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5.1 Light Reflection

Introduction :

Light is a form energy that enables us to see.              

Light is emitted or given by hot objects, like a candle flame, or the glowing filament of light bulb,or the sun.

An object which produces its own light is said to be luminous.

Most of the objects we can see do not produce their own light – they are non-luminous. What we can see is the light is reflected off them.

Light seems to travel in straight lines. A picture of light as rays travelling in straight lines from a luminous source or bouncing off objects, or being absorbed or transmitted by them, is very useful one. We can think of light ray as a very thin beam of light. A real beam contains many rays. But by drawing just a few of them we can get a clear idea of what happening to the whole beam.                             

When rays of light strike any surface the rays are reflected , unless the surface is black, when they are absorbed.

The reflection depend on how smooth the surfaces are.Good mirrors reflect well over 90% of the light that reaches them, with only a small amount being absorbed.

The surface of most objects are very irregular. Light hitting this sort of surface is scattered in all directions. This is called diffuse reflection.

On the other hand,the surface of a polished sheet of glass or metal can be made extremely smooth. The reflection from this is the regular.

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Common terminology of reflection of light on a plane mirror

 

Normal                   :    A line at right angles to the mirror’s surface.

 

Incident ray           :     A ray of light that is directed onto the mirror’s surface.

 

Reflected ray        :     A ray that is reflected by the mirror’s surface.

 

Angle of                                 :               The angle between the incident

incidence                                              ray  and the normal

 

Angle of                                 :               The angle between the reflected

reflection                                               ray  and the normal

 

                  clip_image004

AO          :               Incident ray

ON          :               Normal

OB          :               Reflected ray

  i             :               Angle of incidence             

  r             :               Angle of reflection              

 

Laws of reflection

 

The first law          :     The angle of incidence is equal to the angle of reflection.

 

The second law  :      The incident ray, the normal and the  reflected ray all lie in the

same plane.

Characteristics of an Image Formed by a Plane Mirror by drawing Ray Diagram

 

     clip_image006

 

(1)     Draw two normal’s at the surface of the mirror.                         

(2) Draw  two incident rays and each of them is directed to the normals.

          (3)  Measure the angles of incidence respectively  by using protractor.

(4)     By using the Laws of reflection  (The angle of incidence is equal to the angle of reflection draw two  reflected rays in front  the mirror.

(5)     Extrapolate the reflected rays behind the with dotted line. The point of intersection is the location of the image .

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The characteristics of an image formed by a plane mirror are :

(1)     The size of the image= the size of the object

(2)     The distance of image  = the distance  of object

(3)     Virtual image (appears to be behind the mirror)

(4)     Laterally inverted (left and right are interchanged)

(5)     Upright

Example 1

A boy is at a distance 5 m from a plane mirror. The mirror is moved away from the boy by 2 m. What is the distance between the boy and his image.

Solution

Example 2

The diagram shows two plane mirrors set an angle 100o to each other. A ray strikes one of the mirrors as shown at  angle of incidence of 45o . Redraw and complete the diagram showing the path of the ray and calculate  the angle of reflection at which it leaves the second mirrors.

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Solution

 

Common terminology of reflection of light on a curved mirror

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                Concave mirror

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                Convex mirror

 

Pole of mirror , P                 :     The centre point on the curved mirror

 

Centre of curvature , C       :       The geometric centre of a hollow sphere of which the

                                                        concave or convex mirror is a part.

 

Principal axis  ,PFC            :         A line which passes through the centre of curvature ,C

                                                          and the pole of a curved mirror,P

 

Principal focus , F                :        A point through which all rays travelling parallel to the

                                                        principal axis converge to or appear to diverge from  

                                                        after reflection by the mirror.

 

Radius of curvature, r            :    Distance between the pole,P and the

centre of curvature,C.

 

Focal length , f                       :       The distance between the principal focus ,F and the

                                                         pole of the curved mirror,P.

