What Makes Things Visible

Our sense of sight is incredibly important, allowing us to perceive the world around us. We see objects like mountains, trees, people, and the moon and stars. But how exactly is seeing possible?

While our eyes are the organs of sight, they cannot see in complete darkness. This indicates that eyes alone are not sufficient for seeing. We see an object only when light from that object enters our eyes. This light can either be emitted by the object itself (if it is luminous) or reflected by the object (if it is illuminated by another light source).

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Laws Of Reflection

When light falls on a surface, it bounces back. This phenomenon is called reflection. A polished or shiny surface, like a mirror, is particularly effective at reflecting light.

To study reflection, we can consider light as travelling in straight lines called rays. A narrow beam of light consists of many rays. When a ray of light strikes a surface, it is called the incident ray. The ray that bounces back from the surface is called the reflected ray.

At the point where the incident ray strikes the surface, we draw a line perpendicular to the surface. This line is called the normal. The angle between the incident ray and the normal is the angle of incidence ($\angle i$). The angle between the reflected ray and the normal is the angle of reflection ($\angle r$).

Experiments show that reflection follows specific rules known as the Laws of Reflection:

1. The angle of incidence is always equal to the angle of reflection ($\angle i = \angle r$).
2. The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane. This means they can all be represented on a flat surface, like a sheet of paper. If you bend the paper containing the reflected ray out of the plane of the incident ray and normal, you will no longer see the reflected ray.

A plane mirror forms an image of an object. This image has certain characteristics:

• It is virtual (cannot be obtained on a screen).
• It is erect (upright).
• It is of the same size as the object.
• It appears to be located at the same distance behind the mirror as the object is in front of it.
• It undergoes lateral inversion, meaning the left side of the object appears as the right side of the image, and vice versa (seen when looking at text in a mirror).

Image formation in a plane mirror occurs because reflected rays from a point on the object, when extended backward, appear to meet at a point behind the mirror where the virtual image is formed.

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Regular And Diffused Reflection

The nature of the reflecting surface affects how parallel rays of light are reflected:

• Regular Reflection: Occurs when parallel rays of light strike a smooth, polished, and regular surface (like a plane mirror). All the reflected rays are parallel to each other. Images are formed by regular reflection.
• Diffused or Irregular Reflection: Occurs when parallel rays of light strike a rough or irregular surface (like cardboard, a wall, or a piece of paper). Although the laws of reflection ($\angle i = \angle r$, etc.) are still obeyed at each individual point on the surface, the irregularities cause the reflected rays to scatter in different directions and not be parallel. Diffused reflection is why we can see objects from different angles even if they don't emit light. Diffused reflection does not mean the laws of reflection have failed; it is a consequence of the surface's irregularity.

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Do We See All Objects Due To Reflected Light?

Most objects around us are visible because they reflect light from a source into our eyes. These are called illuminated objects. Examples include the moon (reflects sunlight), tables, chairs, and people. Objects that produce their own light and are visible because their light directly enters our eyes are called luminous objects. Examples include the sun, a burning candle, and an electric lamp.

Reflected Light Can Be Reflected Again

Light that has been reflected from one surface can be incident on another surface and be reflected again. This principle is used in devices like periscopes and in everyday situations like using multiple mirrors.

In a hairdresser's shop, a mirror held behind your head allows you to see the reflection of the back of your head in the mirror in front of you. Light from the back of your head reflects off the mirror held behind you, and this reflected light then falls on the mirror in front, reflecting into your eyes. This is an example of light being reflected twice.

A periscope uses two plane mirrors placed parallel to each other at an angle of 45° to the incoming light. Light from an object enters the top mirror, is reflected down, hits the bottom mirror, and is reflected horizontally into the viewer's eye. This allows viewing objects that are not in the direct line of sight, such as over obstacles or from a submerged submarine.

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Multiple Images

A single plane mirror forms one image. However, when two or more plane mirrors are placed at an angle to each other, they can form multiple images of a single object.

The number of images formed depends on the angle between the mirrors. For two mirrors placed at an angle $\theta$, the number of images ($n$) formed can be calculated using the formula:

$ n = \left(\frac{360^\circ}{\theta}\right) - 1 $ (if $\theta$ divides $360^\circ$ exactly and the object is placed symmetrically)

If the object is placed asymmetrically, or if $360^\circ/\theta$ is not a whole number, the calculation is slightly more complex, but the principle is based on repeated reflection.

For specific angles:

• Two mirrors at right angles (90°): $n = (360/90) - 1 = 4 - 1 = 3$ images.
• Two mirrors parallel to each other (angle 0° or 180°, considered as $\theta \to 0$): An infinite number of images are formed due to repeated reflections back and forth between the mirrors.

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Kaleidoscope

The principle of forming multiple images using inclined mirrors is used in a fascinating optical toy called a kaleidoscope. A kaleidoscope typically consists of three rectangular mirror strips joined to form a prism, placed inside a tube. Small pieces of coloured glass or beads are placed at one end. When viewed through a hole at the other end, the mirrors create beautiful, symmetrical patterns formed by multiple reflections of the coloured pieces. Rotating the tube changes the arrangement of the coloured pieces, generating endless, unique patterns. Kaleidoscopes are used by designers and artists for inspiration.

Sunlight — White Or Coloured

Sunlight, which appears white, is actually composed of multiple colours. This was demonstrated by Isaac Newton using a prism, and can also be seen in natural phenomena like rainbows.

When white light passes through a medium like a prism or water droplets, it splits into its constituent colours: violet, indigo, blue, green, yellow, orange, and red (often remembered by the acronym VIBGYOR). This phenomenon is called dispersion of light.

