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How are shadows formed?

Class 7 ScienceNCERT chapter: Light: Shadows and Reflectionsइसे हिंदी में पढ़ें

Exam-ready answer

Light travels in straight lines. When an opaque object comes in the path of light, it does not let light pass through it, so a dark region forms behind the object on a screen. This dark region is its shadow. A shadow needs three things: a source of light, an opaque object and a screen like a wall.

Explanation

Light from a torch or the Sun moves in straight lines in all directions. When an opaque object such as a hand, a book or a tree stands in its way, the rays that hit the object are blocked. Rays that pass beside the object continue straight on. Behind the object there is a region that light cannot reach, and this region appears dark on a surface. That dark patch is the shadow.

A transparent object like clear glass lets almost all light pass, so it forms practically no shadow. A translucent object like butter paper or frosted glass lets some light through, so its shadow is faint. A shadow is always dark, whatever the colour of the object: a red ball and a blue ball both cast dark shadows. The shadow shows only the outline of the object, not its colour or details.

The size and shape of a shadow change with the position of the light source. When the object is moved closer to the torch, its shadow on the wall becomes bigger. Outdoors, shadows are short around noon when the Sun is high and long in the morning and evening when the Sun is low. This changing shadow was used in sundials to tell the time of day.

Points to remember

  1. Shadow: the dark region formed behind an opaque object when it blocks light.
  2. A shadow forms because light travels in straight lines (rectilinear propagation of light).
  3. To get a shadow we need a source of light, an opaque object and a screen.
  4. Opaque objects form dark shadows, translucent objects faint ones, and transparent objects almost none.
  5. A shadow is always dark, whatever the colour of the object; its size changes with distance from the source.

Exam tip

Draw a diagram with a torch, an opaque object and a screen, showing straight rays passing the edges of the object and the dark shadow behind it. Write 'light travels in a straight line' as the key reason.

Questions students also ask

Why is a shadow always dark even if the object is coloured?
A shadow is the region where light from the source does not reach. Since no light reaches there, no colour can be seen, so the shadows of red, green and blue objects all look the same dark shade.
Why are shadows longest in the morning and evening?
In the morning and evening the Sun is low in the sky, so its light falls on objects at a slant. Each object then blocks light over a longer stretch of ground, making a long shadow. Around noon the Sun is high and shadows are shortest.
How is a shadow different from an image in a mirror?
A shadow is a dark outline formed where light is blocked. An image in a mirror forms when light bounces back from a shiny surface, so it shows the colour and details of the object. A shadow shows only the shape of the outline.

Chapter labels help you find the topic in your textbook. Check your own edition: chapter names and order can change. These are Know.it explanations, not official NCERT solutions.