Ans. Light is a form of energy which enables us to see objects. We see an object only when light from the object enters our eyes. This light may be given out by the object itself (if it is luminous) or may be reflected from it (if it is non-luminous).

For example, the Sun and a glowing bulb are luminous objects as they emit their own light. A book, chair or tree is non-luminous, but we can still see them because they reflect light falling on them from a luminous source like the Sun or an electric lamp. Thus, light acts as a link between our eyes and the objects, making the world visible to us.

Ans. Objects can be grouped in different ways depending on how they interact with light.

• Luminous objects: These give out their own light. Examples – Sun, burning candle, glowing bulb.
• Non-luminous objects: These do not emit light but can be seen when light reflects from them. Examples – book, table, tree.
• Transparent objects: Allow light to pass through them completely; we can see clearly through them. Examples – clear glass, clean water.
• Translucent objects: Allow light to pass partially; objects behind them appear blurred. Examples – oiled paper, frosted glass.
• Opaque objects: Do not allow light to pass through; they form shadows. Examples – wood, stone, metal box.

These differences are important for understanding visibility and the formation of shadows in this chapter.

Ans. The property of light by which it travels in a straight line is called rectilinear propagation of light. This is an important basic property used to explain many phenomena such as shadows and pinhole images.

We can demonstrate this by the following activities:

• Take three cardboards with small pinholes in the centre. Arrange them in a straight line and place a candle in front of the first hole. When we look through the holes, we can see the candle flame only if all the holes are exactly in a straight line. If any one cardboard is shifted a little, the flame is no longer visible. This shows that light travels in straight paths.
• When an opaque object is placed in the path of light, a sharp shadow is formed on the screen behind it. If light could bend around the object, the shadow would not be sharp. The straight boundary lines of the shadow also indicate that light moves in straight lines.

These simple observations confirm the rectilinear propagation of light.

Ans. A shadow is a dark region formed on a surface when an opaque object blocks the path of light coming from a source. The shadow shows the outline of the object on the screen.

The following conditions are necessary for the formation of a shadow:

• Source of light: An object can form a shadow only if there is a source of light like the Sun, lamp or torch.
• Opaque object: The object must be opaque or at least translucent so that it can block the light partially or fully.
• Screen: A surface such as a wall, ground or screen is needed where the shadow can fall and be seen.

When light travels in a straight line and is obstructed by the opaque object, the region behind it receives no light and appears dark. This dark region is the shadow. Shadows help us know the shape of objects and are also used in sundials to estimate time.

Ans. When light falls on the surface of a body and bounces back into the same medium instead of passing through it, the phenomenon is called reflection of light. Mirrors work on this principle.

Important terms used in reflection are:

• Incident ray: The ray of light that strikes the reflecting surface.
• Reflected ray: The ray of light that bounces back from the reflecting surface.
• Normal: An imaginary line drawn perpendicular to the surface at the point where the incident ray falls.
• Angle of incidence (∠i): The angle between the incident ray and the normal.
• Angle of reflection (∠r): The angle between the reflected ray and the normal.

These terms are used to state and verify the laws of reflection for plane mirrors.

Ans. The two laws of reflection are:

• First law: The incident ray, the reflected ray and the normal at the point of incidence all lie in the same plane.
• Second law: The angle of reflection is always equal to the angle of incidence (∠r = ∠i).

Verification: Place a plane mirror on a drawing sheet and draw its outline. Draw a normal at a point on the mirror. With a ray box, send an incident ray making a fixed angle with the normal and mark the path. Observe the reflected ray and mark its path. Measure ∠i and ∠r with a protractor. They are found to be equal. Repeating this for different incident angles gives the same result. The incident ray, reflected ray and normal are seen on the same sheet, showing that they lie in one plane. Thus both laws are verified.

Ans. In regular reflection, a beam of parallel light rays falls on a smooth, polished surface and is reflected as a beam of parallel rays. The reflected rays go in one direction and form clear images. Plane mirrors and still water produce regular reflection.

In diffused reflection (or irregular reflection), light falls on a rough surface and the rays are reflected in many different directions because each small part of the surface has a different slope. Reflected rays are scattered and no clear image is formed. A wall, paper or curtain gives diffused reflection.

Both types are important. Regular reflection helps in seeing sharp images in mirrors, while diffused reflection from walls and surroundings makes objects visible from all directions and provides uniform daylight in a room.

Ans. No, diffused reflection does not mean that the laws of reflection are violated. The laws of reflection hold true for every small point on a rough surface.

