reflector telescopes uses reflection and its optics are made up of mirrors. The refractor telescope uses refraction by a combination of lenses.

In general all telescopes do the same thing, focusing faint distant light into an enlarged, brighter image. A refractor telescope uses an objective lens and a reflector telescope a primary mirror to collect as much light as possible from a far-away object. The object lens or primary mirrors focuses all of that light in one position inside the telescope. The eyepiece lens then magnifies the image at that position

So why chose one telescope over another? It depends on the user's purpose. Lenses do something that mirrors do not, bending light of different wavelengths to slightly different degrees, which is why light passing through a prism separates the light into a rainbow of colors. When our refractor telescope is focused on red light, it won't be perfectly focused for something that is blue, blurring the image.

This effect becomes worse beyond visible light for light with very long wavelengths, or much shorter wavelengths if we want to create only one image instead of having several different refractor telescopes tuned to different wavelengths of light. To make an image with a broad spectrum of wavelengths we'd need a reflector telescope. This is what the Hubble Telescope is, for exactly that reason. Astronomers probing the origin of the universe maybe prefer reflector telescopes, but for the amateur astronomer gazing at the Moon, Mars, and Neptune, a refractor telescope will do just fine.