Chapter Notes - 1, Light, Science, Class 8 | EduRev Notes

Class 8: Chapter Notes - 1, Light, Science, Class 8 | EduRev Notes

The document Chapter Notes - 1, Light, Science, Class 8 | EduRev Notes is a part of the Class 8 Course Science Class 8.
All you need of Class 8 at this link: Class 8

What Makes Things Visible?

Light is a form of energy, which induces the sensation of vision in our eyes and makes us able to see various things present in our surroundings. The light ray may be objects self-light or reflected light.

Luminous objects are objects which emit light of their own.
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.

Reflection of Light

  • The ray of light which falls on the mirror surface is called an incident ray. 
  • The point at which the incident ray strikes the mirror is called the point of incidence. 
  • The ray of light which is sent back by the mirror is called the reflected ray. 
  • The ‘normal’ is a line drawn at right angles to the mirror surface at the point of incidence. 
  • The angle between the incident ray and normal is called the angle of incidence. 
  • The angle between the reflected ray and normal is called the angle of reflection.

Reflection of Light by a Plane Mirror

Try yourself:Beam of light striking the reflecting surface is called
View Solution


Laws of Reflection

  • The incident ray, the reflected ray, and the normal (at the point of incidence), all lie in the same plane.
  • The angle of reflection is always equal to the angle of incidence.


Regular and Diffused Reflection

Light undergoes either diffuse or regular reflection.

Regular Reflection
  • In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction.
  • Regular reflection of light occurs from smooth surfaces like that of a plain mirror.
  • Images are formed by regular reflection of light.
Diffused Reflection
  • In Diffuse reflection, a parallel beam of incident light is reflected in different directions.
  • The diffuse reflection of light takes place from rough surfaces.
  • A sheet of paper produces diffuse reflection of light. No image is formed in diffuse reflection of light.
    Diffused Reflection
Reflected Light can be Reflected Again

Characteristics of image formed by a plane mirror:

  • The image formed by a plane mirror is virtual & Erect.
  • The image formed in a plane mirror is the same distance behind the mirror as the object is in front of it.
  • The image formed in a plane mirror is of the same size as the object.
  • The image in a plane mirror is laterally inverted.

Plane Mirror

Uses of a plane mirror:

  • To see ourselves
  • To make some instruments like periscope
  • In shops for decoration
Multiple Images
  • When two or more mirrors are placed at an angle to one another, we can see multiple images of an object.
  • 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.
  • If two plane mirrors are inclined at an angle q, then the number of images formed in them is given by the formula: 
    No. of images formed = (3600/ q) - 1
  • 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.
  • When the two mirrors are kept opposite and parallel to each other, we will get an infinite number of images formed.
Kaleidoscope

The kaleidoscope is an instrument which produce multiple reflections of coloured glass pieces (or coloured plastic pieces) and creates beautiful patterns. 

Kaleidoscope changing pattern of colourful images

Construction: The Kaleidoscope consists of three long and narrow strips of plane mirrors inclined at 600  to one another forming a hollow prism, and fitted into a cardboard tube. One end of the cardboard tube is closed by an opaque disc (cardboard disc) having a small hole at its centre. The other end of the cardboard tube is closed with circular discs of glass. The inner disc being of transparent glass (clear glass) and the outer disc of ground glass (translucent glass). A number of small pieces of different coloured glass (or plastic) and having different shapes are kept between the two glass discs (which can move around freely in the space between the two glass discs). When we hold the kaleidoscope tube towards light and look inside it through the small hole, we see beautiful patterns of coloured glass.

Patterns formed by KaleidoscopeSunlight - White or Coloured

The splitting up of white light into seven colours on passing through a transparent medium like a glass prism is called dispersion of light. The seven colours of the spectrum of white light are violet, indigo, blue, green, yellow, orange and Red.

Dispersion of Light 

Try yourself:Band of seven colours is called
View Solution


What is Inside our Eyes?
  • The main parts of the human eye are Cornea, Iris, Pupil, Ciliary muscles, Eye lens (which is a flexible convex lens), Retina and Optic nerve.
  • Cornea is made of a transparent substance 
  • The light coming from an object enters into the eye through cornea. Function of cornea is to protect the eye.
  •  Iris is the colored part of the eye. The iris has a hole at its centre which is called pupil. Pupil appears like a dark spot in the centre of iris because no light is reflected from it. Iris controls the amount of light entering the eye by adjusting the size of pupil.
  • The eye-lens is a convex lens made of a transparent and flexible material.
  • The eye-lens is held in position by ciliary muscles. The function of ciliary muscle is to hold the lens & changes its size according to the need. 
  • The retina is a screen on which the image is formed in the eye.
  • The optic nerve carries the image formed on retina to the brain in the form of electrical signals.

Human Eye

Rods and Cones

  • Rods are the rod-shaped cells present in the retina of an eye which are sensitive to dim light.
  • Cones are the cone-shaped cells present in the retina of an eye which are sensitive to bright light. Cones also cause the sensation of color of objects in our eyes.

Rods and Cones present in the Retina of the Eye

Try yourself:Which one of the following statements is correct regarding rods and cones in the human eye?
View Solution


Blindspot

At the junction of optic nerve and retina in the eye, there are no light sensitive cells {no rods or cones due to which no vision is possible at spot. This is called blind spot.
Blindspot
Persistence of Vision 
The image of an object seen by our eyes persists {or remains}on the retina for about 1/16th of a second even after the object has disappeared from our view. The ability of an eye to continue to see the image of an object for a very shot duration even after the object has disappeared from view, is called persistence of vision.
Persistence of Vision

Defects of the Eye

  • Myopia (short-sightedness) : In this defect the person is able to see the near by objects but not able to see the far-off objects.
    This defect can be corrected by using spectacles containing Concave lens.
  • Hypermetropia (Far-sightedness) : In this defect the person is able to see the far-off objects but not able to see the near by objects.
    This defect can be corrected by using spectacles containing Convex lens.

    Myopia and Hypermetropia

  • Cataract: Cataract develops when the eye lens of person becomes cloudy (or even opaque) due to the formation of membrane over it. Cataract decreases the vision of the eye gradually. It can even lead to total loss of vision of the eye. The opaque lens is removed from the eye of the person by surgical operation & a new artificial lens is inserted in its place.

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.
  • 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 foodstuff 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.

Visually Challenged Persons can Read and Write

Some persons, including children, can be visually handicapped. They have very limited vision to see things. Some persons cannot see at all since birth. Some persons may lose their eyesight because of a disease. Such persons try to identify things by touching and listening to voices more carefully. They develop their other senses more sharply.

However, additional resources can enable them to develop their capabilities further.

What is the Braille System?

  • Braille is a system to read and write for visually impaired people.
  • Consists of 63 dot patterns or characters.
  • These patterns are embossed in braille sheets that can be recognized on touch.
  • The dots are raised in order to make it easier to touch.
The document Chapter Notes - 1, Light, Science, Class 8 | EduRev Notes is a part of the Class 8 Course Science Class 8.
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