 

 

Relationship between f and r

 

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Relationship curvature of mirror and f

 

The more curved the mirror , the shorter its focal length,f.

                          clip_image017

 

Characteristics of an Image Formed by a curved Mirror by drawing Ray Diagram

 

We must draw any two of three rays to determine the position of the image.There are three

                                Ray 1                               

                                For concave mirror    :                           

                   A ray parallel to the principal axis is reflected  to pass through F.

                                clip_image019

                               

For convex mirror :                               

                                A ray parallel to the principal axis is reflected as if it came from F.

 

                                clip_image021

                                                Ray 2                               

                                                For concave mirror :                                               

    A ray through F is reflected parallel to the principal axis.

clip_image023

 

                                For convex mirror :

 

A ray towards F is reflected parallel to the principal axis.

 

                clip_image025

                                Ray 3                               

                                For concave mirror :

 

A ray through C is reflected back along its own path.

 

                                clip_image027

For convex mirror :

 

A ray towards C is reflected back along its own path.

 

                                clip_image029

Image formed by a Concave Mirror

A            u < f  ( Object between F and P )

 

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solution:

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Characteristics of image  :

1. Virtual

2. upright

3. Magnified

Application :

Shaving mirror

B            u = f ( Object,O is at F )

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 Solution:

clip_image037

Characteristics of image  :

1.at infinity

2.virtual

3.upright

4. magnified

Application :

Spotlight,light reflector

C      f < u < 2f  or  f  < u  <  r ( Object O is between F and C.)

clip_image039

 Solution:

clip_image041

Characteristics of image  :

1.Real

2. Inverted (songsang)

3.Magnified

D            u = 2f  or u = r ( Object ,O is at C)

clip_image043

 Solution:

clip_image045

Characteristics of image  :

1.real

2.inverted

3. same size

Application :

Light reflector in a projector

 

E             u > 2f  or  u > r ( Object, O is beyond C )

clip_image047

 Solution:

clip_image049

Characteristics of image  :

1. Real

2. Inverted

3. diminished

 

F  u = ¥ ( Object ,O very far from the lens)

 

clip_image051

 Solution:

                                                                clip_image053

 

Characteristics of image  :

  1. real

  2. inverted

  3. diminished

Application :

Telescope reflector

 

Conclusion :

The distance object,u

Characteristics of image

u   <  f

 

u    = f

 

f  <u <  2f

 

u  =  2f

 

u  >  2f

 

u = ¥

 

 

Image formed by a Convex Mirror

               

A            u < f  ( Object between F and P )

clip_image055

 Solution:

clip_image057

Characteristics of image  :

1. Virtual

2. upright

3.Diminished

Application :

Car Review mirror

 

B      f < u < 2f  or  f  < u  <  r ( Object O is between F and C)

 

clip_image059

Solution

clip_image061

Characteristics of image  :

1. Virtual

2. upright

3.Diminished

Application :

Car Review mirror

Conclusion:

The object distance,u

Characteristics of image

u   <  f

 

f  <u <  2f

 

 

Uses of the reflection of light

               

Plane mirror

(1)     A mirror periscope can be built by mounting two plane mirrors in a cardboard tube.

(2)     A  rear view mirror and a side mirror of a car in order to enable the driver see objects at the back and side of the car.

(3)     Plane mirror acts as an anti-parallax error mirror in electrical instruments such as the voltmeter and the ammeter.

 

Concave mirror

                (1)   A shaving mirror or a make-up mirror used

a wide-aperture concave mirror with a large radius of curvature.

(2)     A torchlight or a headlight of motor vehicles used concave parabolic mirror to produce a beam of parallel light rays.

 

Convex mirror

(1)     Convex mirror are mounted at a sharp corner of a road so that the drivers are able to see oncoming cars on the blind side of the corner.

(2)     Convex mirror hung on the corner of ceilings in supermarkets serve to provide a wide field of view activities happening in the shopping area.

 

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