NCERT Science Notes - Class 8
Chapter 13 - Light

Welcome to AJs Chalo Seekhen. This webpage is dedicated to Class 8 | Science | Chapter 13 - Light. The chapter delves into the fascinating properties and behavior of light. This chapter covers essential concepts such as reflection, refraction, and dispersion of light. Students will learn about the laws of reflection, the formation of images by plane mirrors, and the functioning of the human eye. Engaging activities and experiments, such as observing how light bends through different mediums and creating simple optical instruments, make learning interactive and fun. This chapter aims to provide a comprehensive understanding of light, its various phenomena, and its significance in our daily lives.

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NCERT Science Notes - Class 8
Chapter 13 - Light

How is the World Known?

• Senses: The world is largely known through our senses.
  • Definition: Senses are the physiological mechanisms through which we perceive external stimuli. These include sight, hearing, touch, taste, and smell.

• Importance: The sense of sight is one of the most important senses.
  • Definition: Sight is the ability to perceive objects through the eyes by detecting light.

• With sight, we can see:
  • Natural objects: Mountains, rivers, trees, plants.
  • Man-made objects: Chairs, people, and other items around us.
  • Sky objects: Clouds, rainbows, birds.
  • Night objects: The moon and stars.
  • Reading: We can also see printed words and sentences.

• Seeing is made possible through the sense of sight, which involves the eyes detecting light and processing it to create images of the world around us.

13.1 - Force of Friction

What Makes Things Visible?

• Common Thought: You might think that we see objects simply because of our eyes.
• Clarification: However, eyes alone cannot see objects, especially in the dark.

• We can see an object only when light from the object enters our eyes.
• Types of Light:
  • The light may be emitted (produced by the object itself, like a light bulb).
  • The light may be reflected (bounced off the object, like sunlight hitting a tree).

• Definition: Reflection is the bouncing back of light when it hits a surface.
• Mirrors: A polished or shiny surface can act as a mirror.
  • Function of a Mirror: A mirror changes the direction of the light that falls on it.

• Explanation: The direction of reflected light depends on the angle at which it hits the surface. This is what we will explore further.

13.2 - Laws of Reflection

1. Reflection of Light:
   • Definition: The process where light bounces off a surface is called reflection.
   • Mirror's Role: A mirror reflects light in a specific direction, which is determined by certain laws.

• A white sheet of paper (for surface visibility).
• A comb (with all openings except one blocked using black paper).
• A torch (as a light source).
• A plane mirror (to observe the reflection).

1. Setup: Fix the white sheet on a flat surface like a drawing board or table.
2. Comb Preparation: Close all the openings of the comb except for one in the middle using a black paper strip.
3. Positioning: Hold the comb perpendicular to the paper.
4. Light Source: Shine light from the torch through the single open tooth of the comb.
5. Observation: You will see a ray of light on the paper coming through the comb.
6. Mirror Placement: Place a plane mirror in the path of this light ray.
7. What to Observe: After placing the mirror, you will notice that the light ray is reflected off the mirror.

Incident and Reflected Rays

1. Incident Ray:
   • The light ray that strikes any surface is called the incident ray.
2. Reflected Ray:
   • The light ray that comes back from the surface after reflection is known as the reflected ray.

Ray of Light: Idealization and Beam

• Ray of Light: A ray of light is an idealization.
• Narrow Beam: In reality, we observe a narrow beam of light, which consists of several rays.
• Simplification: For ease, we refer to this narrow beam as a ray of light.

1. Drawing the Diagram:
   • Draw lines showing the plane mirror, incident ray, and reflected ray on paper with the help of others.
   • Remove the mirror and comb after positioning the lines.
2. Normal Line:
   • Draw a line at a 90º angle to the line representing the mirror at the point where the incident ray strikes the mirror.
   • Normal Line: This 90º line is known as the normal to the reflecting surface.

1. Angle of Incidence (∠i):
   • The angle between the normal and the incident ray is called the angle of incidence.
2. Angle of Reflection (∠r):
   • The angle between the normal and the reflected ray is known as the angle of reflection.
3. Measuring Angles:
   • Measure both the angle of incidence and the angle of reflection.
   • Repeat the activity multiple times with different angles of incidence.

