Class 8 Science

# Light

## Extra Questions

Question 1: How does light enable us to see something?

Answer: When light falls on an object, some of the light is reflected back to our eyes. Thus, we are able to see an object because of light being reflected from the object.

Question 2: Write the two laws of reflection.

Answer: Laws of reflection are as follows:

First Law of Reflection: Angle of reflection is equal to angle of incidence.

Second Law of Reflection: The incident ray, reflected ray and normal at the point of incidence; all lie in the same plane.

Question 3: What do you understand by regular reflection?

Answer: When all the reflected rays are parallel to each other; this case of reflection is called regular reflection. Regular reflection happens from a smooth surface, e.g. a mirror. We get to see clear images when image is formed by regular reflection.

Question 4: What do you understand by irregular reflection?

Answer: When the reflected rays are not parallel to each other, this case of reflection is called irregular reflection. Irregular reflection happens from a rough surface. The laws of reflection are obeyed in this case as well but because of the irregularities in the surface, the reflected rays are not parallel. We get to see somewhat blurred images when image is formed by irregular reflection. The relative clarity of an image depends on the relative smoothness of the reflecting surface.

Question 5: Write a short note on multiple reflections?

Answer: When two or more mirrors are placed at some angles to each other, we get to see multiple images. Let us take an example in which two mirrors are placed opposite to each other. If an object is placed between them, its image is formed in both the mirrors. The image in one mirror would act as an object for another mirror and this sequence would continue. This will result in the formation of multiple images.

Question 6: What is blind spot?

Answer: The junction of retina and the optic nerve is called the blind spot. There is no photosensitive cell at this spot, and hence no image is formed at this spot on the retina.

Question 7: Explain the formation of image in eyes.

Answer: Light rays enter the eye through pupil and pass through lens. Lens focuses light rays on retina, and makes real, inverted and smaller image on retina. Signals from retina are carried by optic nerve to the brain. Thus, we are able to see an object.

Question 8: What are the common vision defects?

Answer: Myopia, Hypermetropia, Cataract and Presbyopia

Question 9: Write a brief note on Braille system.

Answer: The Braille scripts are written in the form of raised dots. This system is composed of 63 characters. Each character is written on a grid of 6 cells. This can be written with the help of Braille slate and stylus. A reader needs to touch the Braille script to read it.

Question 1: How will you trace image formation in plain mirror?

Answer: Image Formation in Plane Mirror

In this figure, PQ is a plain mirror and O is an object. OA and OC are the incident rays; coming from the object. The incident rays are reflected from the mirror and reflected rays emerge as AB and CD. If AB and CD are extended behind the mirror, they appear to meet at point ‘I’. This is the point where image is formed. If OM is object distance and MI is image distance, then in plain mirror, OM = MI. This means image distance is equal to object distance in case of a plain mirror.

Question 2: Explain the structure of human eye.

Answer: The human eye has following main parts:

Cornea: The eyeball is covered with a tough layer. This layer is transparent on the front. This transparent portion is called cornea.

Iris: Iris is a thin circular structure. It works like the shutter of a camera. It controls the amount of light entering the eye. The colour of iris imparts distinct colour to the eyes of an individual. A person with blue iris has blue eyes.

Pupil: There is a hole in the centre of iris. This is called pupil. Light enters the eye through the pupil. When the light is bright, iris contracts and thus allows less light into the eye. When the light is dim, iris dilates and thus allows more light into the eye.

Lens: Lens is present behind the pupil. The lens in the human eye is a convex lens. The size and thickness of the lens change as per the distance of an object.

Retina: Retina is at the back of the eye and marks the inner layer of the eyeball. Images are formed on the retina and thus it works like a screen. There are photosensitive cells in the retina. These cells are of two types, which are as follows:

• Cone Cells: The cone cells are sensitive to bright light. They also give the sense of colour.
• Rod Cells: The rod cells are sensitive to dim light.

Optic Nerve: The optic nerve emerges from the back of the eye ball. This nerve goes to the brain.

Blind Spot: The junction of retina and the optic nerve is called the blind spot. There is no photosensitive cell at this spot, and hence no image is formed at this spot on the retina.

Question 3: Suggest some steps for care of eyes.

Answer: Some tips for care of eyes are as follows:

• Do not read in too bright or too dim light.
• Do not look directly at a bright object or at the sun.
• Do not keep the book too close to your eyes, while reading. Don’t keep the book too far either.
• If something gets into the eye, do not rub the eye. Wash it with cold water.
• In case of any problem (like itching or burning sensation) consult an ophthalmologist. A doctor who specializes in the disease of eyes is called an ophthalmologist.

## Crop Production

The science of farming is called agriculture. Farming includes cultivation of plants and rearing of animals for food and other beneficial items.

## Microorganisms: Friend or Foe

An organism which cannot be seen by naked eyes is called microorganism or microbe. We need a microscope to see a microbe.

## Synthetic Fibres

ibres which are man-made are called synthetic fibres, e.g. nylon, acrylic, etc. A synthetic fibre is made of multiple units of a chemical substance.

## Metals Non-metals

Metals are lustrous, sonorous, ductile, and are good conductor of heat and electricity.

## Coal & Petroleum

Resources which from nature are called natural resources. Examples: coal, petroleum, air, water, soil, etc.

## Combustion & Flame

Combustion is a chemical process in which a substance reacts with oxygen to give off heat. Some substances may also give light along with heat during combustion.

## Conservation of Plants and Animals

The part of the earth where all living organisms are present is called biosphere. Biosphere is the narrow zone between hydrosphere, troposphere and atmosphere.

## Cell: The Fundamental Unit of Life

The basic structural unit of a living organism is called cell. All living beings are made up of cells.

## Reproduction in Animals

The process by which an organism produces its offspring is called reproduction. Reproduction is necessary to continue the lineage of an organism.

## Reaching the Age of Adolescence

Adolescence is the period of life when many changes take place in the body, leading to reproductive maturity. Adolescence begins at around 10 years of age and lasts till 18 or 19 years of age.

## Force & Pressure

A push or pull on an object is called force. Pressure is the force acting on a unit area on an object.

## Friction

When one surface is moving over another surface, a force comes into play and opposes their relative motion. This force is called friction or force of friction.

## Sound

Sound is a kind of wave which gives us the sense of hearing. Sound is produced by a vibrating body.

## Chemical Effects of Electric Current

When electric current is passed through a conducing solution, some chemical reaction takes place in the solution. This is called chemical effect of electric current.

## Natural Phenomena

Transfer of charge from one object to another is called charging. The process of transfer of charge to the earth is called earthing.

## Light

When light falls on a shiny surface, most of it bounces back. This phenomenon is called reflection of light.

## Solar System

The solar system is made up of the sun, its planets, satellites, asteroids, comets, dwarf planets, etc. All other members of the solar system keep on revolving around the sun.

## Pollution

When air is contaminated with unwanted substances and becomes harmful for the living and non-living, this is called air pollution.