Light - Class 8: Notes

(i) Light is a form of energy, which includes the sensation of vision in our eyes and make us able to see various things present in our surrounding.
(ii) When light falls on an object, some part of light is reflected back to our eyes. Then, we are able to see an object because of light being reflected from the object.

Properties of Light:
(i) It is form of energy.
(ii) It travels in straight line.
(iii) Light can form shadows.

Laws of Reflection: The two laws of reflection are:
(i) The angle of incidence is equal to the angle of reflection.

˂ i = ˂ r

(ii) The incident ray, the normal to the mirror at the point of incidence and the reflected ray, all lie in the same plane.
These laws of reflection are applicable to all types of reflecting surface including spherical surfaces.

Formation of Image by Plane Mirror:
In figure PQ is a plain mirror and O is an object. Incident rays OA and OC are coming from the object. These incident rays are reflected from the mirror and reflected rays emerge as AB and CD. If reflected rays AB and CD are extended behind the mirror, they appear to meet at point ‘I’. This is that point where image is formed.


Some features of image formed by a plain mirror:
(i) Obtained Image is always formed behind the mirror.

(ii) Obtained image is virtual, which means it cannot be obtained on screen.
(iii) Image is erect and laterally inverted.
(iv) Image size is equal to object size.
(v) Distance of object from the mirror = Distance of image from the mirror.
Distance of the object and image is the same from the mirror.

Regular and Diffused Reflection:
1. Regular Reflection:
(i) When the reflection surface is smooth and well – polished, the parallel rays falling on it are parallel to another one, the reflected light goes in one particular direction and also parallel to each other. This is regular reflection. E.g. plane mirror, reflection from still water.


2. Diffused and Irregular Reflection:
When the reflecting surface is rough, the parallel rays falling on it are reflected in different direction. Such as reflection is known as diffuse or irregualar reflection. For example, reflection of light from the wall of a room or tree etc.

Multiple Images:
(i) When two or more mirrors are placed at an angle to one another, we can see multiple images of an object.
(ii) For example in the hair dresser shop two mirrors are placed opposite to each other. If a man is sit between them, His 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 give result in the formation of multiple images.

(iii) The number of images formed depends on the angle between the two mirrors. This can be calculated by using the following formula:

Number of Images = 

(iv) If the given two mirrors are at a right angle to each other, 3 images will be obtained. If the given mirrors are at 60° angle, we shall get 5 images.
(v) When the two mirrors are kept opposite and parallel to each other, we will get infinite number of images formed.

(i) The idea of number of images formed by mirrors placed at an angle to one another is used in a kaleidoscope to make numerous beautiful patterns.
(ii) To make a kaleidoscope, get three rectangular mirror strips about 15 cm long and 4 cm wide each. Join them together to form a prism. Fix them in a circular cardboard tube or tube of a thick chart paper.

Human Eye:
We see objects only when light coming from objects enters our eyes. Eye is one of our most important sense organs which give us the sense of vision. 

Structure of Human Eye:


1. Cornea: It is located at the front portion of the eye. It is transparent window that bulges outwards and responsible for the maximum refraction (bending) of the light that enters the eye. 

2. Iris: It is a thin circular, colour part structure. It works like the shutter of eye. It controls the amount of light entering the eye.

3. Pupil: It is an adjustable circular opening that is located at the centre of the iris. Light enters into eye through the pupil. When the light amount is high, iris contracts and allows less light into the eye. When the light amount is low, iris dilates and allows more light into the eye.

4. Lens: It is made of a jelly transparent material and is a biconvex structure. It is present behind the pupil. The size and thickness of the lens change as per the distance of an object by the ciliary muscle.

5. Retina: The screen of eye is referred to as retina because the light ray comes through the pupil and passes through the lens and converges on a screen called retina. Retina is the light – sensitive inner lining of the back of the eye.
Retina contains several nerve cells. There are two kinds of cells
(i) Cone cells, which are sensitive to bright light and sense colour.
(ii)Rods cells, which are sensitive to dim light.

6. Optic Nerve: It is located at the back of the eye. Optic nerves are the bundle of over one million nerves fibres that carry visual messages from the retina to the brain.