Relation Between Angle of Incidence and Angle of Reflection:

1. Observation:
   • When measuring the angle of incidence (∠i) and the angle of reflection (∠r), it is observed that they are approximately equal.
2. Law of Reflection:
   • Statement: The angle of incidence is always equal to the angle of reflection.
   • This relationship holds true if the experiment is conducted carefully.
   • This is known as the first law of reflection.

1. What Happens if Light is Thrown Along the Normal?
   • Normal Line: The normal is the line drawn perpendicular (90º) to the surface at the point of incidence.
   • If light is thrown along the normal:
     • The angle of incidence (∠i) will be 0º.
     • The light will reflect back along the same path, meaning the angle of reflection (∠r) will also be 0º.
     • The light ray will not deviate from its path since it hits the mirror at 90º.

Activity 13.2: Observing the Reflection on a Bent Surface

Reflection in a Plane:

1. Plane Representation:
   • When the entire sheet of paper is spread flat on the table, it represents one plane.
   • In this plane, the incident ray, the normal at the point of incidence, and the reflected ray are all aligned.
2. Effect of Bending the Paper:
   • When the paper is bent, it creates a new plane that is different from the plane of the incident ray and the normal.
   • As a result, the reflected ray becomes invisible.

• Observation:
  • The inability to see the reflected ray when the paper is bent indicates that:
    • The incident ray, the normal, and the reflected ray must all lie in the same plane for the reflection to be visible.
• Conclusion: This observation supports another law of reflection.

• Paheli and Boojho performed these activities outside the classroom, utilizing the Sun as the light source instead of a torch.
• Recommendation: You can also use the Sun for similar observations of reflection.

• These activities can also be conducted using the Ray Streak Apparatus, which is available in the kit prepared by NCERT.

Features of the Image Formed by a Plane Mirror:

1. Erect or Upside Down:
   • The image formed by a plane mirror is erect (upright).
2. Size of the Image:
   • The image is of the same size as the object.
3. Distance of the Image:
   • The image appears at the same distance behind the mirror as the object is in front of it.
4. Obtaining the Image on a Screen:
   • The image cannot be obtained on a screen; it is a virtual image.

Activity 13.3: Formation of an Image by a Plane Mirror

Setup:

• Source of Light: Place a source of light labeled O in front of a plane mirror labeled PQ.
• Incident Rays: Two rays, OA and OC, are incident on the mirror.

1. Drawing Normals:
   • Draw normals to the surface of the mirror PQ at the points of incidence A and C.
2. Drawing Reflected Rays:
   • Using the law of reflection:
     • The angle of incidence is equal to the angle of reflection.
     • Draw the reflected rays:
       • AB is the reflected ray from point A.
       • CD is the reflected ray from point C.
3. Extending Reflected Rays:
   • Extend rays AB and CD further:
     • Check if they meet in front of the mirror.
     • If they do not meet, extend them backwards (towards the mirror).
4. Marking the Intersection Point:
   • If the reflected rays appear to meet at a point when extended backwards, mark this point as I.
5. Viewing the Image:
   • For a viewer’s eye located at point E:
     • The reflected rays appear to come from point I.
     • Since the rays do not actually meet at point I, but appear to do so, we conclude that a virtual image of point O is formed at point I.

• Virtual Image: The image formed at point I is virtual and cannot be obtained on a screen.
• Lateral Inversion:
  • In an image formed by a mirror:
    • The left side of the object appears on the right side in the image, and the right side appears on the left.
    • This phenomenon is known as lateral inversion.

13.3 - Regular and Diffused Reflection

Activity 13.4: Exploring Reflection on Irregular Surfaces

1. Setup:
   • Imagine parallel rays of light incident on an irregular surface (refer to Fig. 13.6).
2. Using Laws of Reflection:
   • Remember that the laws of reflection apply at each point of the surface.
   • Construct the reflected rays at various points on the irregular surface.
3. Observations:
   • Check whether the reflected rays are parallel to one another.
   • You will find that the reflected rays are directed in different directions (see Fig. 13.7).