A rough surface consists of many tiny, uneven surfaces, each tilted at a different angle. When light strikes each tiny part, it is reflected according to the laws of reflection (∠i = ∠r). However, because each portion is differently oriented, the direction of reflected rays is different from point to point. As a result, the rays are scattered in many directions, giving diffused reflection.

Therefore, scattering of light is due to irregularities of the surface, not due to the failure of the laws of reflection. The laws remain valid at every point on the surface.

Ans. The image formed by a plane mirror has the following characteristics:

• Virtual: The image cannot be obtained on a screen because the reflected rays only appear to meet behind the mirror.
• Erect: The image is upright; the top of the object appears at the top in the image.
• Same size: The size of the image is exactly the same as the size of the object.
• Laterally inverted: The left side of the object appears as the right side in the image and vice versa.
• Equal distance: The image is formed as far behind the mirror as the object is in front of it.

These properties are often asked in exams and help us understand why plane mirrors are widely used for dressing mirrors and in optical devices like periscopes.

Ans. Lateral inversion is the phenomenon in which the left side of an object appears on the right side in its image formed by a plane mirror, and the right side appears on the left.

For example, the word “AMBULANCE” is often written in reversed order on the front of an ambulance. When a driver of a car ahead looks into the rear-view mirror, the mirror reverses the letters again due to lateral inversion, and the word “AMBULANCE” appears correctly. This helps the driver quickly recognise the ambulance and give way.

Thus, lateral inversion explains why mirror images appear reversed from left to right and why certain words are written in reverse for safety purposes.

Ans. In a plane mirror, the image is formed behind the mirror at the same perpendicular distance as the object is in front of it. If the boy is standing 2 m in front of the mirror, his image will be formed 2 m behind the mirror.

The total distance between the boy and his image is the sum of the object distance and image distance from the mirror. Therefore, distance between boy and his image = 2 m (in front) + 2 m (behind) = 4 m.

The image will be virtual, erect and of the same size as the boy due to the properties of plane mirrors.

Ans. Plane mirrors are commonly used as dressing mirrors because:

• They form an image of the same size as the object, so we see ourselves in correct proportion – not enlarged or reduced.
• The image is erect and virtual, which is comfortable for viewing our face and body features clearly.
• The distance of the image behind the mirror equals our distance from the mirror, giving a realistic sense of depth.

These properties together make plane mirrors ideal for grooming, makeup and checking appearance in homes, shops and salons.

Ans. When light is reflected successively from two or more reflecting surfaces, the phenomenon is called multiple reflection. Due to multiple reflection between two mirrors, several images of an object can be seen.

Activity:

• Place two plane mirrors facing each other and make an angle between them (say 60°).
• Keep a small object like a candle or toy between them.
• On looking into one of the mirrors, we see a number of images of the object.

The light from the object reflects from one mirror to the other repeatedly, creating multiple images. As the angle between the mirrors decreases, the number of images increases. When mirrors are parallel, many images are formed, giving the impression of an endless row.

Ans. A kaleidoscope is an optical device that produces beautiful, symmetrical patterns using multiple reflection of light.

Construction:

• Three plane mirrors of equal size are joined along their length to form a triangular tube (usually an equilateral triangle in cross-section).
• One end of the tube is closed with a transparent glass plate. Small pieces of coloured glass or beads are placed on this end.
• The other end is closed with a plain glass having a small hole at the centre to look through.

Working:

When we look through the hole, light enters the tube, falls on the coloured glass pieces and gets reflected multiple times between the three mirrors. Each reflection produces more images, and we see a pattern that changes when the tube is rotated or the glass pieces move.

Uses: Kaleidoscopes are used by designers and artists to get ideas for patterns on wallpapers, fabrics, tiles and other decorative items.

Ans. A periscope is an instrument that allows us to see objects which are not in the direct line of sight, using the principle of reflection of light.

Principle: It works on the principle of reflection of light from plane mirrors. Light from the object is reflected twice and reaches the observer’s eye.

Construction:

• A simple periscope consists of a long tube bent at two right angles.
• Two plane mirrors are fixed inside the tube at 45° to the horizontal, facing each other.
• The upper mirror receives light from the object; the lower mirror reflects the light towards the observer’s eye.

Working/Use: Light from the object falls on the upper mirror, is reflected to the lower mirror and finally to the eye. Thus, a person can see over a wall or from inside a trench or submarine without being seen. Periscopes are widely used in submarines and by soldiers in war fields.

