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Light-Nature.

posted Jan 29, 2015, 2:49 AM by Ranmini Perera   [ updated Mar 1, 2018, 1:30 PM by Upali Salpadoru ]
Nature of Light
light 2.PNG
17th Century  

Fig.1 Can you see the light rays coming from sun to moon?

Fig.2.  Sir Isaac Newton. Eng.

very minute particles, which I name as corpuscles,  traveling at a tremendous speed. if anyone could prove that light slowed down in other materials, then my particle theory would fail”

 

Fig.3. Christian Huygens-Dutch.

“ light propagate as  waves  even in space"

Public:-

" If Newton says they are particles  it has to be correct”

 19th Century 
  Fig.4. Thjomas Young:


 Demonstrated the diffraction of light which is a characteristic property of waves.
He even measured the wave length of light.
 20th Century 
 
 Fig.5. Max Plank :-
 " According to my quantum theory light consists of small units of energy which may be described as 'Photons".

                        


  Fig.6. Albert Einstein:  " I too agree."
Photo electricity can only be explained as considering light to be coming in small bundles of energy which we
 call 'photons'.
Louis de broglie,
Arthue Compton,
Niels Bohr,
Werner heisenberg.
 Light exhibits a dual nature.
It can behave as particles as well as waves.

        




Laws of Reflection.


   Rough surface



    Fig.7. Regular Reflection.          Fig.8. Irregular Reflection. (diffuse reflection)
                                                           In fig.8 the reflected rays are shown in red.

    The two Laws of reflection.

1.  The angle of incidence is equal to the angle of Reflection.
2.  The incident ray, the normal and the reflected ray are in the same plane.

     Refraction of light

When a ray enters from one medium, such as air to another medium like water or glass it bends towards the normal.

Law of Refraction. (Snell's Law)

The ratio of the sine of angle of incidence is to the sine of angle of incidence is a constant between any two media.

 The ratio is called the refractive index.
                           Sine of incident angle
Refractive index = ------------------------
                           Sine of reflected angle


          Sine  i.
    μ = -----------
           Sine  r

The  refractive index of water is 1.3 whereas that of glass is 1.5.
   Diamond  2.4
   Vacuum  1.000
   Air          1.003



Fig.12. A ray of light refracting by glass and emerging out is shown here.

A ray of light cannot be seen but if it goes along a white paper it can be seen as the illuminated region.





Dispersion of white light

We have given the name white light to the pure sun light we get after dawn and before dusk. At sun set and sun rise we do not get white light. There are many ways to show that sunlight is made uo of several colours. Easiest way is to look at the rainbow.

Passing the light through a prism van easily demonstrated as shown here.

 
 

Fig.13. Dispersion of sun liht producing a continuous spectrum.

why do we see some objects coloured even with white light ?

The colours that materials reflect  vary.  A cherry absorbs all colours except red. What we see is the reflected colour or a mixture of colours to which we give various names.

 




Fig. 14. Objects absorb many colours and reflect a few.

 Light as waves

 Light energy may be taken as particles, which are called photons  or waves.  Tje frequency and wave length of different bcolours is shown here.




 

 Fig.15.  Wave properties of colours.

  Total Internal Reflection.

b
 
Rays are coming up in a tank of water. Numbers 1 and 2  undergo refraction only. 3  shows some reflection also. Number 4 gives the highest possible reflection.
What happens if you increase the angle of incidence further ?
There will be no refraction and there will only be reflection.
48 degrees is the critical angle for water  to air.
 



Finding the Critical angle.
  For water the Critical angle is the refracted angle when the angle of incidence is 90 deg.
  Example 1
  Calculate the angle of refraction when a ray is incident on water making an angle of incidence equal to 30 deg.
                             Sin i. / sine r  = 1.3
                                            sine i. - sin 30  -  0.5
                              0.5 / sine r   = 1.3
                                  sine r       =  0.5/1.3   =  0.3846
                                           r     = 22.6 deg.
                               
