How Shadows are Formed

Objective

To study how shadows are formed.

 

Theory

If you shine a torch on the wall and bring your hand in front of the light beam, you can see dark patches formed on the wall. This dark region surrounded by light is referred to as a shadow.  

Shadows are formed when an opaque object comes in the path of light. Since the light travels in a straight line, whenever you place an opaque object in the path of light, the light rays in that region is blocked. Hence, the shadows are formed in the shape of the object. In fact, you can even make accurate guess of the object from its shadow. 

Essential requirements for a shadow are: 

  • A source of light: 

   Since shadows are formed when the light from a source is blocked, a source of light is necessary for the formation of a shadow. The source of light can be natural (such as sun) or artificial (such as a torch light, bulb etc.). Shadows are not produced in dark rooms. 

  • An opaque object:  

    An opaque material does not allow light to pass through it. If you use a transparent or translucent object, the light is not effectively blocked as these materials allow the light to pass through them. 

  • A screen: 

The characteristics of the shadow formed depend on the position and intensity of the source of light. For instance, while walking on the road at noon when the sun is directly overhead, you can see a short and dark shadow. On the other hand, you might find that your shadows are longer in the early morning or late afternoon. Our forefathers made good use of this fact when they devised the sundial as an instrument to show time. A sundial consists of a horizontal circular plate and a projection called a gnomon. The sundial indicates the time of day by the position of the shadow of gnomon exposed to the sun's rays. As the day progresses, the sun moves across the sky, causing the shadow of the gnomon to move and indicating the passage of time.  Sundials were the only means of telling the time prior to the development of clocks. In fact, the world’s largest sundial is situated at Jantar Mantar in Jaipur, Rajasthan. 

 

Learning Outcomes

  • Students learn to identify the shadow.  
  • Students understand how shadows are formed.
  • Students learn the conditions for formation of a shadow.
  • Students learn to relate the size and shape of the shadow in terms of distance between the light source and the object.