1. Diffused Reflection:
   • When the parallel rays reflected from a rough or irregular surface are not parallel, this type of reflection is called diffused or irregular reflection.
   • Cause: Diffused reflection occurs due to irregularities in the reflecting surface, such as that of cardboard.
   • Note: Diffused reflection does not occur due to the failure of the laws of reflection; rather, it is a result of the surface texture.
2. Regular Reflection:
   • Reflection from a smooth surface, like that of a mirror, is referred to as regular reflection (see Fig. 13.8).
   • Characteristics:
     • In regular reflection, the reflected rays remain parallel, allowing for the formation of clear images.
     • Images are formed distinctly through regular reflection.

Seeing Objects Due to Reflected Light

1. Reflected Light:
   • Nearly everything we see around us is visible due to reflected light.
   • For example, the Moon receives light from the Sun and reflects it, allowing us to see it.
2. Illuminated Objects:
   • Objects that shine in the light of other objects are called illuminated objects.
   • Examples of Illuminated Objects:
     • The Moon
     • Planets
     • Trees and buildings under sunlight
     • Any objects visible in the light of a lamp or candle.
3. Luminous Objects:
   • Some objects give off their own light; these are known as luminous objects.
   • Examples of Luminous Objects:
     • The Sun
     • Fire (like a flame of a candle)
     • Electric lamps
     • Stars

• Question: Can the reflected rays be further reflected if they strike another mirror?
• Answer: Yes, if the reflected rays from one mirror strike another mirror, they can be reflected again. This principle is based on the laws of reflection, which hold true at every point of a reflecting surface. Thus, light can continue to reflect between multiple mirrors, producing a series of reflections.
  
  

13.5 - Multiple Images

Single Image Formation by Plane Mirrors:

• A plane mirror typically forms only one image of an object.

• When two plane mirrors are used together, they can create multiple images.

1. Setup:
   • Take two plane mirrors and set them at right angles (90°) to each other, with their edges touching (see Fig. 13.10).
   • Use adhesive tape to hinge them.
2. Observation:
   • Place a coin between the mirrors.
   • Count how many images of the coin you can see (refer to Fig. 13.10).
3. Varying Angles:
   • Change the angle between the mirrors to different degrees (e.g., 45°, 60°, 120°, 180°).
   • Place an object, such as a candle, between the mirrors.
   • Record the number of images observed for each angle.
4. Parallel Mirrors:
   • Finally, set the two mirrors parallel to each other.
   • Place a candle between them and determine how many images are formed (see Fig. 13.11).

• The number of images formed by mirrors placed at an angle can be calculated using the formula: $$\text{Number of images} = \frac{360°}{\text{Angle between mirrors}} - 1$$Number of images=Angle between mirrors360°​−1
• Example:
  • For mirrors at 90°: $$\frac{360°}{90°} - 1 = 4 \text{ images}$$
  • For mirrors at 45°: $$\frac{360°}{45°} - 1 = 7 \text{ images}$$

• This concept of forming multiple images is used in a kaleidoscope, which creates beautiful patterns through the reflection of light.
• DIY Kaleidoscope: You can make a kaleidoscope at home to observe the fascinating patterns created by multiple reflections.

Kaleidoscope

Activity 13.6: Making a Kaleidoscope

Materials Needed:

• Three rectangular mirror strips:
  • Length: 15 cm
  • Width: 4 cm
• Circular cardboard tube or thick chart paper tube (slightly longer than the mirror strips).
• Cardboard disc with a hole in the center.
• Transparent plastic sheet (to cover the hole).
• Circular plane glass plate.
• Small pieces of colored glass (e.g., broken pieces of colored bangles).
• Ground glass plate.

1. Prepare the Mirror Prism:
   • Join the three mirror strips together to form a prism (see Fig. 13.12(a)).
2. Assemble the Tube:
   • Insert the prism arrangement into the circular cardboard tube.
   • Ensure the tube is slightly longer than the mirror strips.
3. Seal One End:
   • Close one end of the tube with a cardboard disc that has a hole in the center (for viewing).
   • Paste a transparent plastic sheet under the cardboard disc for durability.
4. Add the Glass Plate:
   • At the other end (the end touching the mirrors), fix a circular plane glass plate (see Fig. 13.12(c)).
   • Place several small pieces of colored glass on this glass plate.
5. Close the Other End:
   • Close this end of the tube with a ground glass plate.
   • Ensure there is enough space for the colored pieces to move around freely.

