Optical Instruments

Welcome to class!

In today’s class, we will be talking about optical instruments. Enjoy the class!

All optical instruments make use of lenses or mirror and operate on the principle of refraction and reflection of light.

Some of these instruments are:

The Human Eye
Camera
Microscope
Telescope
Slide Projector

THE HUMAN EYE

The human eye is a pair located in the eye sockets of the brain which is held in position by some muscles that connect the blood vessels with nerve cell.

The diagram of the human eye is shown in the figure above

(1) The Cornea

This is a protrusion in the front view; it is responsible for the refractive power of the eye. It has refractive power twice that of the eye lens.

(2) Aqueous Humour

This is located between the cornea and the lens. Front liquid for the initial transmission of light is stored on it. It helps to maintain the shape of the eyeball.

(3) Iris

Located just before the lens: it acts as a control for the amount of light passing to the retina (by expanding or contracting) via the lens

(4) Lens

This is located between the aqueous and vitreous humour and is responsible for the accommodation property of the eye.

(5) Ciliary Muscle

Located at the edges of the lens, varies the complexity of the lens and hence its focal length.

(6) Suspensory Ligament

It acts as a support for the lens.

(7) Citreous Humour

Located between the lens and the retina. Back liquid for the final transmission of light is stored on it. It also helps to maintain the shape of the eyeball.

(8) Retina

Located behind the lens at the edge of the vitreous humour light is focused on the retina and image is formed on it especially at a point called the fovea. It also contains typical brain cells as well as specialized lights cell example cones and rods. The cones are for daylight vision while rods are for night vision.

(9) Choroid and Sclerotic

Are the inner first and second layers of the retina respectively and supports the retina for effective functioning.

(10) Blind Spot

Located at the central part of the outer part of the optic nerve. It corresponds to the point where nerve fibres leave the eye to the brain (and it has no nerves in itself)

(11) Optic Nerve

These are axons (Connecting verves) that connect the eyes to the brain. There are about 10⁶nerve fibres.

DEFECTS OF VISION AND CORRECTION OF DETECTS

Eye defect is a disease of the eye which is experienced when light refraction coming from the object through the lens is not properly focused on the retina. It may be before or beyond the retina, wrong image, untrue colour image or blurred image.

Some of the eye defects are:

(i) Myopia or shortsightedness

(ii) Hypermetropia or longsightedness

(iii) Astigmatism

(iv) Presbyopia

Myopia or shortsightedness

The person cannot see the distant object clearly only near objects. Light from a far object is focused in front of the retina.

Causes

This arises when the eyeball is too long

Correction

Myopia (Short-sightedness) is corrected by using a concave spectacle lens or diverging spectacle lens which diverges the ray of light entering the eye so that the ray is brought in to focus

NB:

Diverging lens causes rays to diverge as though coming from F.

Hypermetropia or longsightedness

The person cannot see near objects clearly but only far objects. Light from a nearby object is focused behind the retina.

Causes

This arises when the eyeball is too short

Correction

This can be corrected by using a convex spectacle lens or converging spectacle lens which converges the rays of light entering the eye so that the rays are brought in to focus.

NB:

Converging lens reduces the divergence of rays so that they appear to come from O.

Astigmatism

The refracting surface of the eye has widely varying curvatures in different planes.

Causes

This arises due to old age or loss of power of the lens.

Correction

Cylindrical lenses are used

Presbyopia

This is a lack of power of accommodation

Causes

Hardening or inelasticity of the eye lens due to old age.

Correction

This can be corrected using a bifocal lens (upper convex lens and lower concave lens)

THE LENS CAMERA

We use the came in taking photograph by placing the object in front of it. The image is formed on a light-sensitive paper called the film. The amount of light entering the camera is controlled by the size of the aperture. The shutter controls the length of time for where the film is exposed to light. The structure and function of the camera are very similar to that of the human eye.

COMPARISON OF EYE AND CAMERA

Similarities between the Human Eye and the Camera

S/N	Human Eye	Lens Camera
1	Has a converging lens system	Has a converging lens
2	The image formed on the light-sensitive retina	The image formed on light-sensitive film
3	An inverted image is formed	An inverted image is formed
4	Amount of light entering the eye is controlled by the size of the pupil	Amount of the light entering the camera is controlled by the Aperture
5	Inner walls are pigmented black. (Tight)	The walls are black (light)

Differences between the Human Eye and the Camera

S/N	Human Eye	Lens Camera
1	The focal length of the lens is variable.	The focal length of the lens is fixed
2	The image distance is fixed	The image distance is variable
3	Biological organ	Mechanical device
4	The eyelids remain open continuously as the retina forms series of constantly changing pictures with continuous motion.	The shutter works on the simulation of light before photographs can be taken.

In our next class, we will be talking about Microscope. We hope you enjoyed the class.

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Optical Instruments