Rectilinear propagation of light :
Light travels in a straight line. In vacuum or air, light travels with the velocity of 3 × 108 m/s.
Examples of rectilinear propagation of light in everyday life :
(i)     When the sunlight enters through a small hole in a dark room, it appears to travel in straight lines.
(ii)     The light emitted by the head light of a scooter at night appears to travel in straight lines.
(iii)     If we almost close our eyes and try to look towards a lighted bulb, it appears to give light in the form of straight lines, which travel in various direction.

Shadow
A shadow is a dark outline or image cast by an opaque object that blocks light coming from a source of light.

(a)     The cause of formation of shadows is 
        Rectilinear propogation of light (light travels on straight lines)

(b)     Essentials of a shadow:
         (i)  Source of light
        (ii) Presence of opaque object in the path of light
        (iii) Screen on which shadow is formed

(c)     Location of a shadow: 
       Shadow fills the space between the opaque object and the screen. It is the volume and not area on the screen.

 (d) Formation of shadow :
It is formed when light hits the opaque object which does not let the light pass through. Everywhere else around the opaque object, the light continues in a straight path until it bounces off the ground or wall behind the object. The wall or ground behind the opaque object is the screen. On this screen is a dark patch, or shadow, with the same outline as the object surrounded by light. The colour of the opaque object does not affect the colour of the shadow that is formed.

Shadow sticks :
A shadow stick is a vertical pole placed in the ground. Sunlight casts its shadow on to a level surface below.(e.g. a sheet of card or just level ground)

As the Sun moves from A to C, the shadow shortens and then lengthen accordingly
The length and position of the shadow then depends on both the time of year and the time of day. Local noon can be found from the time when the shadow is shortest. At this time the Sun is highest in the sky and crossing the meridian.    

However, shadow sticks are not good clocks - the azimuth of the Sun’s shadow at a given time changes throughout the year with the Sun’s declination.

The shape, size and other characteristics of a shadow depend upon :

(i) Position and distance of the source of light with respect to the object.

 (ii) The distance between the object and the surface on which the shadow falls.

(iii) The size of the source of light.

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