Coloured Shadows: Explanation, Formation and Experiment

Coloured shadows refer to the different colours of the shadow of an object other than black colour.

• A shadow is a dark region caused by an object blocking light from a light source. 
• It occupies the whole three-dimensional volume behind an object with light in front of it. 
• The cross-section of a shadow is a two-dimensional shape.
• When different colours of light strike an object, some of it is absorbed and the rest is reflected. 
• The wavelengths of light that are not absorbed by the object determine the colour of the reflected light. 
• For example, a red object absorbs all wavelengths of light except red, therefore it reflects red light.
• If the surface where the shadow appears is also illuminated with coloured light, the shadow will appear to be a combination of the two colours. 
• For example, if a red object produces a shadow on a green surface illuminated by blue light, the shadow will appear cyan (a combination of blue and green).

Key Terms: Shadow, Wavelength, Dispersion of light, Colours, Light, Prism, Colored shadows, Primary colours

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What is Shadow?

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Shadows are formed when light is blocked by an opaque object (an object through which light does not pass). 

• The emitted light from a source is unable to pass through a solid surface, producing a shadow on the ground. 
• The shape and size of the shadow are determined by the position of the light from the solid object. 
• The produced shadow is much darker than its surroundings.
• The shape and size of the shadow of the same object may vary. 
• The shadow formed by an object can sometimes be misleading. An ice cream cone, for example, can be shaped into a triangle.

Also Read:

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Coloured Shadows

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The fact that visible light has wavelengths ranging from 400 to 700 nanometers provides scientific support to colour. 

• When light incident on an object, part of it is absorbed and the rest is reflected. 
• We know that the white light we see is a mixture of various colours such as violet, indigo, blue, green, yellow, orange, and red. 
• The process of dispersion splits the white light into its constituent colours. 
• This process can be observed by using a prism.
• When lights of different colours strike the same place, preferably on a white surface, the light reflecting off that place equals the sum of the lights that incident on it. 
• As a result, we observe multiple coloured shadows on the white surface with varying colour combinations. This is referred to as the coloured shadow.

Colour Sensitivity

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Formation of Coloured Shadows

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The retina of the eye is responsible for concentrating light and helps in vision. 

• The retina consists of rods and cones that help in both day and night vision. 
• Cones are classified into three categories, each of which is sensitive to red, green, and blue light. 
• These three colours are known as additive primaries of light. 
• Seven different types of colours may be formed by combining these three colours in various combinations. 
• Blue, red, green, yellow, magenta, cyan, and black are the seven different colours. 
• Cones selects colours based on the sensitivity of each cone and are responsible for our multicoloured vision.
• Green cones help in the perception of blue, green, purple, yellow, and orange colours. 
• Red cones help in the perception of purple, blue, green, yellow, orange, and red colours. 
• We can observe a wide spectrum of colours formed when colours overlap by using the sensitivities of the three cones of the eye. 
• A representation of colour sensitivity is displayed below.

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Coloured Shadow Experiment

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This basic experiment will demonstrate how to produce coloured shadows using primary colour-producing sources (light bulbs). This experiment will be carried out in a dark room.

Coloured shadow experiment

Materials Required

• 3 light bulbs of colours green, red, and blue
• A white background
• A couple of objects

Procedure

• Arrange the bulbs so that they all fall on the same region of the white screen.
• Turn on all of the light bulbs and adjust their positions until you get the brightest and whitest light on the screen.
• Adjust the distance between the screen and any thin opaque object until you see three separate colours (green, blue, and red) on the screen.
• Remove the object and turn off one of the coloured lights. The changes in colours are easily visible. Place the object again and see how the colour of the shadow changes.
• Try moving the object closer to the screen to notice how the shadows overlap. The combination of colours can be seen here.
• For example, if you block two colours of light, you will see the shadow of the third colour.

Also, try these combinations:

• When the green and red lights are blocked, a blue shadow appears.
• A dark shadow results from blocking all three lights.
• When you block one of the three lights, you get a shadow that is a combination of the other two colours.
• When you combine blue and green colours, you get cyan.
• When you combine blue and red colours, you get magenta.
• When you combine green and red colours, you get yellow.

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Things to Remember

• The shadow of an object is the area where light from a light source is blocked by the object.
• The shadow of an object may be dark or coloured.
• When the light used to illuminate the object and the surface on which the shadow can be seen is coloured, the shadow appears coloured.
• The shape of the shadow depends on the shape of the object producing the shadow as well as the angle of the light source. 
• The size of a shadow is determined by the distance between the object, the light source, and the surface on which the shadow is produced.
• The process of splitting white light into different colours is known as dispersion.

Also Read:

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Previous Year Questions

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6. An electric dipole of dipole moment is placed in a uniform external electric field…. [KEAM]
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