Ans. When an object is placed between two plane mirrors inclined at an angle, multiple images are formed due to repeated reflections. As the angle between the mirrors decreases, the number of images seen increases.

When mirrors are placed at a wide angle, light from the object gets reflected fewer times from one mirror to the other. When the mirrors are closer (smaller angle), light can bounce back and forth many more times, producing more images. In the extreme case, when mirrors are parallel (angle close to 0°), light can reflect indefinitely, giving a very large number of images, appearing like an endless row.

This shows that the number of images in multiple reflection depends on the angle between the mirrors.

Ans. Sunlight appears white to our eyes, but it is actually made up of many colours. We can show this by passing sunlight through a glass prism.

In a simple activity, a prism is placed in the path of a narrow beam of sunlight entering a dark room. On a white screen placed on the other side of the prism, we observe a band of different colours – violet, indigo, blue, green, yellow, orange and red. This band of colours is called a spectrum.

The formation of this spectrum shows that white sunlight is actually a combination of these seven colours. The prism bends different colours by different amounts, separating them and making the hidden colours visible.

Ans. Dispersion of light is the phenomenon in which white light splits into its component colours when it passes through a transparent medium like a glass prism.

Activity:

• Place a prism on a table near a window in such a way that a ray of sunlight falls on one of its faces.
• Put a white screen on the other side of the prism.
• Adjust the prism until a coloured band is seen on the screen.

The band of seven colours (VIBGYOR) appearing on the screen is the spectrum. It shows that white light is made up of different colours, each bent by a different amount by the prism. This separation of colours from white light is dispersion.

Ans. A rainbow is a natural spectrum appearing in the sky, usually after rain when the Sun is shining. It is formed due to dispersion, refraction and reflection of sunlight by tiny water droplets in the atmosphere.

When sunlight enters a water droplet, it is refracted (bent) and dispersed into different colours. Inside the droplet, the light is reflected from the inner surface and then refracted again when it comes out. Due to this double refraction and one internal reflection, the different colours emerge at slightly different angles. Many droplets together produce a large, circular arc of colours in the sky.

A rainbow is seen only when the Sun shines during or after rain because water droplets in the air are necessary to act as tiny prisms. The observer must have the Sun behind and droplets in front to see the rainbow in the opposite part of the sky.

Ans. A spectrum is the band of different colours obtained when white light is dispersed by a prism. It shows that white light is actually a mixture of many colours.

The seven colours of the visible spectrum in order are: Violet, Indigo, Blue, Green, Yellow, Orange and Red. The first letters together form the word VIBGYOR, which helps in remembering the correct sequence.

Understanding the spectrum and VIBGYOR is important because:

• It explains the coloured patterns seen in rainbows and CD surfaces.
• It shows that different colours of light bend differently in a medium.
• It forms the basis of further study of optics, colour formation and the working of various optical instruments.

Ans. (Students should draw the diagram in their notebook.) The main parts and their functions are:

• Cornea: The transparent front part of the eye that allows light to enter and begins the focusing process.
• Iris: The coloured part of the eye surrounding the pupil; it controls the size of the pupil and thus the amount of light entering the eye.
• Pupil: The small black opening in the centre of the iris through which light enters the eye.
• Eye lens: A transparent, flexible, convex lens that focuses light rays onto the retina to form an image.
• Retina: The light-sensitive layer at the back of the eye that receives the image and converts it into electrical signals.

These signals are then carried by the optic nerve to the brain, which interprets them and enables us to see.

Ans. Our sense of vision is the result of coordinated work of the eyes and the brain.

When we look at an object, light reflected from it enters the eye through the cornea and pupil. The eye lens focuses this light on the retina, forming a real, inverted image. The retina contains light-sensitive cells that convert this optical image into electrical signals. These signals travel through the optic nerve to the brain.

The brain processes these signals, interprets them and finally gives us the sensation of an upright object at its correct position. Thus, we “see” not only with our eyes, but also with the help of our brain, which understands the information sent by the eyes.

Ans. To keep our eyes healthy, we must follow some simple rules:

• Always read or write in sufficient light, neither too dim nor too bright, to avoid eye strain.
• Maintain a proper distance from books, TV and mobile screens; do not hold them very close to the eyes.
• Do not rub your eyes with dirty hands; if something enters the eye, wash with clean water and consult a doctor if irritation persists.
• Eat a balanced diet rich in vitamin A (carrots, green leafy vegetables, milk, etc.) which is good for eye health.
• Have regular eye check-ups, especially if you have difficulty in seeing the board at school or feel frequent headaches while reading.

