The Human Eye and Colourful World Class 10 Notes

The human eye works as a camera. Light from an object enters through the transparent cornea, passes through the pupil whose size is controlled by the iris, and is then focused by the eye lens, a convex lens of adjustable curvature.The inverted, real and diminished image is formed on the retina, which is the light-sensitive screen at the back of the eyeball. Nerve impulses then travel from the retina to the brain, where the image is seen upright because the brain inverts it back, and that is why the world appears the right way up.

• Cornea: the transparent front surface of the eyeball through which light first enters
• Iris: the coloured muscular part that controls the size of the pupil in bright and dim light
• Pupil: the small opening in the centre of the iris that limits the amount of light entering the eye
• Eye lens: a convex lens of adjustable curvature that does the actual focusing
• Ciliary muscles: the muscles that change the curvature, and therefore the power, of the eye lens
• Retina: the screen at the back of the eyeball on which the inverted image is formed

Accommodation is the ability of the eye to change the curvature of its lens so that objects at different distances are brought into focus on the retina. When the eye looks at a distant object the ciliary muscles relax and the lens becomes thin and less curved, so its power is at its least.When the eye looks at a near object the ciliary muscles contract, the lens becomes thicker and more curved, and its power increases. The nearest point on the eye without any muscular effort is the far point, which for a normal eye is at infinity, and the nearest point that can be seen distinctly is the near point.

• The least distance of distinct vision, or near point, is 25 cm for a normal eye and is denoted by D.
• The far point of a normal eye is at infinity, since the eye needs no effort to focus on distant objects.
• Accommodation is brought about by the ciliary muscles and not by the iris, which only controls light.
• The power of the eye is least for a distant object and greatest for an object at the near point.
• Relaxation of the eye means the lens is at its thinnest and is used for the far point.
• The range of vision of a normal eye is therefore from about 25 cm to infinity.

A defect of vision is a condition in which the eye cannot form a sharp image on the retina, either because the eyeball is the wrong length for the power of the eye lens or because the lens itself is too strong or too weak.Myopia, or short-sightedness, arises when the eyeball is too long or the lens is too curved, so the image of a distant object forms in front of the retina and distant objects look blurred. Hypermetropia, or long-sightedness, arises when the eyeball is too short or the lens is too flat, so the image of a near object forms behind the retina and near objects look blurred.

• Myopia, short-sightedness: image forms in front of the retina, distant objects are blurred, near objects are seen clearly.
• Correction of myopia: a concave or diverging lens of suitable negative power, which diverges the rays and moves the image back onto the retina.
• Hypermetropia, long-sightedness: image forms behind the retina, near objects are blurred, distant objects are seen clearly.
• Correction of hypermetropia: a convex or converging lens of suitable positive power, which converges the rays and brings the image forward onto the retina.
• The concave lens for a short-sighted person has a negative power and a negative focal length, while the convex lens has both positive.
• In each case the correcting lens is chosen so that the image of the object forms exactly on the retina.

Presbyopia is the loss of near vision that comes with age. It is a physiological and unavoidable change, caused by the gradual weakening of the ciliary muscles and the stiffening of the eye lens, so the lens can no longer become curved enough to focus on a near object.A person with presbyopia holds a book at arm's length to read it, which is a useful clue in a question. The correction is a convex lens, and bifocal lenses with a convex upper portion for near vision and a concave lower portion for distant vision are commonly prescribed for such eyes.

• Cause: weakening of the ciliary muscles with age, so the lens cannot be made sufficiently convex.
• Symptom: near objects cannot be seen distinctly, while distant vision remains normal.
• Correction: convex reading glasses, or bifocals with a convex top for reading and a concave bottom for distance.
• It is an age-related change and not a defect of the eyeball's length, unlike myopia and hypermetropia.
• Children have short eyeballs, so they are normally hypermetropic, and the eye grows as they do.
• The near point of a child is therefore further away than 25 cm, and it comes closer with age.

The choice between a concave and a convex mirror is made by the job the mirror has to do. A concave mirror converges parallel light and can form a real image, so it is used where a large, bright and concentrated image is wanted.A convex mirror diverges reflected light and always forms a virtual, erect and diminished image, so it is used where a wide field of view is wanted and where the size of the image does not matter.

• Shaving and make-up mirror: a concave mirror, because it forms a magnified image when the face is placed within the focal length.
• Dentist's mirror: a concave mirror, used to examine the back of a tooth, since it forms an enlarged image at that small distance.
• Headlamp or headlight reflector: a concave mirror with a polished parabolic surface that reflects parallel light from the bulb into a strong parallel beam.
• Solar cooker: a concave mirror that concentrates the sun's rays at its focus, where the temperature becomes high enough to cook food.
• Rear-view mirror in a vehicle: a convex mirror, since it gives a much wider field of view and shows a larger area behind the vehicle.
• Security mirror in a shop: a convex mirror fitted high in a corner, which lets the shopkeeper watch the whole floor from one point.

A convex lens converges light and is used wherever a real, inverted and sharp image is needed, or wherever a magnified upright image of a small object is required. A concave lens diverges light and is used to correct short-sightedness and to reduce the size of an image.A camera works on the same principle as the eye. Its objective is a convex lens that forms a real, inverted and diminished image on a film or a sensor placed at the focus, and the camera is focused by changing the distance between the lens and the screen rather than by changing the curvature of the lens.

• Camera: a convex lens forms a real, inverted and diminished image on the film or on the sensor.
• Spectacles for hypermetropia: a convex lens, which converges light so that the image forms on the retina.
• Spectacles for myopia: a concave lens, which diverges light so that the image forms on the retina.
• Magnifying glass: a convex lens used with the object placed within the focal length, giving a magnified, virtual and erect image.
• Viewfinder of a camera: a small concave lens that gives an upright, virtual and diminished image, so that the scene can be framed comfortably.
• Reading glass for presbyopia: a convex lens, used in a pair of bifocals for the same reason.

The questions from this chapter fall into three clear groups: a diagram question on the eye and its defects, a five-mark application question on mirrors and lenses, and short questions that test the near point and accommodation.Almost all of them can be scored by writing a fixed set of points, and none of them needs a numerical. Accuracy of vocabulary matters more here than calculation, so the terms retina, ciliary muscles, accommodation, near point and power of the lens should be used exactly.

• Draw a labelled diagram of the human eye and show the formation of an image on the retina.
• What is accommodation, and how is it brought about in the eye?
• State the least distance of distinct vision for a normal eye and explain the role of the ciliary muscles.
• What is presbyopia, what causes it, and how is it corrected?
• Why is a convex mirror fitted as a rear-view mirror in vehicles instead of a concave one?
• State two uses each of a convex lens and a concave lens, giving the reason in each case.