Light (Geometrical Optics)

Introduction

Our eye is the most important optical instrument gifted us by the God.

Without eye, all other optical instruments would have no value . It is an instrument which makes man to not only looks at objects but to appreciate the beauty behind the object. 

The Human Eye

The human eye is an organ that reacts to light in many circumstances. As a conscious sense organ the human eye allows vision; rod and cone cells in the retina allow conscious light perception and vision, including color differentiation and the perception of depth. The human eye can distinguish about 10 million colors.

Parts of human eye

The eye is made up of a number of parts, including the iris, pupil, cornea, and retina. The eye has six muscles which control the eye movement, all providing different tension and torque. The eye works a lot like a camera, the pupil provides the f-stop, the iris the aperture stop, the cornea resembles a lens.

Parts of the Eye

Sclera. The sclera is the white of the eye. 

The Cornea. The cornea is the clear bulging surface in front of the eye. 

Anterior & Posterior Chambers. The anterior chamber is between the cornea and the iris. 

Iris/Pupil. 

Lens. 

Vitreous Humor. 

Retina.

The Cornea

The cornea is the clear bulging surface in front of the eye. It is the main refractive surface of the eye.

Iris/Pupil

Iris is heavily pigmented

Sphincter muscle to constrict or dilate the pupil

Pupil is the hole through which light passes

Lens

Transparent body enclosed in an elastic capsule

Made up of proteins and water.

Retina

It acts as a screen to obtain the image of the object. It contains number of cells in the form of rods and comes which are sensitive to light . 

Working 

The eyes are very complex structures, and we actually don’t have a top-of-the-line eye in evolution. We have a very good eye and we have something very special. Predators, such as we, have binocular vision; it’s extremely important. The predator needs to be able to judge distances very accurately. We can judge a little bit of that distance by the size of that object and how far away it is from us. 

We can look with our two eyes and focus on a very near object and the brain knows and interprets unconsciously that the angle is very sharp here and that that object is closer. We can move that object farther away and look at it in the distance where our eyes are almost parallel, and the brain will know that distance, because it’s very good at judging distance.

If you lose vision in one eye, you lose the ability to have depth perception. And if you hold a finger out, close one eye and try to touch it in one shot, you’ll probably miss. You’ll then be able to make corrections and hit it. And we run into this sometimes as surgeons when we started doing laparoscopic surgery, which was done through a fisheye lens on a television screen, and all of a sudden our depth perception was gone and we had to learn how to judge distance purely by size. Neurosurgeons who work through a very small hole can only get one eye looking down that hole at a time. And they have to learn to judge distance. 

How to Find Your Blind Spot

There are more structures here you really want to know about. The optic disc is where the big optic nerve comes through the brain and enters the eye. The nerve then is spread out on both sides, actually in 360 degrees all around the retina. All the impulses are going to be channeled from the retina back here to the brain. It also carries the blood supply. Here’s a single artery and a single vein supplying all the blood for the eye—bad situation, isn’t it? We only have one supply vessel from which the capillaries of the choroid radiate. If you knock off this vessel, that’s it for the eye. Nature has chosen not to put too many vessels in this area because every vessel takes up room where we might be receiving vision. So we ended up with only one blood supply vessel.

Defects of vision 

Defects of Vision and their Correction.

Three common refractive defects of the eye:

Myopia.

Hypermetropia.

Presbyopia.

Myopia and Hypermetropia.

Refractive eye defects can also be corrected using contact lenses or through specific surgical procedures.

Defects of images formed by lenses

The defects in images formed by lenses are called aberrations. Images formed by a lens are defective because of the following reasons.

1) Lens maker's formula is derived on the assumptions that incident rays are paraxial and aperture of the lens is small.
2) Object may be extended and need not have point size always.
3) Due to dispersion, the focal length of the lens changes with color.
4) Refractive index of the lens changes with wavelength of the incident light.

Aberrations are of two types
1) Monochromatic aberrations
2) Chromatic aberrations

Monochromatic aberrations

Monochromatic aberration is also called as spherical aberration. The failure of paraxial and marginal rays to form a common image for pointed object in front of a lens on the principal axis is called spherical aberration.

Methods to reduce spherical aberration

1) Using ‘stops' which reduce the effective lens can minimize spherical aberration. Aperture. In this attempt, care should be taken to maintain the brightness of the image.
2) A crossed lens can be employed so as to minimize spherical aberration in longitudinal direction.
3) Spherical aberration can be minimized by using two Plano-convex lenses separated by a distance equal to the difference in their focal lengths
4) Since spherical aberration is positive for convex and negative for concave lenses, a suitable combination must be chosen to minimize the aberration effect.

Chromatic aberration

When a beam of white light passes through a lens, the failure of different colors of light to form a common image is called chromatic aberration. It is caused due to variation of refractive index of the lens with the color.

Minimization of chromatic aberration

1) A chromatic combination of two lenses in contact.

A concave lens produces chromatic aberration in the opposite sense as compared to a convex lens. Therefore we can combine a convex lens with a suitable concave lens to minimize the chromatic aberration. A combination of a convex and concave lens made of different materials cemented together suitably to minimize chromatic aberration is called an achromatic doublet. A widely used achromatic doublet is designed with convex lens of low focal length made of crown glass and concave lens of great focal length made of flint glass cemented with Canada balsam.


