What exactly are shadows?

Shadows are dark regions formed when an opaque object blocks the path of light. This formation is possible only because light only travels in a straight line.

Shadows
An opaque object blocks the light falling on it. This creates an area of darkness on the side of the object away from the source of light. A translucent object also creates a faint area of darkness. An area of darkness formed by an opaque object obstructing light is called a shadow. The following three things are required for a shadow to form (Fig. 13.4):

  • a source of light
  • an opaque object
  • a screen or face behind the object.

Formation of shadows

A shadow will not form if any of these is absent. This explains why we cannot see a shadow in the dark. It is only when light rays are obstructed by an opaque object that we get a shadow of the object. When an opaque object is placed in path of light, a dark portion is formed on the opposite side of object on the screen. This dark portion is shadow.
(1) The size of shadow depends on the distance of source of light and on the angle at which the light rays fall on the object.
(2) If the source of light is closer to the object, a larger shadow is formed than when the source of light is far from the object.
(3) The shadow can be seen only on a screen. The ground, walls of the rooms, a building, or other such surfaces act as a screen for the shadows you observe in everyday life
(4) Shadows sometimes gives lot of information about the object.
(5) Shadows are in black in colour and this is not changed by the colour of the opaque objects. So a red and yellow rose will have same black shadows.

Nature of a Shadow
The shadow is categorized into two parts based on the brightness of the dark patches:-
i. Umbra: It is the darkest part of the shadow present in the middle of it, which forms due to the absence of light.

ii. Penumbra: It is the faintest part of the shadow present at the corners of it. It is formed when the light is not completely blocked (Figure 7).

 Figure 7: Umbra and Penumbra
 
Characteristics of a Shadow

  • An opaque screen is required to form a shadow. If there is no screen, then no shadow will form.
  • The colour of the shadow is always black.
  • A shadow casts the shape of the opaque object only.

The size of the shadow varies according to the presence of light coming from the different angles:-

  • If the distance between the source of light and an opaque object is decreased, then the size of the shadow forms large dark patches.
  • If the distance between the source of light and an opaque object is increased, then the shadow is small.
  • If the distance between the object and the screen is decreased, then the shadow is small.
  • If the distance between the object and the screen is increased, then the shadow is large.