A substance which gives light is known as a light source. Sun, a glowing bulb,lighted candle etc. are some sources of light. Any object which burns or glows acts as a source of light.
How can we see objects?
- Make your room dark by shutting the door and windows; put on the light.
- Look at any one of the objects in the room.After that, hold a plank or a writing pad in front of your face.
- The object is visible when there is no obstruction between your eyes and the object.
- If we keep obstructions like plank or writing pad, they do not allow some thing that is coming from the object to reach us.
- When we put on the bulb, light falls on the object, bounces from the object and reaches us.
- We can see an object only when light falls on it and bounces back to our eyes.See the picture given below and observe the direction of the arrowheads.
Try to see your shadow in moonlight on a full moon day. It is also possible to get your shadow at night.
- We need light to get the shadow of any object.
The substances like paper, plank, wood, iron etc. don't allow light to pass through them. These objects form shadow.These are called Opaque substances.
The substances like glass and air allow light to pass through them and hence
we don't get their shadows. Such substances are called Transparent.
The substances such as polythene cover and oily paper partially allows the light to pass through them. Their shadows are unclear. These are called Translucent substancess.
Example of all 3 substance in 1 image:
Is it possible to guess the colour of the object by observing it's shadow? If not why?
Shadow is an area where light is absent. Hence, the shadow is colourless irrespective of colour of the object.
We have seen that we can't guess the object by observing it's shadow.
Example of irrespective colour of object shadow:
Another example of Irrespective shadow of Objects:
Making a pinhole camera:
A pvc pipe, about 8 cm in diameter and length 30 cm.
A pvc pipe, about 7 cm in diameter and length 20 cm and one black drawing sheet.
oil - 1 ml, two rubber bands, a pin, and A4 sheet.
If you cannot get pvc pipes, take a thick sheet of paper and roll it to form tubes.The diameter and length of the tubes should be the same as that given for the pipes.
Cut a piece of black paper and put it like a cap at one end of the big pvc pipe and it with a rubber band.Put the white paper like a cap at one end of the thinner pvc pipe. Fix it with a rubber band.
Now make a hole in the middle of black paper cap with the help of a pin. Put 2 to 3 drops of oil on the white paper cap so that it becomes translucent.
Insert the thin pipe into the big pipe.
Your pinhole camera is ready.
Arrange a lighted candle in front of the pinhole of the camera. Move the thinner pipe forward and backward to get a clear picture of the candle on the screen of the thin pipe.
This picture is to be observed from the back of the thin pipe.
The flame of the candle appears inverted on the screen. This is not the shadow of the candle. It is its image.
By observing the image,try to understand how light enters into the pinhole camera. This will explain the reason for inversion of image.
The light from the candle travels straight in all directions from each point of the flame of the candle. But only the light coming in some particular directions can enter into the camera through its pinhole.
Light which comes from the point at the top of the flame goes straight towards the bottom of the screen and light which comes from the point at the bottom of the flame goes straight towards the top of the screen, as shown in image.In this way, the light coming in a particular direction from each point of the flame, will be able to enter into the pinhole, and light going in other directions is blocked by the black sheet.
This leads to the formation of an inverted image
The formation of inverted image on the screen of the pinhole camera explains that light travels in a straight line.
Now look at a tree through the pinhole camera:
- We get the full image of the tree in the pinhole camera.But when we put a candle in front of the pinhole camera,we get the image of the flame only.
Fun with a magnifying lens:
Take a magnifying lens and try to form an image of a tree on a white drawing sheet.
The image formed through the magnifying lens is clearer than that formed with a pinhole camera.
Difference between Image and Shadow:
We see our face in the mirror everyday. We know that shadows are not coloured but an image has colours that are same as that of the object. Also, a shadow shows only the outline of the object but an image shows the complete object as it is, just like a photograph..
Observe the Reflection:
Make your class room dark by closing doors and windows. Ask one of your friends to hold a mirror in his hand.Take a torch and cover its glass with a black paper leaving only slit in the middle. Now switch on the torch and adjust it so that light falls on the mirror in your friend's hand. Ask your friend to adjust the mirror so that the patch of light falls on another friend standing in front of him at some distance.
When light falls on any object, it rebounds back. This is called reflection.
You can reflect sunlight using mirrors and play with it. But make sure that the reflected light does not enter your eyes.
What we have learnt:
- We need light to see objects.
- A substance which gives light is known as a source of light.
- Shadows are formed when opaque objects obstruct the path of light.
- In addition to light and object we also need a screen to obtain the shadow of an opaque object.
- Colour of objects cannot be determined by looking at their shadows.
- Light travels in a straight line.
- Light gets reflected when it falls on any object.
- People came to an understanding that light travels in a straight line by observing the shapes of shadows.
- An image is different from a shadow.