physics by ashutosh mishra - knowledge bite · the principal axis of the mirror is taken as the...

PHYSICS BY ASHUTOSH MISHRA

VISION ACADEMY | Add. - R.N.220, 2ndFLOOR, KALPANA PLAZA, BIRHANAROAD, KANPUR-208001 India

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Light-Light is a form of energy which produces in us the sensation of vision and is associated with electric

and magnetic fields perpendicular to each other. It activates the sense of sight and enables an observer to

see various things around us.

Kinds of light sources-There are two main kinds of light sources given below:

1) Natural sources-These include all those sources which are natural and have their own light e.g., sun,

firefly, stars, moon etc.

2) Artificial Sources- These include all those sources which are man-made, but have the ability of

emitting light e.g., candle, moon, electric lamp etc.

Ray-Ray is a straight line path followed by light.

Beam-A bunch of rays is called a beam.

Optical Medium- Anything through which light energy can pass

partially or wholly is called optical medium.

Refraction of Light-The phenomenon of bending of light as it goes from one medium to another and results

in a change in the path of light is called refraction. It is caused due to different speed of light in different

medium.

“When a ray of light passes from an optically rarer medium to a denser medium, it bends towards the

normal and when it travels from a denser medium to a rarer medium, it bends away from the normal.”

Law of Refraction- There is mainly two laws of refraction given

below:

I Law- The incident ray, the normal to the refracting surface at

the point of incidence and the refracted ray all lie in the same

plane.

II Law- For a given pair of media and for light of a given

wavelength, the ratio of the sine of the angle of incidence to

the sine of the angle of refraction is a constant. (Snell’s Law)

= Constant = n (Refractive index)

Principle of Reversibility - The principle of reversibility of light

states that the path of a ray of light is reversible.



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According to this principle, if a ray of light travels from a certain path, it will follow exactly the same path,

while travelling reverse.

Reflection of light-The phenomenon of bouncing back the light

rays when they fall on the surface of the object is called

reflection of light.

Law of Reflection- There is mainly two laws of refraction given

below:

I Law-The incident ray, the normal at the point of incidence

and the reflected ray all lie in the same plane.

II Law- The angle of incidence is equal to the angle of reflection i.e., i = r in case of regular surfaces and is

unequal in case of irregular surfaces.

Refractive Index-The refractive index of any optical medium is defined as the ratio between the velocity of

light in air or vacuum to the velocity of light in that medium. It is denoted by n.

Where c is speed of light in air and vacuum and v is speed of light in medium.

Refractive index of a medium is a ratio hence it has no unit.

Factor effecting refractive index – There are following factor effecting refractive index given below:

1. Nature of material - smaller the speed of light in a medium relative to air, higher is the refractive index.

2. Colour or wavelength of light - refractive index increases with the decrease in wavelength. > .

3. Temperature - On increase in temperature, the speed of light in medium increases, so the refractive

index of medium.

Lateral displacement- The perpendicular distance of separation

between the emergent ray and original path of the incident ray is

called lateral displacement.

Lateral displacement increases with

a) An increase in the thickness of the medium.

b) An increase in the angle of incidence.



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c) An increase in the refractive index of the medium.

d) A decrease in the wave length of light.

Real and Apparent depth of objects in water- the rays of light from

the fish are refracted away from the normal as the leave the water.

This bending of the light is not registered by the eye rather it seems as

if the rays come from an imaginary fish higher in the fish bowl.

Therefore the apparent position of the fish is higher than the real

position.

Note- Same type of effect is observed in other objects other than fish

and by this phenomenon we can also explain banding of stick in water

bath.

Partial Refraction- If light ray incident on a transparent material than

the major part of light ray refracted but a minor part of light ray

reflected this phenomenon is known as Partial Refraction.

Note- Partial refraction is a phenomenon which is responsible for that

we should see transparent materials in presence of light.

Prism- “Prism is a peace of a triangular

glass block or any other transparent

material having two refracting surface

bounded with some angle known as

prism angle.”

Refraction of Light through Prism- Figure shows the front surface of a prism where A is a prism angle and a

light ray incident on a surface AB and by following path QR and emergent from a surface AC. Now,

In quadrilateral AQTR,



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A + T =

In triangle QTR,

By eq. (1) and eq. (2),

In triangle UPR,

For minimum deviation angle i.e.,

And than,

We know that,

{ }

And,

{ }



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We know that,

(

)

Critical Angle- If we gradually increase i, the r will also increases.

