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● Light is a form of energy that affects the eye to produce the sensation of vision.
Two types of sources of light are:
(1) Natural sources such as sun, stars. and jugnu.
(2) Artificial sources such as fire, electric lamp, electric tube light, a burning. candle, a kerosene lamps heated bodies, etc.
● Bodies- as luminous bodies and non luminous bodies
Bodies which themselves emit light are called luminous bodies. Examples: torch, electric lamps electric tube light, burning candle, kerosene lamp, sun, stars. Jugnu etc.
Bodies which do not emit light by their own, are called non-luminous bodies. Examples: moon, earth, table, book, chair etc.
As medium- transparent , translucent and opaque
A Medium which allows the passage of light through it easily, is called a transparent medium. Examples: glass, air, water etc.
A medium which allows only a small amount of light to pass through it, is called a translucent medium. Examples: butter paper, tracing paper etc.
A medium which does not allow any light to pass tough it, is called an opaque medium. Examples: wood, metals etc.
● Rectilinear propagation of Light -Light travels in a straight line path.
● A simple application of Rectilinear propagation of Light- Pin hole camera .The image formed in a pin hole camera is inverted .
● The shadow of an opaque object is the dark patch obtained on the screen when that opaque object is placed in the path of light.
- Shadow is formed because light travels in a straight line path.
- The shadow is similar to the shape of the object.
- The part of the shadow where no light reaches from the source is completely dark and is called the umbra.
- The part of the shadow where light reaches from only a portion of the source is partially dark and is called the penumbra
- .●Cases-
- By point source
- There is only umbra in the shadow of an opaque object due to a point source. The umbra is bigger in size than that of the object.
- The umbra increases in size if the. screen is moved away from the object.
- By Extended source smaller than object-
- The shadow of an object due to a light source smaller than the object contains an umbra surrounded by a penumbra. The umbra is bigger in size than that of the object. Both the umbra and penumbra increase in size as the screen is moved away from the source.
- By Extended source bigger than object -
- The shadow of an opaque object due to a light source bigger than the object contains an umbra (which is much smaller in size than the object) surrounded by a penumbra. The umbra diminishes while the penumbra increases in size if the screen is moved away from the object.
- ●Lunar and solar eclipses are the examples of formation of shadows in nature.
- A lunar eclipse
- is caused on a certain full moon night when the earth comes in between the sun and the moon so that the earth casts its shadow on the moon.
- A solar eclipse
- is caused on a certain new moon’s day when the moon comes in between the sun and the earth so that the moon casts its shadow
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