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Light - Intro.

posted Nov 25, 2015, 10:09 AM by Upali Salpadoru   [ updated Jul 16, 2016, 10:22 PM ]
Ali has two interesting questions regarding the nature of light.
1.    Can light be seen?
2.    Light exists as a wave or particles.
If you say “it can be seen’ he’ll say ‘It cannot be seen. If anyone says ‘it cannot be
seen’ he'll say it can be seen.
 Fig.1 What do you see here?

Look at the picture carefully.


Do you see the light rays or the tree?


" The branches of the tree” . 


 “ Light rays through the branches”.

Fig.2.  Who is right ?
Both are right .      “How come ?"

If you switch on an electric light in a dark room , there will be light. 

But can you see the ight?................

“ No ! ”              

Can be quite a correct answer.

You’ll see the glowing filament, the glass cover (bulb)  the walls of the room and the objects inside but you will fail to point your finger to show light.
Fig.3  A girl is flashing a light at night to a wall. Do you see light?

It is a well known fact that we see the moon with the help of sun’s rays. But you cannot see the light rays in the night sky on its way to the moon.

Then why do we say both are right

" Normally when we say we see something what we mean is that an image of the object forms in the retina of the eye.  But seeing has another meaning. If you take the meaning of seeing as sensitivity to light, definitely we are sensitive to light. Therefore you can also say that we see light".



 Fig. Do you see a shadow of the coconut tree ? .    
      What we know about shadows.
i.    A shadow roughly outlines the object.
ii.    It may be bigger, smaller or the same size as the object.
iii.    It cannot have a colour. (grey and black are not colours.)
iv.    Length and breadth may not be shown to same scale.
vi.    It may be just the opposite of an image. (An  Image is formed by illumination while a shadow   is by covering light)
vii.    An image is visible while the shadow is invisible as the un lit region

The shape of the shadow depends primarily on the source of light.
The emanating beam can be diverging as from a bulb, a point source, 
 Parallel as from a very distant source like the sun, or
 converging as from a convex lens.

Fig.   A large source of light gives two distinct shadows.


Most of the light falling on objects are absorbed changing to internal energy(heat)  Some are always reflected. There is a visible difference between reflection from a smooth surface and a rough surface. Surfaces like paper, walls reflect in all directions while, metal sheets water surface will reflect in a regular manner.

All reflections observe two simple laws.
1.    The incident angle is equal to reflected angle.
2.    The incident ray, the normal and the reflected ray are in the same plane.

Fig.8. Yellow represent light rays. (The thick line is a beam)  i=incident angle,  r =. Reflected angle

Fig.9. Reflecting from a smooth, plane  silvery surface.

 The image formed has the following properties.

 i.  Virtual (not real , as it cannot be obtained on to a screen)

 ii. Formed behind the opaque surface.

 iii. The image distance = objet distance.

 iv. The size of image = size of object.

 v. The leaf on the right is on the left side of image. (laterally inverted.)


  • 1. Making a transparent polythene opaque.









(showing a clear polythene sheet)   Can you see through this?

Yes, Very clear. It’s like glass.

Without using any other material, who can make this prevent seeing through.

Yes , tilt it  ,

May be part of the light coming will reflect or deflect. But it is not a perfect job.

Ya ! I know. Give it to me. ( folds the paper into2 then 4 & again and again)

(Applause)   Well done Nelly.

Every surface , even a transparent object reflects a certain amount of light. As you fold a sheet, you increase the number of surfaces. Every surface will reflect and it will become translucent and silvery after that.


            To learn about Refraction and diffraction click.  Light nature and L.Interference.L.-Interference.