An eyepiece, or ocular lens, is a type of lens that is attached to a variety of optical devices such as telescopes and microscopes. It is so named because it is usually the lens that is closest to the eye when someone looks through the device. The objective lens or mirror collects light and brings it to focus creating an image. The eyepiece is placed near the focal point of the objective to magnify this image. The amount of magnification depends on the focal length of the eyepiece.

An eyepiece consists of several "lens elements" in a housing, with a "barrel" on one end. The barrel is shaped to fit in a special opening of the instrument to which it is attached. The image can be focused by moving the eyepiece nearer and further from the objective.

Telephoto lens

A telephoto lens, in photography and cinematography, is a specific type of a long-focus lens in which the physical length of the lens is shorter than the focal length. This is achieved by incorporating a special lens group known as a telephoto group that extends the light path to create a long-focus lens in a much shorter overall design. The angle of view and other effects of long-focus lenses are the same for telephoto lenses of the same specified focal length. Long-focal-length lenses are often informally referred to as telephoto lenses although this is technically incorrect: a telephoto lens specifically incorporates the telephoto group

A telephoto lens works by having the outermost (i.e. light gathering) element of a much shorter focal length than the equivalent long-focus lens and then incorporating a second set of elements close to the film or sensor plane that extend the cone of light so that it appears to have come from a lens of much greater focal length. The basic construction of a telephoto lens consists of front lens elements that, as a group, have a positive focus. The focal length of this group is shorter than the effective focal length of the lens. The converging rays from this group are intercepted by the rear lens group, sometimes called the "telephoto group," which has a negative focus.

Use of telephoto lens

Telephoto Lenses. A telephoto lens is one designed for photographing distant subjects like wildlife and sports events. They are also used in portrait photography. A telephoto lens is a type of camera lens designed for taking photographs of subjects at moderate to far distances.

 Difference between zoom and telephoto

Telephoto, roughly, means that the lens has a relatively narrow field of view, thus it can be used to look at things further away. Telephoto lenses can be either zoom or prime. Zoom means that they can change how far they are looking at or prime means they have a fixed amount of magnification and can't be altered

How does a telephoto lens work?

A telephoto lens works by having the outermost (i.e. light gathering) element of a much shorter focal length than the equivalent long-focus lens and then incorporating a second set of elements close to the film or sensor plane that extend the cone of light so that it appears to have come from a lens of much greater.

Zoom lens

A zoom lens is a mechanical assembly of lens elements for which the focal length (and thus angle of view) can be varied, as opposed to a fixed focal length (FFL) lens (see prime lens).

A true zoom lens, also called a parfocal lens, is one that maintains focus when its focal length changes.[1]. Most consumer zoom lenses do not maintain perfect focus, but are still parfocal designs.

Characteristics 

Zoom lenses are often described by the ratio of their longest to shortest focal lengths. For example, a zoom lens with focal lengths ranging from 100 mm to 400 mm may be described as a 4:1 or "4×" zoom. The term superzoom or hyperzoom is used to describe photographic zoom lenses with very large focal length factors, typically more than 5× and ranging up to 19× in SLR camera lenses and 22× in amateur digital cameras.

Use of zoom lens

A zoom lens is a type of camera lens that is offers the photographer a useful range of different focal lengths in a single lens. This is in comparison to a prime lens, which only offers a single focal length. A zoom lens allows for quick and easy re-framing of a scene while staying in the same physical position.

Overhead Projector (OHP)

An overhead projector (OHP), like a film or slide projector, uses light to project an enlarged image on a screen. In the overhead projector, the source of the image is a page-sized sheet of transparent plastic film (also known as 'foils') with the image to be projected either printed or hand-written/drawn.

 These are placed on the glass surface of the projector, which has a light source below it and a projecting mirror and lens assembly above it (hence, 'overhead'). They were widely used in education and business before the advent of computer-based projection.

An overhead projector works on the same principle as a slide projector, in which a focusing lens projects light from an illuminated slide onto a projection screen where a real image is formed. However some differences are necessitated by the much larger size of the transparencies used (generally the size of a printed page), and the requirement that the transparency be placed face up (and readable to the presenter). For the latter purpose, the projector includes a mirror just before or after the focusing lens to fold the optical system toward the horizontal. That mirror also accomplishes a reversal of the image in order that the image projected onto the screen corresponds to that of the slide as seen by the presenter looking down at it, rather than a mirror image thereof. Therefore, the transparency is placed face up (toward the mirror and focusing lens), in contrast with a 35mm slide projector or film projector (which lack such a mirror) where the slide's image is non-reversed on the side opposite the focusing lens.

 Principle of projector

The principle behind a projector is to take an input signal (computer, film, etc), process it and use the projectors illumination system to output it through a lens. The light from the projector is projected on a projection screen or other kind of surface that is applicable.

 Advantages of a projector

Projectors reflect light; TVs emit light. Reflected light is less straining, more comfortable. Projectors produce bigger images. Larger images create easier viewing, less strain.