• Observation: Look through the hole at the closed end of the tube.
• You will see a variety of patterns formed by the reflections of the colored glass pieces.
• Unique Patterns: An interesting feature of a kaleidoscope is that you will never see the same pattern again as the colored pieces shift.

• Designers of wallpapers and fabrics, as well as artists, often use kaleidoscopes to generate ideas for new patterns.

• To make your kaleidoscope visually appealing, you can wrap it in colored paper.

13.6 Sunlight — White or Coloured

Concept:

• Sunlight is often referred to as white light.
• It consists of seven colors, which can be demonstrated through simple activities.

Activity 13.7: Dispersion of Light

Objective: To observe the dispersion of light through a combination of a mirror and water.

Materials Needed:

• Plane mirror of suitable size.
• Bowl (katori).
• Water.
• White sheet of paper (if the wall is not white).
• Direct sunlight (a window with sunlight).

1. Setup:
   • Place the plane mirror in the bowl.
   • Fill the bowl with water.
2. Positioning:
   • Place the arrangement near a window so that direct sunlight falls on the mirror.
   • Adjust the bowl’s position to ensure that the reflected light from the mirror falls on a wall.
3. Observation:
   • If the wall is not white, affix a white sheet of paper to the wall to enhance visibility.
   • Observe the reflected light on the wall; it will display many colors.

• The combination of the mirror and water acts like a prism, which causes the light to break up into its component colors.
• This phenomenon of light splitting into various colors is known as dispersion of light.

• A rainbow is a natural example of light dispersion, where sunlight is refracted and dispersed in water droplets in the atmosphere.

13.7 - What is Inside Our Eyes?

Concept:

• We perceive objects when light from them enters our eyes. The eye is one of our most crucial sense organs, making it essential to understand its structure and function.

1. Shape:
   • The eye has a roughly spherical shape.
2. Outer Coat:
   • The outer layer of the eye is white and tough, providing protection to the interior of the eye.
3. Cornea:
   • The transparent front part of the eye, allowing light to enter.
4. Iris:
   • Located behind the cornea, the iris is a dark muscular structure.
   • It controls the size of the pupil, which is a small opening in the iris.
   • The iris also gives the eye its distinctive color (e.g., green eyes refer to the color of the iris).
5. Pupil:
   • The size of the pupil is regulated by the iris, controlling the amount of light that enters the eye.

• The iris adjusts the pupil size in response to light intensity, helping to regulate the light entering the eye and protecting the retina from excessive brightness.

• Important: Never use a laser torch for activities related to the eye, as it can cause damage.

Activity 13.8: Observing Pupil Size

1. Observation of Pupil Size:
   • Procedure: Look into your friend's eye and observe the size of the pupil.
   • Effect of Light: Shine a torch on the eye to illuminate the pupil, then switch off the torch.
   • Observation Questions:
     • Do you notice any change in the size of the pupil?
     • In which case was the pupil larger?
     • Why do you think this change occurs?
2. Inference:
   • The pupil is larger in dim light and smaller in bright light.
   • Reason: In dim light, the pupil expands to allow more light to enter the eye for better vision, while in bright light, it constricts to limit the amount of light entering the eye and protect the retina.
3. Lens in the Eye:
   • Behind the pupil is a lens that is thicker in the center.
   • Definition: This lens is a convex lens (or converging lens), which focuses light on the retina.
   • The retina is a layer at the back of the eye where the focused light forms images.
4. Retina and Nerve Cells:
   • The retina contains nerve cells that send visual information to the brain through the optic nerve.
   • Types of Cells in Retina:
     • Cones: Sensitive to bright light and responsible for color vision.
     • Rods: Sensitive to dim light and responsible for vision in low-light conditions.

