Light Class 8 Notes Science Chapter 13

What is Light?

Light is a fascinating form of energy that plays a crucial role in our daily lives. It allows us to see the world around us and experience the vibrant colors, shapes, and texturesof our surroundings. 

• When light enters our eyes, it triggers a process that enables us to perceive and understand our environment. 
• Light can come from various sources, such as theSun, which emits its own light, or from objects that reflect light, like the Moon or a shiny surface.
• Understanding the properties and behaviors of light helps us unravel the mysteries of the universe and opens up countless possibilities for scientific discoveries and technological advancements.

Let us learn more about Lightthrough the short notes in this document. 

What Makes Things Visible?

The visibility of objects relies on the presence and interaction of light. Light, being a type of energy, stimulate our visual perception and enables us to observe the world around us. This light can originate from either self-illuminating sources or as reflected light.

• Luminous objects refer to those that emit light independently. For instance: the Sun, light bulbs, and tube lights. Example: Sun, bulb, tube light
• Non-luminous objects are objects which reflect light from other sources. They do not emit light of their own. Example: Moon, tree, table, painting.

Laws of Reflection

Some important points:

• When light hits a mirror, it undergoes reflection.
• The incident ray is the incoming ray of light that strikes the mirror.
• The reflected ray is the outgoing ray that bounces off the mirror's surface.
• We use a line called the "normal" to understand the process.
• The normal is drawn perpendicular to the mirror's surface at the point of incidence.
• Angle of incidence- The angle between the incident ray and the normal is called the angle of incidence. 
• Angle of reflection-The angle between the reflected ray and the normal is called the angle of reflection.

1. First Law:The angle of incidence is always equal to the angle of reflection. This is one of the laws of reflection.
2. Second Law:The incident ray, the normal at the point of incidence and the reflected ray all lie in the same plane.

Image formed by Plane Mirror

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

• Virtual and Upright: The image cannot be projected onto a screen, as it is formed by the apparent intersection of reflected rays. The image appears upright, maintaining the same orientation as the object.
• Same Size as the Object: The size of the image is equal to the size of the object.
• Laterally Inverted: The image is reversed from left to right (lateral inversion). For example, if you raise your right hand, it appears as if your left hand is raised in the image.
• Equal Distance: The distance of the image from the mirror is the same as the distance of the object from the mirror.
• Erect: The image remains vertical, in the same orientation as the object.

Regular and Diffused Reflection

Reflection Types:

• Regular Reflection: Occurs when parallel rays of light bounce off a smooth surface, remaining parallel after reflection (e.g., a mirror). The resulting image is clear and well-defined.
• Diffused Reflection: Happens when parallel rays of light reflected from a surface scatter in various directions due to irregularities on the surface, like rough materials (e.g., cardboard). Although not violating reflection laws, diffused reflection does not form distinct images and spreads light in different directions.

Reflection Effects:

• Regular reflection is responsible for forming clear images.
• Diffused reflection, without forming distinct images, scatters light in various directions.

Understanding these reflection types helps us grasp how light interacts with different surfaces, influencing our perception of the surrounding environment.

Reflected Light Can be Reflected Again

Reflected light can be reflected again, which is called multiple reflection. This happens when a reflected light ray hits another surface and is reflected again.

1. Hair Salon Mirror
At the hairdresser, after cutting your hair, they show you the back using a mirror held behind you.

2. Periscope

Periscope works on the principle of multiple reflection. It consists of two or more mirror, used for the reflection.

Working of periscope

• Reflection from these mirrors helps see objects not directly visible.
• Light enters through one mirror, reflects off it, hits the second mirror, and reflects again to your eyes.
• By bouncing light multiple times, the periscope lets you see things that might be around corners or obstacles.

Multiple images 

What is Kaleidoscope?

A kaleidoscope is an intriguing device consisting of a tube filled with loose colored beads, pebbles, or small objects, accompanied by strategically placed mirrors.

Kaleidoscope View

Working of kaliedoscope

• When someone peers into one end of the kaleidoscope, light enters from the opposite end, reflecting off two rectangular mirrors running along the tube's length.
• Rotating the tube produces captivating displays of shifting colors and intricate patterns.
• Kaleidoscopes provide a delightful and magical experience, allowing exploration of the beauty of colors, shapes, and symmetry in a creative and enchanting manner.

Sunlight — White or Coloured

Sunlight, referred to as white light, consists of seven colours.

Activity to Observe Colours in Sunlight:

• Place a plane mirror in a bowl filled with water.
• Allow direct sunlight to fall on the mirror.
• Adjust the bowl so the reflected light falls on a white surface or wall.
• The reflected light will show multiple colours.

Explanation:

• The mirror and water act as a prism.
• They break the white light into its component colours.
• This phenomenon is called dispersion of light.

Note:A rainbow is an example of natural dispersion of sunlight into its seven colours.

What is Inside our Eyes?

Our ability to see the world around us is made possible by the intricate workings of our eyes. Serving as one of our most vital sense organs, the eye allows us to perceive and interpret the visual stimuli that surround us. Let's explore the fascinating structure and functions of this remarkable organ.

 The eye, roughly spherical, is protected by a white outer coat, shielding the delicate interior. The transparent cornea is at the front, and the colored iris, controlling the pupil's size, lies deeper within.  Human Eye

Different parts of Human Eye are :

Cornea

• The outermost layer is called the cornea.
• It is transparent.

Iris

• Behind the cornea, there is a dark muscular structure called the iris.
• The colour of the iris differs among people (e.g., brown, black, blue).

