CBSE Class 8 Science Light Notes

CBSE Class 8 Science Light Chapter Notes. Learning the important concepts is very important for every student to get better marks in examinations. The concepts should be clear which will help in faster learning. The attached concepts made as per NCERT and CBSE pattern will help the student to understand the chapter and score better marks in the examinations.

LIGHT

WHAT IS LIGHT?
If we enter a dark room, the objects present in the room are not visible. However, if we switch on a bulb, everything in the room becomes visible. Why? The bulb gives out an invisible energy called light. When this energy falls on the objects in the room, it bounces off from the surface of objects. When this energy enters our eyes, the eyes sense it and send a message to the brain. It is finally, the brain which really sees the objects. Eyes are only an aid in seeing the objects around us. Why do we say that light is invisible ? Well, when light energy falls on the objects, we really do not see it. When energy bounces off from the surface of objects and enters our eyes, the sensation produced by this energy, helps our brain to see. Thus, to sum up we can say :
Light is an invisible energy, which causes in us the sensation of vision. When the light falls on any object, it bounces off from the surface of the object in all directions. This is called scattering of light.

DEFINITION
Light is form of energy which enables us to see objects which emit or reflect light. Light is a type of (form of) energy which can produce sensation in our eyes. So we can experience the sensation of vision.
It is travel in straight line in form of particles and waves. With the help of light we see all colours of nature. Our eyes are mostly sensitive for yellow colour and least sensative for violet and red colour. Due to this reason commercial vehicle's are painted with yellow colour, sodium lamps are used in road lights.

PROPERTIES OF LIGHT
Light energy propagates (travels) via two processes.
(i) The particles of the medium carry energy from one point of the medium to another.
(ii) The particles transmit energy to the neighbouring particles and in this way energy propagates in the form of a wave.
(iii) It propagates in straight line.
(iv) It's velocity in vacuum is maximum whose value is 3 × 108 m/sec. (297489978 m/s)
(v) Light does not need a material medium to travel that is it can travel through a vacuum.
(vi) It exhibits the phenomena of reflection, refraction, interference, difraction, polarisation and double refraction.

RAY OF LIGHT
A straight line which shows, the direction of light is called ray of light.

BEAM OF LIGHT
A bunch of light rays or bundale of rays at a point is called beam of light.

HOW WE SEE ?
When a light ray is falling (strike) on the surface of any object which reflect and reached to our eyes. Due to this our eyes feel a sensation then we see the object.

REFLECTION OF LIGHT
When rays of light falls on any object it return back in the same medium from the surface this phenomenon is called reflection of light. Due to reflection of light we can see all the nature.

INCIDENT RAY
The ray of light which falls on a polished surface (or a mirror) is called the incident ray of light.

REFLECTED RAY
The ray of light which gets reflected from a polished surface (or a mirror) is called the reflected ray of light.

NORMAL
The normal is a line at right angle to the reflecting surface.

LAWS OF REFLECTION
(i) The incident ray, the reflected ray and the normal to the surface at the point of incidence all lie in the same plane.
(ii) The angle of incidence (∠i) is always equal to the angle of reflection (∠r) i.e. ∠i = ∠r

• When a ray of light falls on a mirror normally or at right angle it gets reflected back along the same path.

Depending on the nature of the reflecting surface there are two types of reflection :-
(i) Regular (specular) reflection (ii) Irregular (diffused) reflection

 

IRREGULAR REFLECTION (DIFFUSED REFLECTION)

When paraller light rays fall on a rough surface all the rays of light are reflected in all possible (Different) direction this is called diffused or irregular reflection.

 

(vii) Cones (g) controls the size of the pupil

 

 

TRUE OR FALSE

(i) Both incident ray and reflected ray lie in the same plane. [ ]

 

(ii) Diffused reflection is due to the failure of the laws of reflection. [ ]

 

(iii) The image formed by plane mirror is laterally inverted. [ ]

 

(iv) The iris is the coloured part of the eye. [ ]

 

(v) Rods are sensitive to bright light. [ ]

 

(vi) Changing of the thickness of the eye lens is called accommodation. [ ]

 

A smooth, highly polished reflecting surface is called a mirror.

 

When a glass plate is polished on one sided with reflecting material such silver or nickel then is becomes a mirror.

 

From the reflecting surface of mirror there are two types of mirror.

 

(i) Plane mirror (ii) Spherical or curved mirror

 

(i) Plane mirror : A highly polished plane surface is called a plane mirror or if a flat (totally plane) surface of a glass plate is polished one side of reflecting material is called plane mirror.

(ii) Spherical mirror : A mirror whose polished, reflecting surface is a part of hollow sphere of glass is called a spherical mirror. For a spherical mirror, one of the two curved surfaces is coated with a thin layer of silver followed by a coating of red lead oxide paint. Thus one side of the spherical mirror is made opaque and the other side acts as a reflecting surface.

 

For the polishing side there are two type of spherical mirror.

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