8th Physics -Light

 class 8th science

ICSE/ CBSE Notes-

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Light

Introduction of light:

1- Light is an Electromagnetic wave so it does not require any medium to travel.

2- The speed of light in vacuum  is  c = 3x108 m/s 

3- Light make  shadow of  any object

4-Light travels in straight line 

5- If light falls in any surface ,then following cases are happen

• Reflection of  light
•  Refraction of light  
•  Absorption of light

6- Light is a wave as well as particle ,so we can say that light have dual nature.

7- Speed of light is maximum in vacuum.

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Reflection of light:

when a ray of light bouncing back after striking on a polished surface ,this process is called reflection of light 

Reflection of light

• The ray of light which falls on a mirror is called incident ray  (IR)
• The ray which comes back from the surface of a mirror after reflection is known as a reflected ray (RR)
• . A perpendicular line at the point of incidence is called normal.(N)

• Angle of Incidence: The angle between the normal and the incident ray is called the angle of incidence. it is represented by ∠i

Angle of Reflection: The angle between the normal and the reflected ray is called the angle of reflection. it is represented by ∠r

Laws of Reflection

There are  two laws of reflection given as fallows     
    1-The angle of reflection is always equal to the angle of incidence. (∠i = ∠r).

     2-The incident ray, the normal and the reflected ray all lie on the same plane at the point of         incidence.

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Regular Reflection

• It occurs when the surface is smooth.

• Reflected rays move in a particular direction.

• Example: Reflection by the plane mirror

Diffused Reflection or irregular reflection

• It occurs when the surface is a rough surface.

• Reflected rays scattered in random directions.
• Example: Reflection by the road surface.

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Refraction of light 

When light travels from one transparent medium to another transparent medium, it bends from its original path.This phenomenon of bending of light is called refraction of light 

Refraction takes place at the boundary of separation of the two media.

Important terms for Refraction 

1-Incident ray  -The ray which falls on the boundary of separation to enter the other medium.

2-Refracted ray- The ray travels  in the second medium after refraction.

3-Normal- A perpendicular passing through the point at the boundary of separation, where the incident ray falls.

4-Angle of incidence- (i): The angle between the normal and incident ray.

5-Angle of refraction -(r): The angle between the normal and refracted ray.

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Optically denser medium-

Out of two given media ,the medium which have higher value of refractive index  is called optically denser medium.

• When light Ray travels from a denser medium to a rarer medium, it bends away from the normal.
• ie the angle of refraction is greater than the angle of incidence.

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Optically Rarer medium-

Out of two given media , the medium   which have lower value of refractive index is called  optically rarer medium

• When light Ray travels from a rarer medium to a denser medium, it bends towards the normal.
• ie  the angle of refraction is less than the angle of incidence.

NOTE-When a light  Ray passing  from one medium to another medium at a right angle to the surface separating the two media, it does not bend.

• ie Ray simply travels in its original direction.

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