Placing a plane mirror in a bowl of water near a window where sunlight falls on the mirror can act like a prism. The reflected sunlight, when projected onto a wall, can be seen as a spectrum of colours, demonstrating that white sunlight is composed of these seven colours. A rainbow is a natural example of dispersion, where sunlight is dispersed by water droplets in the atmosphere.

What Is Inside Our Eyes?

Our eye is a remarkable sense organ that allows us to see by processing light that enters it. The eye has a complex structure:

• Cornea: The transparent, outer front part of the eye. It protects the eye and refracts (bends) incoming light.
• Iris: Behind the cornea is a dark, muscular structure. The iris gives the eye its unique colour (e.g., blue, brown, green eyes).
• Pupil: A small opening in the center of the iris. The size of the pupil is controlled by the iris and regulates the amount of light entering the eye. In dim light, the iris expands the pupil to let in more light. In bright light, the iris contracts the pupil to reduce the amount of light entering.
• Lens: Located behind the pupil, the eye lens is a convex lens (thicker in the center). It focuses the light onto the retina.
• Retina: A light-sensitive layer at the back of the eye. The focused light forms an image on the retina. The retina contains millions of nerve cells (photoreceptors) that are sensitive to light.
• Optic Nerve: A bundle of nerves that transmits the electrical signals generated by the nerve cells in the retina to the brain. The brain processes these signals to form the perception of sight.

The nerve cells in the retina are of two types:

• Cones: Sensitive to bright light and detect colour. They function best in daylight.
• Rods: Sensitive to dim light and are responsible for vision in low light conditions (black and white vision). They do not detect colour.

At the junction where the optic nerve leaves the eye, there are no photoreceptor cells (rods or cones). This spot is insensitive to light, creating a blind spot in our vision. We don't normally notice this blind spot because the brain fills in the missing information based on the surrounding image and the other eye's vision.

The impression of an image on the retina does not disappear instantly but persists for about 1/16th of a second. This phenomenon, called persistence of vision, is the basis of motion pictures. When a series of still images are flashed in rapid succession (at a rate faster than 16 frames per second), our brain perceives continuous movement.

A normal eye can adjust its focus to see both distant and nearby objects clearly. The most comfortable reading distance for a normal eye is about 25 cm. Visual defects like near-sightedness (difficulty seeing distant objects) and far-sightedness (difficulty seeing nearby objects) can be corrected using appropriate lenses.

Cataracts, common in old age, occur when the eye lens becomes cloudy, causing foggy or severe loss of vision. This can often be treated by surgically removing the opaque lens and replacing it with a new artificial lens.

Care Of The Eyes

Proper care is essential to maintain healthy eyes:

• Visit an eye specialist regularly for checkups.
• Use suitable spectacles if advised.
• Ensure adequate lighting for reading and working; too little or too much light can strain the eyes or cause damage. Avoid looking directly at intense light sources like the sun or laser torches.
• Never rub your eyes; wash them with clean water if dust or particles enter. Seek medical help if irritation persists.
• Maintain the correct reading distance (about 25 cm) to avoid straining your eyes.
• Consume a balanced diet rich in vitamins, especially vitamin A, which is crucial for eye health. Deficiency of vitamin A can lead to vision problems like night blindness. Good sources of vitamin A include raw carrots, broccoli, leafy green vegetables (spinach), cod liver oil, eggs, milk, curd, cheese, butter, papaya, and mango.

Visually Impaired Persons Can Read And Write

Visually impaired individuals, who have limited or no sight, develop their other senses, such as touch and hearing, more acutely to interact with their environment and recognise objects. Additional resources and technologies can help them to enhance their abilities and lead fulfilling lives.

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What Is The Braille System?

The Braille system is a widely used method of reading and writing for visually impaired people. It was developed by Louis Braille, who was himself visually challenged.

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Louis Braille

Louis Braille created this tactile system in 1821. It consists of raised dot patterns that visually impaired individuals can read by touching with their fingertips.

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Braille System Details

The Braille system uses a set of 63 basic dot patterns or characters. Each character is formed within a 'cell' consisting of two vertical rows of three dots each (a 3x2 grid). Different arrangements of raised dots within this cell represent letters of the alphabet, combinations of letters, common words, or grammatical signs. These patterns are embossed onto paper, creating a textured surface that can be read by touch. Readers learn to recognise the different patterns through their fingertips.

The current standard Braille code was adopted in 1932. There are specific Braille codes adapted for different languages, including many Indian languages, as well as for mathematics and scientific notation. Braille texts can be produced manually using a stylus and slate or by machines resembling typewriters and printers.

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Non-optical And Optical Aids For Visually Impaired

Visually impaired individuals use various aids to assist them in daily tasks and learning:

• Non-optical Aids: These include:
  • Visual aids: Devices that magnify text or provide better lighting.
  • Tactual aids: Rely on the sense of touch, like Braille writers and reading materials.
  • Auditory aids: Use the sense of hearing, such as audio books, tape recorders, and talking devices.
  • Electronic aids: Devices like talking calculators and computers with screen readers or voice output. Closed-circuit televisions (CCTV) can enlarge printed material.
• Optical Aids: These involve lenses and optical devices to improve residual vision:
  • Bifocal lenses, contact lenses, and tinted lenses to correct vision problems.
  • Magnifiers to enlarge text.
  • Telescopic aids for viewing distant objects like a blackboard.

These aids, combined with training and support, empower visually impaired individuals to overcome challenges and participate more fully in education, work, and society.

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