Light Shadows – How it is Formed by Different Types of Objects

Shadows 

 Key Concepts

• Shadows
• Shadows formed by opaque, translucent and transparent objects
• Shadow shade
• Factors affecting shadow size and position

Introduction

All the materials that we see around can be grouped into three categories based on the amount of light they let through themselves or the amount of light they block. Those are transparent, translucent and opaque objects. The objects in these categories can be arranged in the descending order of their ability to block light falling on them as follows: 

Opaque > Translucent > Transparent 

Opaque objects block almost all the light that falls on them. 

Explanation

Shadows: 

When light is stopped or blocked by an object, it cannot shine through that object. In such a case, a shadow forms on the other side of the solid object. 

When sunlight is blocked by the trees, a shadow of the trees is formed on the other side. 

Shadow formed by an opaque object: 

A shadow is formed when light is blocked by an opaque object. When a person stands by facing his back towards the Sun, he sees his own shadow in front of him. This is because his body does not let any light pass through it, as it is opaque. Opaque materials cast a black (dark) shadow. The shadow of a watering jug is also formed on the opposite side of the source of light and is black in color, as shown below. 

Shadows formed by a translucent object: 

A translucent object also forms a shadow, as it blocks some amount of light falling on it. When light falls on a pair of sunglasses from one side, a grey shadow is formed on the other side instead of a dark one. This is because sunglasses block some amount of light falling on them and allow some light to pass through them. 

Shadows formed by transparent objects: 

A transparent object lets all the light falling on it pass through itself. Therefore, it does not form a shadow at all when light shines on it. Glass panes of the door of a window do not form a shadow. However, the other attachments to the glass panes in the door form a shadow as they are not transparent. 

Shadow shades: 

• All the light passes through the transparent objects; thus, they do not form a shadow. 
• Some light passes through the translucent objects; their shadows are in the shades of grey. 
• Almost all the light falling on opaque objects is blocked; thus, they form a black shadow. 

Factors affecting shadow size: 

The length of the shadow of an object varies with the distance between the light source and the object. Consider the set-up shown in the figure below wherein a source of light (torch), an object (football) and a screen are placed as shown. The shadow of the football is formed on the screen. However, if the torch is moved back and forth, the size of the shadow is changed. 

The closer the football is moved to the light source, the bigger the shadow becomes. This is because the object blocks more and more light as it moves towards the source. The further the football is moved from the light source, the smaller the shadow becomes. This is because the object blocks lesser and lesser light as it moves away from the source. 

Also, at different times of the day, the shadow of an object changes its position and size, even though the distance between the Sun and the object remains almost the same throughout the day. This is because the Sun changes its position, i.e., it moves from east to west during the day. The shadow of an object placed outside is of different sizes and at different positions throughout the day, as shown in the figure below. 

In the above figure, the shadow of the pole is of maximum length during sunrise. It keeps on decreasing until 12 noon, after which it again keeps on increasing in size. Throughout this process, the shadow of the pole changes position in order to be on the opposite side of the sun.  

Summary

1. On the basis of the amount of light blocked by an object, the types of objects can be
   arranged as, Opaque > Translucent > Transparent.
2. When sunlight is blocked by an object, its shadow is formed on the other side.
3. Opaque objects cast a black (dark) shadow. This is because they do not let any light
   pass through them.
4. Translucent objects cast a grey shadow. This is because they let a small amount of light
   pass through them.
5. Transparent objects cast no shadow. This is because they let all the light pass through
   them.
6. The closer the object is moved to the light source, the bigger its shadow becomes. This
   is because the object blocks more and more light as it moves towards the source.
7. The further the object is moved from the light source, the smaller its shadow becomes.
   This is because the object blocks lesser and lesser light as it moves away from the
   source.
8. When the light source moves, the shadow of an object changes its size and position.
9. When the sun is overhead, the shadow of the object lies underneath and is the smallest.
10. When the sun is on one side of the object, the shadow lies on the opposite side and is
    also longer in size.