1. Understanding the Blind Spot:
   • At the junction of the optic nerve and retina, there are no sensory cells; this area is called the blind spot.
   • Procedure:
     • Draw a round mark and a cross on a sheet of paper, with the spot to the right of the cross (6-8 cm apart).
     • Hold the sheet at arm's length, close your left eye, and focus on the cross.
     • Move the sheet towards you while keeping your eye on the cross.
     • Observation: Notice if the round mark disappears at some point.
2. Repeating the Activity:
   • Close your right eye and look at the round mark. Repeat the movement.
   • Observation: Check if the cross disappears.
3. Inference:
   • The disappearance of either the round mark or the cross indicates the presence of a point on the retina that cannot send messages to the brain when light falls on it, confirming the existence of the blind spot.

• The impression of an image persists on the retina for about 1/16th of a second.
• If still images of a moving object are presented at a rate faster than 16 frames per second, the eye perceives the object as moving.
• This phenomenon is known as persistence of vision.

1. Protective Features:
   • Eyelids:
     • Function: Prevent objects from entering the eye and shut out unnecessary light.
2. Versatility of Vision:
   • The human eye is capable of clearly seeing both distant and nearby objects.
   • The minimum distance at which objects can be seen distinctly varies with age.
   • For a normal eye, the most comfortable reading distance is approximately 25 cm.
3. Vision Defects:
   • Some individuals can see close objects clearly but struggle to see distant objects (often referred to as nearsightedness or myopia).
   • Others may see distant objects clearly but have difficulty with close objects (known as farsightedness or hyperopia).
4. Corrective Measures:
   • Corrective Lenses: Used to address vision defects, allowing individuals to see clearly at all distances.
5. Cataracts:
   • Definition: A condition where the eye lens becomes cloudy, leading to foggy eyesight.
   • Cause: Often occurs with age but can also result from other factors.
   • Symptoms: Can lead to a significant loss of vision, which may be severe.
   • Treatment:
     • Surgical removal of the opaque lens.
     • Insertion of a new artificial lens.
     • Modern technology has made cataract surgery simpler and safer.
       

13.8 - Care of the Eyes

1. Importance of Eye Care:
   • It is essential to take proper care of your eyes to maintain good vision.
   • If there is any eye-related problem, you should consult an eye specialist for proper treatment and regular checkups.
2. Precautions for Healthy Eyes:
   • Use of Spectacles:
     • If prescribed by a doctor, wear suitable spectacles to correct vision problems.
   • Proper Lighting:
     • Too little light can cause eyestrain and headaches.
     • Too much light, such as direct sunlight, powerful lamps, or laser torches, can damage the retina.
     • Always avoid looking directly at the Sun or any powerful light source.
   • Eye Hygiene:
     • Do not rub your eyes, as it may cause irritation or injury.
     • If dust or foreign particles enter your eyes, wash them with clean water.
     • If the problem persists, seek medical help.
   • Reading Distance:
     • Always read at a comfortable distance for clear vision. Avoid holding the book too close or too far from your eyes, as it can strain the eyes.
3. Balanced Diet and Eye Health:
   • A balanced diet is crucial for maintaining healthy eyes.
   • Vitamin A is particularly important for eye health. A deficiency of vitamin A can lead to night blindness and other eye problems.
   • Sources of Vitamin A:
     • Vegetables: Raw carrots, broccoli, and green leafy vegetables like spinach are rich in vitamin A.
     • Animal Products: Cod liver oil, eggs, milk, curd, cheese, and butter are good sources.
     • Fruits: Papaya and mango are also rich in vitamin A.

Did You Know?

1. Variety in Animal Eyes:
   • Different Shapes: Animals have eyes that vary in shape, adapted to their needs and environments.
2. Crab's Eyes:
   • Small Size: Crabs have small eyes.
   • Functionality: These eyes allow crabs to look in all directions, enabling them to sense predators or danger from any angle, even from behind.
3. Butterfly's Eyes:
   • Large Compound Eyes: A butterfly’s eyes appear to be made up of thousands of small eyes, known as compound eyes.
   • Wide Field of View: This structure allows butterflies to see in the front, sides, and even behind them.
4. Night Vision in Owls:
   • Adaptation for Night: Owls have excellent night vision but cannot see well during the day.
   • Special Adaptations:
     • Large Cornea and Pupil: These features allow more light to enter the eye, aiding in vision at night.
     • Rods and Cones: The retina of owls has a large number of rods (sensitive to dim light) and very few cones (sensitive to bright light), making them ideal for night vision.
5. Daylight Birds (Kite, Eagle):
   • Adaptation for Day: Birds like kites and eagles have sharp vision during the day but struggle to see at night.
   • Retina Composition: They have more cones and fewer rods on their retina, which makes their vision suited for bright daylight conditions.