Pupil

• In the iris, there is a small opening called the pupil.

Lens

• Behind the pupil, there is a lens.
• The lens focuses light on the back of the eye on a layer called the retina.

Retina

• The layer at the back of the eye is called the retina.
• The retina contains different nerve cells.
• A blind spot, where vision is absent, exists at the junction of the optic nerve and the retina.

Nerve Cells

• There are two types of nerve cells in the retina:
• Cones: Sensitive to bright light.
• Rods: Sensitive to dim light.

Optic Nerve

• Sensations felt by the nerves are transmitted to the brain through the optic nerve.

Ciliary Muscle

• Helps in changing the shape of the lens.
• This allows the lens to focus properly.

Role of Eyelids:

• Eyelids protect the eyes from foreign objects.
• They also block light when it is not needed.

Retinal Impressions: Images reaching the retina persist for about 1/16th of a second, allowing the perception of motion. Rapid presentation of still images creates the illusion of motion. This property is used to create the illusion of video in movies 

How Movies Are Seen:

• Movies consist of separate pictures displayed in sequence.
• These pictures move across the eye at a rate of 24 pictures per second (faster than 16 per second), creating the illusion of continuous motion.

Eye’s Ability to See:

• The eye can focus on both distant and nearby objects.
• The minimum distance at which objects are seen clearly varies with age.
• The most comfortable distance for reading with a normal eye is about 25 cm.
• Some people can see nearby objects clearly but not distant ones while others see distant objects clearly but not nearby ones. Both defects can be corrected using suitable lenses.

How to Take Care of the Eyes?

• It is necessary that we take proper care of our eyes. If there is any problem we should go to an eye specialist. Have a regular checkup. If advised, use suitable spectacles.
• Too little or too much light is bad for eyes. Insufficient light causes eyestrain and headaches. Too much light, like that of the sun, a powerful lamp or a laser torch can injure the retina. Do not look at the sun or a powerful light directly.
• Never rub eyes. If particles of dust go into eyes, wash eyes with clean water. If there is no improvement go to a doctor. Wash eyes frequently with clean water.Tips to care of the eyes 
• Always read at the normal distance for vision. Do not read by bringing book too close to eyes or keeping it too far.
• Lack of vitamin A in foodstuffs is responsible for many eye troubles. Most common amongst them is night blindness.One should, therefore, include in the diet components which have vitamin A. Raw carrots, broccoli and green vegetables (such as spinach) and cod liver oil are rich in vitamin A. Eggs, milk, curd, cheese, butter and fruits such as papaya and mango are also rich in vitamin A.

Empowering Visually Impaired Individuals through Braille

• Some individuals, including children, face visual impairments that greatly limit their ability to see things.
• Whether they have limited vision or are completely blind from birth or due to a disease, these individuals rely on their other senses, such as touch and hearing, to identify and understand their surroundings.
• While they naturally develop heightened senses, additional resources can further enhance their capabilities.

Braille Alphabets

The Braille System:

• The most widely used and valuable resource for visually impaired individuals is the Braille system.
• This system, adopted in 1932, provides a means of reading and writing through touch.
• Braille includes codes for various languages, mathematics, and scientific notations.
• In many countries, including India, Braille versions of different languages are available.
• Visually impaired individuals learn the Braille system by starting with letters, then progressing to special characters and letter combinations.
• The learning methods involve recognizing the characters by touch, with each character requiring memorization.

Braille Production:

• Braille texts can be produced manually or with the assistance of machines.
• Initially, individuals used manual methods, such as embossing or writing Braille characters by hand.
• Technological advancements have brought about devices resembling typewriters and printing machines designed specifically for creating Braille texts.
• These machines enable more efficient and accurate production of Braille materials.
• This technology makes it easier for visually impaired individuals to access a wider range of reading materials.

Frequently Asked Questions (FAQs) Related to Light

Q1: What is light?

Ans:Light is a type of electromagnetic radiation that is visible to the human eye and can be detected by other organisms. It is a form of energy that travels in waves and does not require a medium to propagate. 

Q2: What are the properties of light?

Ans: Light has several properties, including wavelength, frequency, amplitude, and polarization. It can also be described in terms of its speed, which is approximately 299,792,458 meters per second in a vacuum. Light can be reflected, refracted, and absorbed by different materials, and its behavior can be influenced by external factors such as temperature and pressure. 

Q3: How is light produced? 

Ans: Light can be produced by a variety of sources, including natural phenomena such as the sun, stars, and lightning, as well as human-made devices such as light bulbs, lasers, and LEDs. Some materials, such as phosphors and fluorescent dyes, can also emit light when excited by an external energy source. In addition, certain chemical reactions can produce light, as in bioluminescence. 

Q4: What is the importance of light?

Ans:Light is essential for life on Earth, as it provides energy for photosynthesis in plants and allows animals to see and navigate their environment. It also plays a crucial role in many human activities, such as communication, transportation, and entertainment. Light-based technologies are used in medicine, astronomy, telecommunications, and many other fields, and research in optics and photonics has led to numerous innovations and discoveries. 

Q5: How does light interact with matter?

Ans.When light interacts with matter, it can be absorbed, reflected, or transmitted depending on the properties of the material and the wavelength of the light. Absorption occurs when the energy of the light is transferred to the material, causing it to heat up or undergo a chemical reaction. Reflection occurs when the light bounces off the surface of the material without being absorbed, while transmission occurs when the light passes through the material without being absorbed or reflected. These interactions can be used to study the properties of materials, create images, and manipulate light for various applications.