Light Shadows and Reflections - Revision Notes

 CBSE Class–VI Subject Science

Revision Notes
CHAPTER – 11
Light, Shadows and Reflections

Light: Light is the natural agent that stimulates sight and makes things visible.

  • Its path is only visible when it is scattered by dust particles present in the atmosphere.
  • Light is a form of invisible energy.
  • Light itsel is not visible .
  • It makes other objects visible.
  • Light and the eyes both are necessary to see the things.
  • The sensation which helps us to see things is called sight or the vision.

SOURCES OF LIGHT 

  • Sun,
  • stars
  • electric torch, 
  • candle flame

Light is classified into two:

(i) Emission of light: Classifying objects on the basis of emission of light.

LUMINOUS OBJECT  :- objects which emit their own light are called luminous objcts.Ex. A candle, electric torch.

NON-LUMINOUS OBJCTS :- Objects which do not have their own light and are seen by the light scattered by them are called non-luminous objects.. Example: moon, chair, table, etc.

  • Optical media :- Any substance which allows ligt to pass through it either partially or wholly is calle an otical media.·
  • Transparent: All substances that allow light to pass freely through them and through which objects can clearly be seen are called transparent substances.Example: glass, water, air, etc.
  • Translucent: A substance that allows light to ass through it only partially is calle a translucent medium or substance.. Example: butter paper, tissue paper, etc.
  • Opaque: Objects that do not allow light to pass through them. Example: book, brick, etc.

Shadow: A  shadow is the 'region of absence of light'. Light from a source is cut off by an obstacle and shadow is formed.

  • Opaque object cast a dark shadow.
  • Translucent objects produce a weak shadow.
  • Transparent objects do not cast a shadow at all.

Types of shadow:

The kind of shadow depends upon the size of he source of light.

Due to smaller (point) light source: Only one dark shadow is formed and this is known as umbra.

Due to larger(extended) light source: Two shadows are formed-a dark one in the centre and a light one on the outside. Dark shadow is called umbra and the faint or lighter shadow is called penumbra.

  • The size of umbra decreses and penumbra increses as the the screen is moved away from the object.

Colour of shadow :- Whatever be the colour of the  opaque objcet , the shadow formed is always of the same colour , that is , black.

LENGTH OF THE SHADOW :- The length of the shadow differs depends upon the angle at which light falls on a body.

  • Eclipse: A shadow formed in space that makes the sun or the moon invisible for some time.Eclips is the overshading of a bright obect.
  • Solar eclipse: The moon comes between the sun and the earth, so that the earth (in the shadow) darkens during the day.
  • The solar eclips occurs only on a 'NEW MOON DAY' .
  • Lunar eclipse: The moon and the sun are in a straight line such that the earth is in the between the sun and the moon, the shadow of the earth falls on moon and the moon cannot be seen.
  • The lunar eclips occurs on a 'FULL MOON ' NIGHT.

(ii) Reflection of light: The process of sending back the lig