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

Science Class 8

Created by: Dr Manju Sen

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.
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LIGHT
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 surrounding. The light ray may be objects self-light or reflected light.

Luminous objects are objects which emit light of their own.
Example: Sun, bulb, tubelight

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 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 incident ray and normal is called the angle of incidence. 
  • The angle between reflected ray and normal is called the angle of reflection.

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

Fig: Reflection of Light on mirror

Laws of reflection of light:
  • 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.
Chapter Notes - 1, Light, Science, Class 8 | EduRev Notes


TYPES OF REFLECTION
Light undergoes either diffuse or regular reflection.
Regular Reflection:
(1) In regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction.

(2) Regular reflection of light occurs from smooth surfaces like that of a plain mirror.

(3) Images are formed by regular reflection of light.

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

Diffused Reflection:

(1) In Diffuse reflection, a parallel beam of incident light is reflected in different directions.

(2) The diffuse reflection of light takes place from rough surfaces.

(3) A sheet of paper produces diffuse reflection of light. No image is formed in diffuse reflection of light.

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

Characteristics of image formed by a plane mirror:

(1) The image formed by a plane mirror is virtual & Erect.

(2) The image formed in a plane mirror is the same distance behind the mirror as the object is in front of it.

(3) The image formed in a plane mirror is of the same size as the object.

(4)The image in a plane mirror is laterally inverted.

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

Fig: Plane Mirror Reflection Image

Uses of plane mirror:
1. To see ourselves
2. To make some instruments like periscope
3. In shops for decoration

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.

Chapter Notes - 1, Light, Science, Class 8 | EduRev Notes(iii) 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
(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.

 

Kaleidoscope: The kaleidoscope is an instrument which produce multiple reflections of colored glass pieces (or colored plastic pieces) and create beautiful patterns. 


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

Fig: Kaleidoscope changing pattern of colorful 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 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 colored 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 colored glass.

Chapter Notes - 1, Light, Science, Class 8 | EduRev Notes 
Fig. Patterns formed by Kaleidoscope


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

Chapter Notes - 1, Light, Science, Class 8 | EduRev NotesFig: Dispersion of light 

The Human Eye

  • 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.

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

Fig: The 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.

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

Fig: Rods and cones present in retina of an eye
Blind spot
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.


Chapter Notes - 1, Light, Science, Class 8 | EduRev Notes
Fig: Eye Blind Spot
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.

Chapter Notes - 1, Light, Science, Class 8 | EduRev Notes 
Fig: Persistence of Vision 
Defects of the Eye:
(a) 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.
(b) 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.c)


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

Fig: Myopia and Hypermetropia the Defects of the Eye

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.




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