CLASS VIII | SCIENCE | CHAPTER 16
Notes prepared by Subhankar Karmakar
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- Luminous Objects:
The objects their own light are called luminous objects.
- Non Luminous Objects:
The objects which do not emit their own light are called non luminous objects.
We can see the non luminous objectsbecause they reflects light into our eyes. Non luminous objects are also called illuminated objects.
- Reflection of light
The process of sending back light rays which fall on the surface of an object, is called reflection of light.
- Incident Ray:
The ray of light which falls on the mirror surface is called incident ray.
- Point of Incidence:
The point at which the incident ray strikes the mirror is called the point of incidence.
- Reflected Ray:
The ray of light which is sent back by the mirror is called reflected ray.
- Normal at the point of incidence:
The normal is a line drawn at right angles to the mirror surface at the point of incidence. It can be defined as a line which is perpendicular to the mirror surface at the point of incidence.
- Angle of incidence:
The angle between incident ray and normal is called the angle of incidence.
- Angle of reflection:
The Angle between reflected ray and normal is called the angle of reflection.
- Laws of reflection of light:
There are two laws of reflection. They are as follows.
- According to the first law of reflection, the incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.
- According to the second law of reflection, the angle of reflection is always equal to the angle of incidence.
- Special case:
When a Ray of light falls normally or perpendicularly on the surface of a plane mirror the ray is reflected back along the same path.
- 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 surface like that of a plane mirror or highly polished metal surfaces.
- Diffuse 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 like that of paper, cardboard chalk, table, chair, walls and unpolished matter objects.
Both regular reflection as well as diffuse reflection obey laws of reflection.
Formation of image in a plane mirror:
Suppose a small object O is placed in front of a plane mirror MM'.
- We take two diverging incident rays OA and OB coming from the object O. These rays strike the mirror at point A and point B.
- Draw two normal AN and BN' at point A and point B.
- ∠OAN and ∠OBN' are two angles of incident.
- Draw ∠NAX = ∠OAN and ∠N'BY = ∠OBN'. Therefore, AX and BY will be the reflected rays respectively.
- Extend the rays XA and YB beyond the mirror and they intersect each other at point I.
- I will be the image of the object O.
LATERAL INVERSION
In an image formed by a plane mirror, the left side of object appears on the right side in the image whereas the right side of object appears on the left side in the image. This change of sides of an object and its mirror image is called lateral inversion.
Characteristics of image formed by a plane mirror:
- 1. The image formed by a plane mirror is virtual or unreal.
- 2. The image formed by a plane mirror is behind the mirror.
- 3. The image formed in a plane mirror is the same distance behind the mirror as the object is in front of it.
- 4. The image formed in a plane mirror is of the same size as the object.
- 5. The image formed by a plane mirror is erect.
- 6. The image in a plane mirror is laterally inverted.
- Multiple reflection:
A reflected ray can be reflected again and again. This property of light is extensively used in optical instruments.
We shall discuss two optical instruments here.
1. Periscope and 2. Kaleidoscope
- Periscope:
A Periscope is a long, tubular device through which a person can see objects that are out of their direct line of sight. It works on the reflection of light from two plane mirrors are parallel to one another.
Usefulness: A Periscope gives us a higher view than normal. By using a periscope, we can see the objects on the other side of which cannot be seen by us directly.
Construction: A Periscope consists of a long tube T having two plane mirrors M₁ and M₂ fitted at its two ends. The two plane mirrors are fitted in such a way that they are parallel to one another and their reflecting surfaces face each other. Each plane mirror makes an angle of 45° with the side of the tube. There are two holes in the Periscope tube, one hole is in front of the top mirror M₁ and the agar hole is in front of the bottom mirror M₂.
Working of a Periscope:
Light ray from the object enters the Periscope through the upper hole and gets reflected by the top mirror vertically downwards. This reflected ray again strikes the bottom mirror of the Periscope and reflected again along a horizontal direction and enters the eyes of the viewer. Thus the object can be seen behind an obstacle.
Some of the uses of Periscope:
- 1. A Periscope is used to see over the heads of a crowd.
- 2. A Periscope is used by soldiers sitting in a trench or bunker to observe the enemy activities outside over the ground.
- 3. Epidiascope is used by a navy officer sitting in a submarine to see ships over the surface of water in the sea even though the submarine itself may be submerged under water.
- Multiple images:
When two plane mirrors are kept inclined at an angle , they can form multiple images of an object. The image of object formed in one plane mirror acts as object for the other plane mirror. It has been found that if two plane mirrors are inclined at an angle x, then the number of images formed in them is given by the formula :
No. of images formed = (360°/x) - 1
If an object is placed between two parallel plane mirrors facing each other, then theoretically, an infinite number of images should be formed.
- Kaleidoscope:
The Kaleidoscope is an instrument or toy containing inclined plane mirrors which produce multiple reflections of coloured glass pieces and create beautiful patterns.
The coloured glass pieces act as objects and the inclined plane mirrors form multiple images of these glass pieces by repeated reflections, which look like beautiful patterns.
The coloured glass pieces act as objects and the inclined plane mirrors form multiple images of these glass pieces by repeated reflections, which look like beautiful patterns.
(a pattern produced by kaleidoscope)
- Dispersion of light:
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 formation of spectrum shows that white sunlight is made up of seven colours. The seven colours of the spectrum of white light are: Violet, Indigo, Blue, Green, Yellow, Orange and Red.
- Sunlight - White or Coloured
The sunlight is referred to as white light. The white sunlight actually consists of seven colours. The fact that white sunlight consists of lights of seven different colours can be shown by using a glass prism as follows.
Rainbow in sky is a natural phenomenon showing the dispersion of sunlight.
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