13.9 - Visually Impaired Persons Can Read and Write

1. Visually Impaired Persons:
   • Definition: Visually impaired persons have very limited or no vision. This includes people who are born blind or lose their eyesight due to disease or injury.
2. Compensatory Abilities:
   • Use of Other Senses: Visually impaired individuals often develop their other senses, such as touch and hearing, to help them identify objects and navigate their environment.
3. Types of Vision Impairment:
   • Congenital Blindness: Some people are blind from birth.
   • Acquired Blindness: Others may lose their eyesight later in life due to accidents, diseases, or injuries.
4. Use of Additional Resources:
   • Support Tools: Visually impaired people can further develop their capabilities with the help of specialized tools and resources designed to assist them in reading, writing, and performing everyday tasks.

1. Non-Optical Aids:
   • Types:
     • Visual Aids: Magnify words, adjust light intensity, and help place material at proper distances for better visibility.
     • Tactual Aids: Use the sense of touch, such as Braille writer, slate, and stylus, allowing visually impaired persons to take notes, read, and write.
     • Auditory Aids: Use the sense of hearing. Examples include cassettes, tape recorders, talking books, and other sound-based devices.
     • Electronic Aids: These include talking calculators, computers, and closed-circuit television (CCTV). CCTV enlarges printed material with contrast and illumination adjustments. Audio CDs and voice boxes connected to computers are also useful for listening and writing text.
       
       
2. Optical Aids:
   • Types:
     • Bifocal and Contact Lenses: Correct various visual limitations.
     • Tinted Lenses: Help with specific vision conditions.
     • Magnifiers: Used to enlarge text or objects for better clarity.
     • Telescopic Aids: Assist in viewing distant objects, like the chalkboard or classroom demonstrations.

13.10 - What is the Braille System?

1. Definition:
   • The Braille System is a method used by visually challenged individuals to read and write through touch.
2. History:
   • Inventor: Louis Braille, who was visually challenged, developed the system in 1821.
   • Modern System: The present Braille system was adopted in 1932.
3. Braille Code:
   • Braille includes codes for common languages, mathematics, and scientific notation.
   • Indian Languages: Many Indian languages can also be read using the Braille system.
4. Structure:
   • The system consists of 63 dot patterns or characters.
   • Character Representation: Each character can represent a letter, a combination of letters, a word, or a grammatical symbol.
   • Dot Arrangement: The dots are arranged in cells of two vertical rows with three dots each.
   • Raised dots are embossed on Braille sheets for recognition by touch.
5. Learning Process:
   • Visually impaired individuals learn the Braille system starting with letters, moving on to special characters and combinations.
   • Memorization: Each character must be memorized.
6. Production of Braille Texts:
   • Methods: Braille texts can be produced manually or by machine.
   • Devices: Typewriter-like devices and printing machines have been developed to print Braille texts.

Notable Achievements of Visually Impaired Individuals

1. Diwakar (India):

• A child prodigy known for his remarkable performances as a singer.

• Born completely visually impaired.
• Achieved the Sangeet Prabhakar degree from Allahabad.
• Excelled as a lyricist, singer, and music composer.

• Visually impaired himself.
• Founder of the Association for Special Education and Rehabilitation of the Disabled in India.
• Represented India at UNESCO on Braille-related issues.

• An American author and lecturer.
• Lost her sight at 18 months old.
• Despite this, she completed her graduation and became an inspiring figure.
• Wrote several books, including her famous autobiography, "The Story of My Life" (1903).

NCERT Science Notes - Class 8 | Science | Chapter 13 - Light

NCERT Science Notes - Class 8 | Science | Chapter 13 - Light