Lenses are one of the best human inventions ever. Any transparent piece of material can be called a simple lens. The magic of illusion is done by combining a lot of simple lenses in different positions and sizes. Lenses are not just about the convex type and the concave type that you had studied in your primary school. They are much more than what you think. Let us have a look at two interesting inventions that use the principle of a lens as they work.
A mirror is actually a lens with one of its surfaces covered with a material like aluminum or silver.Two-way mirrors are objects that act like a transparent glass when viewed from one side and like a mirror when viewed from the other side. They are also called as half mirrors.
To understand two-way mirrors, one must understand the working of an ordinary mirror:
When you stand in front of a mirror, the light that is incident on you strike the surface of the mirror. Again, the photons in the light bounce back to you. This is called reflection. The lens in your eyes receive this reflection and your brain reconstructs it into a mirror image. This is how you see your reflection.
But this does not happen when you stand in front of a rock. This is because rough surfaces scatter light i.e. the light on the rough surface is thrown at random angles instead of bouncing back to the original source. Smoother the surface better is the reflection.
When you stand in front of a glass, the light bouncing from you escapes the glass. This is called refraction. Glasses are smooth but they cannot reflect light completely without an external support that can prevent refraction. When you stand in front of an aluminum sheet, you can see your image but not clear as what you see in the mirror. This is because the surface of the aluminum is quite rough.
So glasses have a smooth surface but they cannot prevent the light from escaping. Aluminum can arrest the light from escaping but has a rough surface. But if you want a complete reflection, the light should neither escape nor scatter. So our ancestors came up with an idea of covering the surface of the aluminum sheet with a layer of glass. By doing this one can get a material that can produce a perfect reflection by avoiding both refraction and scattering. This arrangement is called a mirror.
Now let us get back to the topic "How do two-way mirrors work?”
Like an ordinary mirror, the two-way mirrors also contain a sheet of metal but in a much thinner size. This arrangement will make one-half of the light reflect (will show your reflection) and the other half refract (will show you the other side of the mirror). It is not just about the thickness of the metal coating on the mirror but the lighting conditions too. The space out of the reflecting side must be much brighter than the refracting side.
You can understand this better if you have noticed your reflection on the windows of buildings during the daytime. These windows let you see through them when the light inside the building is brighter than that outside the building (which happens at night when the buildings are illuminated by artificial lamps). The same windows show you your reflection when the light outside is brighter than the light inside the building (during the day). The two-way mirrors work the almost the same way except for giving a reflection that is better than the window glasses.
The hologram is a realistic three-dimensional image of an object.The difference between an ordinary 3D image and a hologram is that the hologram imitates the real object when viewed from any angle. For example, when you project a 3D image of a face on a screen, you will see the same details when viewed from any angle. You will see the same image of the face even if you watch it standing behind the screen. But when you see a face in the hologram of a head, you can walk around the head to watch the hair from behind just like how you see a real head. If you had ever seen the diamond robbery scene in the movie Dhoom 2 you would understand this better. The fake diamond that Hrithik replaced is a hologram.
Below is the video clip from the movie.
So how is this hologram made? In photography, the lens collects the light bounced from the object at the focus and puts it on a photosensitive plate. This plate is designed in such a way that it reconstructs the image of the object using the light that is bounced off the object. Holography almost works on the same principle except for using laser light instead of ordinary light.
Laser light produces better clarity in the image due to the nature of its beam. The photons in the ordinary light beam travel like the school children rushing out of the school after the evening bell. But laser light travels in a very organized manner like the soldiers marching in a parade.
The laser light is focused onto a two-way mirror. As discussed earlier, the two-way mirror reflects one-half of light and refracts the other half. The reflected part is again redirected using a full mirror to hit the object at the focus. Hence this beam is called the object beam. The object beam is allowed to fall on a photosensitive plate after hitting the object. The other half of the laser beam which was refracted through the two-way mirror is redirected to fall in the plate. The plate merges the two beams and makes the hologram of the object.
To understand how this happens, let us do a small experiment. Just look at your room with both your eyes opened. Now close your left eye and see the same.
You will find a few things to be missing when compared to the image that you saw with your both eyes opened. Now do the same with your right eye closed. The two eyes see different scenes and they focus them onto the brain. The brain actually merges the two small scenes and makes you think that you are looking at just one big scene.
The same thing happens in holography. The photosensitive plate merges the details in the object beam and reference beam to form a single detailed image called a hologram. The image produced on the photosensitive plate due to the object beam is just a photograph. The photograph is converted into a hologram when the reference beam with extra details is merged with the object beam in the plate.