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Chapter 11: Light: Shadows and ReflectionsClass 7 Science — summary, notes, extra questions & NCERT solutions
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Which of these is a luminous object?
Summary
This chapter explores how light behaves, forming shadows and images. Objects that give out their own light, such as the Sun, stars, lightning, fire, and fireflies, are luminous, while objects that only reflect light, like the Moon and a mirror, are non-luminous. Activities with aligned matchbox holes and a bent pipe show that light travels in a straight line. When light meets materials, transparent materials let it pass almost completely, translucent materials let it pass partially, and opaque materials do not let it pass at all. Because light travels straight, an opaque object placed in its path blocks the light and forms a dark patch called a shadow; observing a shadow needs a source of light, an opaque object, and a screen. Opaque objects form darker shadows, translucent ones lighter shadows, and the colour of a shadow does not change with the colour of the object, while its shape and size depend on the positions of object, source, and screen. A shiny surface or mirror changes the direction of light that falls on it — this is reflection. The image formed by a plane mirror is the same size as the object, erect, cannot be caught on a screen, and is laterally inverted (left appears right), which is why AMBULANCE is written reversed on vehicles. A pinhole camera, where light from an object passes through a tiny hole, forms an inverted image on a screen. The chapter ends by using these ideas to build useful items: a periscope, which uses two plane mirrors in a Z-shaped box to see over obstacles, and a kaleidoscope, in which three mirrors create beautiful repeating patterns.
Luminous and non-luminous objectsLight travels in a straight lineTransparent, translucent, and opaque materialsShadowsReflection, plane-mirror images, pinhole camera, periscope, kaleidoscope
Key terms
Luminous object
An object that emits its own light, such as the Sun, stars, fire, or a firefly.
Non-luminous object
An object that does not emit its own light but may reflect it, such as the Moon or a mirror.
Transparent, translucent, opaque
Materials through which light passes almost completely, partially, or not at all, respectively.
Shadow
The dark patch formed when an opaque object blocks light travelling in a straight line; it needs a source, an object, and a screen.
Reflection of light
The change in the direction of light when it falls on a shiny surface or mirror.
Lateral inversion
The left-right reversal seen in the image formed by a plane mirror.
Pinhole camera
A device in which light from an object passes through a tiny hole to form an inverted image on a screen.
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An object that emits its own light, such as the Sun, stars, fire, or a firefly.
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Practice quiz · Light: Shadows and Reflections
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Light: Shadows and Reflections
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