“At certain stage r=90o at this stage the corresponding incident angle (i) is called critical angle ic.”

Total Internal Refraction- “If incident angle is more than the critical angle than the whole light ray is reflected

back. This phenomenon is called Total internal refraction.”

We know that,

At critical angle, and ,

{ }



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Conditions for Total Internal Reflection – There are some important conditions for Total Internal refraction

given below:

1) The ray must be travel from denser medium to rarer medium.

2) The angle of incidence must be grater then critical angle .

Optical Fibre- An optical fibre is a very thin fibre made

of glass or plastic having radius of order of a

micrometre (10-6m).A bundle of such a thin fibre form

a light pipe. It is used to transmit signal through light

pipe by the phenomenon of total internal refraction.

Total Internal Reflection in Prism – The application of Total Internal Refraction in prism is mostly used in

devises like binocular, Periscope etc.

Prism with angle (45 ) –

Case 1-This is the ray diagram of total internal reflection in prism

having prism angle (45-90-45) If the ray of light incident normally on

the base of prism. This is used in binocular.

Case 2- This is the ray diagram of total internal reflection in prism

having prism angle (45-90-45) if the ray of light incident normally on

the one of refracting surface of prism. This is used in in a slide

projector

90

45 45



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Prism with angle (60 ) – This is the

ray diagram of total internal reflection in prism

having prism angle (60-90-30) if the ray of light

incident normally on the one of refracting surface

of prism.

Sparkling of Diamond – Sparkling of diamond is

caused due to Incident rays can face multiple Total Internal Reflection inside a diamond before emerging

from the top of the diamond. Critical angle is very small for diamond due to high refractive index (n=2.42).

Image formed by Plane Mirror -

The image formed is Virtual (Imaginary), Erect, Laterally inverted and the size of image is equal to that of

object.

Reflection of light by spherical Mirrors -

Mirrors, whose reflecting surface are curved

inward or outward spherically are called

spherical mirror. If it is curved inward then the

mirror is concave mirror and If it is curved

outward then the mirror is convex mirror.



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Basic Definition Related to Spherical Mirror –

1. Principal axis - Line joining the pole and Centre of curvature of the spherical mirror.

2. Pole - The geometrical central point of the reflecting spherical surface is called a Pole. It is denoted by

(P).

3. Aperture - The width of reflecting spherical surface.

4. Centre of curvature - The reflecting surface of a spherical mirror form a part of sphere. It has a Centre,

which is known as Centre of curvature. It is denoted by (C).

5. Radius of curvature - The separation between the pole and the Centre of curvature is called Radius of

curvature and is denoted by (R).

6. Focus point – It is the point on the principal axis, where all parallel rays meet after reflection. It is

denoted by (F).

7. Focal length - The length between the pole and focus point. It is denoted by (f).

Relationship between focal length and Radius of curvature - {

}



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Sign Convention for Spherical Mirrors – In sing convention, the pole (P) of the mirror is taken as the origin.

The principal axis of the mirror is taken as the x-axis of the coordinate system. The conventions are as

follows –

1. The object is always placed to the left of the mirror. i.e. the light from the object falls on the mirror from

the left-hand side.

2. All distances parallel to the principal axis are measured from the pole of the mirror.

3. All the distances measured to the right of the origin (along + x-axis) are taken as positive while those

measured to the left of the origin (along – x-axis) are taken as negative.

4. Distances measured perpendicular to and above the principal axis (along + y-axis) are taken as positive.

5. Distances measured perpendicular to and below the principal axis (along –y-axis) are taken as negative.

Mirror Formula and Magnification - In a spherical mirror,

The distance of the object from its pole is called the object distance (u).

The distance of the image from the pole of the mirror is called the image distance (v and the focal length is

(f) then the relation is given by,

Magnification – Magnification by a spherical mirror is given by the ratio of the height of the image (h’) to

the height of the object (h). It is denoted by m.

Summited by: Ashutosh Mishra

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physics by ashutosh mishra - knowledge bite · the principal axis of the mirror is taken as the...

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