These measures protect our eyes from infection, strain and long-term vision problems.

Ans. The Braille system is a method of reading and writing used by visually impaired persons. It uses patterns of raised dots that can be felt with the fingertips.

In Braille, each letter, number and punctuation mark is represented by a special arrangement of six raised dots in a rectangular cell. Visually impaired people run their fingers gently over these dots to identify the characters. By combining many such cells, words and sentences are formed.

Braille allows visually impaired learners to read textbooks, write notes, and study subjects like Science and Mathematics. It provides them a way to become educated, independent and confident like other students.

Ans. The Braille system was developed by Louis Braille, a Frenchman who lost his eyesight in childhood. He devised this script so that blind persons could read and write through touch.

Main features of modern Braille used in India:

• Uses patterns of raised dots arranged in cells of six positions.
• Has codes not only for letters but also for numbers, punctuation and mathematical symbols.
• Is adapted to Indian languages as well as English, so students can learn in their mother tongue.
• Braille books are printed with raised dots on thick paper, allowing repeated reading.

Modern Braille has opened the doors of education for visually impaired children across the country.

Ans. We should never look directly at the Sun, welding arcs or other powerful light sources because the intense light and heat can damage the delicate cells of the retina permanently. This may lead to serious vision problems or even blindness.

Precautions:

• Use proper filters or eclipse glasses specially designed for viewing solar eclipses; ordinary sunglasses are not safe for this.
• Wear good quality sunglasses when going out in strong sunlight to reduce glare.
• Avoid staring at very bright lamps, lasers or welding arcs.
• Follow the advice of eye specialists and teachers regarding eye safety in laboratories and workshops.

These precautions protect our eyes from excessive light and help maintain healthy eyesight.

Ans. When we stand in front of a plane mirror and see ourselves, several concepts from this chapter work together.

First, light from a source (like a tube light) falls on our body and is reflected in all directions. Some of this reflected light falls on the plane mirror. The mirror again reflects the light according to the laws of reflection (∠i = ∠r). The reflected rays from the mirror enter our eyes.

Inside the eye, the lens focuses these reflected rays on the retina, forming an image. The retina converts it into signals which are sent to the brain. The brain interprets these signals as an image of ourselves appearing behind the mirror. Thus, light, reflection from a plane mirror and the working of the eye together enable us to see our own image.

Ans. The statement is not completely correct. We see objects around us mainly due to diffused reflection, not only regular reflection.

In normal conditions, light from the Sun or a bulb falls on walls, furniture, floor, books and other objects. These surfaces are usually not perfectly smooth, so they reflect light in many directions (diffused reflection). Some of these scattered rays enter our eyes, making the objects visible from different positions.

Regular reflection occurs from smooth surfaces like mirrors and still water, giving clear images. But if only regular reflection occurred everywhere, we would see bright spots and dark regions, not a uniformly lit room. Therefore, both types of reflection are important, but diffused reflection plays a major role in everyday vision.

Ans. Our eyes are sense organs that can detect light. However, they need light to function. Without light, we cannot see anything.

In a completely dark room, there is no light to be reflected from objects. As a result, no light enters the eyes, and the retina receives no signal to send to the brain. Thus, even a person with perfect eyesight cannot see objects in total darkness.

When light is present, it falls on objects, gets reflected and enters our eyes. The eye lens focuses this light onto the retina, and the brain interprets the signals as visual images. Hence, light is essential for vision, just as the eye and brain are essential for interpreting what we see.

Ans. The important concepts from Chapter 16: Light for CBSE Class 8 exams are:

• Light as a form of energy helping us to see; luminous and non-luminous objects.
• Rectilinear propagation of light and formation of shadows.
• Reflection of light, definitions of incident ray, reflected ray, normal, angle of incidence and angle of reflection.
• Laws of reflection and their verification, regular and diffused reflection with examples.
• Image formation by a plane mirror, characteristics of the image and lateral inversion.
• Multiple reflection, kaleidoscope and periscope – construction, working and uses.
• Sunlight as a mixture of colours, dispersion of light, spectrum, VIBGYOR and formation of rainbow.
• Structure and working of the human eye, care of eyes and the Braille system for visually impaired persons.

Thorough understanding and regular practice of questions from these topics will help students score well in school tests as well as in annual examinations.