Class 10 Science · Chapter 10 NotesThe Human Eye and the Colourful World

The human eye is a sensitive sense organ that works like a camera. Light enters through the cornea, a thin transparent membrane forming the front bulge of the eyeball. Most refraction of light happens at the outer surface of the cornea. Behind the cornea is the iris, a dark muscular diaphragm that controls the size of the pupil. The pupil regulates the amount of light entering the eye. The crystalline lens provides fine adjustment of focal length so that objects at different distances are focused on the retina, a light-sensitive screen at the back of the eye. The lens forms an inverted real image on the retina. Light-sensitive cells in the retina generate electrical signals that travel through the optic nerves to the brain, which interprets them so we perceive objects as they are.

• Corneatransparent front bulge; most refraction occurs here.
• Irisdark muscular diaphragm that controls pupil size.
• Pupilregulates the amount of light entering the eye.
• Eye lensprovides fine adjustment of focal length.
• Retinalight-sensitive screen where the image is formed.
• Optic nervecarries electrical signals to the brain.

The eye lens is made of a fibrous, jelly-like material and its curvature can be changed by the ciliary muscles. This ability of the eye lens to adjust its focal length is called accommodation. When the ciliary muscles are relaxed, the lens becomes thin, its focal length increases, and we can see distant objects clearly. When we look at nearby objects, the ciliary muscles contract, the lens becomes thicker, its focal length decreases, and nearby objects are seen clearly. The focal length cannot be reduced below a certain minimum. The minimum distance at which an object can be seen most distinctly without strain is called the least distance of distinct vision, or the near point. For a young adult with normal vision, it is about 25 cm. The farthest point up to which the eye can see clearly is the far point, which is infinity for a normal eye.

• Accommodationability of the eye lens to adjust its focal length.
• Near point (least distance of distinct vision)about 25 cm for a young adult with normal vision.
• Far pointinfinity for a normal eye.
• Ciliary muscles relaxedlens thin, focal length increases, distant objects seen.
• Ciliary muscles contractedlens thicker, focal length decreases, nearby objects seen.

The eye may gradually lose its power of accommodation, causing blurred vision. The three common refractive defects are myopia, hypermetropia and presbyopia. Myopia, or near-sightedness, is when a person can see nearby objects clearly but distant objects are blurred. The image of a distant object forms in front of the retina. It is caused by excessive curvature of the eye lens or elongation of the eyeball, and is corrected using a concave lens of suitable power. Hypermetropia, or far-sightedness, is when distant objects are clear but nearby objects are blurred. The image of a nearby object forms behind the retina. It is caused by too long a focal length of the eye lens or a small eyeball, and is corrected using a convex lens. Presbyopia occurs with ageing due to weakening of ciliary muscles and diminishing flexibility of the eye lens; the near point recedes. It is corrected using convex lenses, and people with both myopia and hypermetropia may need bi-focal lenses. Cataract, where the lens becomes milky and cloudy, causes partial or complete loss of vision and can be treated by surgery.

• Myopianear-sightedness; image of distant object forms in front of retina; corrected by concave lens.
• Hypermetropiafar-sightedness; image of nearby object forms behind retina; corrected by convex lens.
• Presbyopiaage-related loss of accommodation; corrected by convex lens; bi-focal lenses for both defects.
• Cataractlens becomes milky and cloudy; treated by cataract surgery.

A triangular glass prism has two triangular bases and three rectangular lateral surfaces inclined to each other. The angle between its two lateral faces is called the angle of the prism. When a ray of light passes through a prism, it bends towards the normal at the first surface (air to glass) and away from the normal at the second surface (glass to air). The incident ray PE, refracted ray EF and emergent ray FS are shown in the activity. The peculiar shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray. This angle is called the angle of deviation, denoted by ∠D. Unlike a rectangular glass slab where the emergent ray is parallel to the incident ray, in a prism the emergent ray is not parallel to the incident ray.

• Angle of prismangle between the two lateral faces.
• Incident ray PE, refracted ray EF, emergent ray FS.
• Angle of deviation (∠D)angle between the direction of the incident ray and the emergent ray.
• In a glass slab, emergent ray is parallel to incident ray; in a prism, it is not.

When a narrow beam of white light is passed through a glass prism, it splits into a band of seven colours: violet, indigo, blue, green, yellow, orange and red. The acronym VIBGYOR helps remember the sequence. The band of coloured components of a light beam is called its spectrum, and the splitting of light into its component colours is called dispersion. Different colours bend through different angles; red bends the least and violet the most. Isaac Newton first used a glass prism to obtain the spectrum of sunlight. He placed a second identical prism in an inverted position and found that the colours recombined to give white light, showing that sunlight is made of seven colours. A rainbow is a natural spectrum formed after a rain shower due to dispersion of sunlight by tiny water droplets, which act like small prisms, refracting, internally reflecting and again refracting light.

• Dispersionsplitting of white light into its component colours.
• Spectrumband of coloured components of light.
• SequenceViolet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR).
• Red bends least; violet bends most.
• Rainbownatural spectrum caused by dispersion and internal reflection in water droplets.

Atmospheric refraction is the refraction of light by the earth's atmosphere, which has a gradually changing refractive index. The twinkling of stars is due to atmospheric refraction. Starlight undergoes continuous refraction before reaching the earth, and the apparent position of the star keeps changing slightly because the physical conditions of the atmosphere are not stationary. Since stars are very distant, they act as point sources, so the amount of light entering the eye flickers, making the star appear brighter or fainter. Planets do not twinkle because they are much closer and act as extended sources; the variations from many point sources average out to zero. Atmospheric refraction also causes advance sunrise and delayed sunset by about 2 minutes, and the apparent flattening of the Sun's disc at sunrise and sunset.

• Atmospheric refractionrefraction of light by the earth's atmosphere.
• Twinkling of starsdue to varying apparent position and amount of starlight.
• Planets do not twinklethey are extended sources, variations average out.
• Advance sunrise and delayed sunsetabout 2 minutes earlier and later due to atmospheric refraction.

The earth's atmosphere contains minute particles such as smoke, water droplets, dust and air molecules. When a beam of light strikes these fine particles, the path of the beam becomes visible because light is scattered. This phenomenon is called the Tyndall effect. It can be seen when sunlight enters a smoke-filled room or passes through a canopy of a dense forest. The colour of scattered light depends on the size of the scattering particles: very fine particles scatter mainly blue light, while larger particles scatter longer wavelengths, and if particles are large enough, the scattered light may appear white. The blue colour of the clear sky is because air molecules and fine particles are smaller than the wavelength of visible light and scatter blue light more strongly than red. Red light has a wavelength about 1.8 times greater than blue light. At very high altitudes, scattering is not prominent, so the sky appears dark. Red is least scattered by fog or smoke, which is why danger signal lights are red.

• Tyndall effectscattering of light by colloidal particles makes the path visible.
• Blue skyfine particles scatter blue light more than red.
• Red light has wavelength about 1.8 times greater than blue light.
• Danger signals are red because red is least scattered by fog or smoke.