   Example 2.
  Calculate the critical angle for glass whose refractive index is 1.5.
                              Sine i.  / sine r  =  1.5
                                                         sine i. = 90 deg. =  1.0
                                   1/ sin r       =  1.5
                                       sine r.    =   1/ 1.5    =  0.6666
                                              r     =  41.8 deg.

Why do the clouds appear white while the sky is blue?

This is due to another phenomenon of light. Actually sky has no colour, astronauts say the sky is black even when the sun is visible.

When the white light falls on a cloud several things can happen. Light can get reflected, refracted and may even get split up into colours. What happens will be varying according to the position of the sun, visual angle, particle size and shape.   

Another phenomenon that can occur is scattering. Unlike in diffuse (or irregular) reflection here the rays can even pass through as in the diagram shown here.

White light rays coming from the sun is shown in white lines.

When white light collides on fine dust particles and the molecules of Nitrogen and oxygen, the light waves having short wave lengths get mostly scattered. This is the reason for the sky to be blue.

As the red light has a longer wavelength they can avoid the particles and travel a greater distance. This is the reason for the setting sun to be red and the clouds to turn colour in the evening.,

 
                                                    
Wave length is in nano meters. Blue can get scattered about 9 times more than the red as they have a shorter wave length.




         For specialised sections click :- Light Lenses.
                                                   :- Sense of Vision.

        

 Q. 1.0

  Which theory explains the phenomenon in column one.

Phenomenon

Wave theory can explains

Particle theory explains

Reflection



Refraction



Interference



Diffraction



Photo electric effect.



                                                                                                          10 Marks
        Q 2.0 
      How can you describe what mainly happens to light on following occasions?

1  Entering clear water making an angle of 10˚.

2 Candle light incident on a wall.

3   A diver in a pool looking up and sees the bottom.

4  One sees his face through a mirror.

5   Traveling a great distance into the atmosphere?

 3 x 5 = 15 Marks 

Q.3.0     

Use one or two words to supply the answer.

1  According to the law of reflection if a ray makes an incident angle of 42˚ what will be the reflected angle?

2 In order to see a flower what must be formed on the retina of your eye?

3 What should be the angle of incidence of a ray to enter the water just to prevent refraction and to minimise reflection?

4 What colours are mostly absorbed by the green leaves for photosynthesis?

5 What colour in the visible spectrum can travel the greatest distance in the atmosphere?

       3 x 5 = 15 Marks

Q.4.0

1    Calculate the refracted angle if a ray enters making an incident angle of 20 on to a water surface?  ( Refractive index for water = 1.3)  (You will need a calculator to get the sine values)

2        Calculate the critical angle for water.

            2 x 10 = 20 Marks

Q. 5.0

       

The diagram illustrates the pinhole camera. Select the correct answers.

1 Does it produce a shadow or an image?

2 Is it real or virtual?

3 Upright or inverted ?

4 Enlarged, diminished or same size?

5 If the girl stands up with the camera will she see the tree bigger, smaller or same size?

    4x 5 = 20 Marks

      Q.6.0

1 Which could not be a valid reason for the sea to be blue?

A. The sun shines brightly on the sea.     

B.   Sea water absorbs red more than the others.  

C. Blue light scatters more by the suspended fine particles. 

D – Reflects sky colour.

2 Which one cannot be right?

A = Light consists of tiny packets of energy. 

B= Light is a wave.  

C= Light waves are similar to sound waves.

D – Light has particle wave dual nature.

3 Select the incorrect statement.

A. =Our eyes are sensitive to electromagnetic waves.

B. =All electro magnetic waves have the same speed.

C  = light year “ is a measure of a distance.

D =  Light falling on a black surface produces heat.

 

4. What causes the bent appearance of a stick in water?

A= Reflection  B= Total internal reflection   C = Refraction. D – Diffraction.

 

5 What is the correct arrangement for a solar eclipse.

A -  Sun – Earth – Moon,    B- Earth – Sun – Moon.  C -  Moon – Sun – Earth

D – Sun – Moon - Earth . 

         4 x 5 = 20 Marks
          For the answers click:-Answer page - Physics L to K.


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