7. Blind Spot: It is the small region where the optic nerve and the retina meet. It has no sensory cells. Hence no image is formed at this spot on the retina.

Working of Eye:
(i) Light reflects off from the objects and enters the eyeball through a transparent layer of tissue at the front of the called the cornea.
(ii) The cornea bends the light rays through the pupil- the dark opening in the center of the colored portion of the eye.
(iii) The adjusted light passes through the eye’s natural crystalline lens.
(iv) The eye lens converges all the light rays on the retina and images is formed on retina real, small and inverted.
(v) The retina converts these light rays into electrical signals that are relayed to the brain via the optic nerve. The brain processes the information it receives, so that in turn, we can see.

Persistence of Vision:
(i) The impression of an image does not vanish immediately from the retina. An image persists there for about 1/16th of a second. This feature is called persistence of vision.
(ii) If still images of a moving object are flashed on the eye at a rate faster than 16 per second, then the eye perceives this object as moving.
Examples: Due to this persistence of vision, when we see many still images in a sequence, they give the illusion of moving images. Movies and animation are made by exploiting this property of the human eye.

Care of Eyes:
(i) Eyes are very important sense organ and they are sensitive too. So, It is necessary to take proper care of your eyes. Some points for care of eyes are as follows:
(ii) If there is any problem you should go to an eye specialist.
(iii) Too bright or too dim light is bad for eyes. Insufficient light causes eyestrain and headaches.
(iv) Do not look at the sun or a powerful light directly.
(v) Never rub your eyes. If particles of dust go into your eyes, wash your eyes with clean water. If there is no improvement go to a doctor.
(v) Always read the book and news paper at the normal distance for vision. Do not read by bringing your book too close to your eyes or keeping it too far.

Vision Defects:
Some of the vision defects are as follows:

1. Myopia: A person with myopia can see near object clearly while distant objects appear blurred. This disease is also called short sightedness or near sightedness. Spectacles with concave lenses can improve the vision in a myopic person. 

2. Hypermetropia: A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. This disease is also called long sightedness. Spectacles with convex lenses can improve the vision in a hypermetropic person.

3. Cataract: During old age, the crystalline lens becomes milky and cloudy in a cataract patient and thus the person cannot see clearly. Cataract is treated by surgery. In cataract surgery, the cloudy lens is removed and an artificial lens is transplanted in the eye.

Visually Challenged Person:
(i) Some people including children face with disability of vision. This disability of vision can be partial or complete. Such persons are called visually challenged persons.
(ii) A visually challenged person usually show a marked development of other senses; like the sense of hearing and sense of touch. However, additional resources can enable them to develop their capabilities further. Resources can be of two types: Non-optical aids and optical aids.

1. Optical Aids: Optical aids help a person who is partially visually challenged. These aids enlarge an image or a text so that they could be visible. Bifocal lenses, contact lenses, tinted lenses, magnifiers and telescopic aids, magnifying devices and telescopic devices come under this category.

2. Non-optical Aids: Non-optical aids help a person who is completely visually challenged. Non-optical aids include visual aids (magnify words), tactual aids (using the sense of touch), auditory aids (using the sense of hearing) and electronic aids.
(i) Visual aids can magnify words; provide suitable intensity of light and material at proper distances.
(ii) Tactual aids, including Braille writer slate and stylus, help the visually challenged persons in taking notes, reading and writing.
(iii) Auditory aids include cassettes, tape recorders, talking books and other such devices. Electronic aids, such as talking calculators, are also available for performing many computational tasks.
(iv) Closed circuit television, also an electronic aid, enlarges printed material with suitable contrast and illumination.
(v) Now a day, use of audio CDs and voice boxes with computers are also very helpful for listening to and writing the desired text.

Louis Braille:
The most popular resource for visually challenged persons is known as Braille. Braille code is used for common languages, mathematics and scientific notation. Louis Braille was born in 1809 in France. He became blind due to an accident in childhood. He developed the Braille script in 1824.

Louis Braille                                                 

Braille Script: This system has of 63 raised dot pattern or characters. Each character or pattern represents a letter, a combination of letters, a common word or a grammatical sign. Dots are arranged in cells of two vertical rows of three dots each. A reader needs to touch the Braille script to read it. Example of dot patterns used in Braille System:


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