Answer key

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Last edited 26 days ago by Learn LoopHQ.

Chapter: 06. Light

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Differentiate between the umbra and penumbra regions of a shadow.

A shadow often consists of two distinct regions: the umbra and the penumbra. The umbra is the central, darkest part of the shadow, which receives no light whatsoever from the source. It is an area of total darkness. In contrast, the penumbra is the region of partial darkness that surrounds the umbra. This area receives some light from the source, making it lighter than the umbra but still darker than the fully illuminated surroundings. The formation of both umbra and penumbra is typically observed with extended light sources.

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What is rectilinear propagation of light? Describe an activity to demonstrate this property.

Rectilinear propagation of light is the fundamental property stating that light travels in a straight line unless its path is obstructed or its direction is changed by an object. To demonstrate this, you can use three cardboards with holes made at the same height. Align these cardboards and place a burning candle at one end; you will be able to see the flame through all holes from the other end. If you then slightly move any one cardboard, misaligning the holes, the candle flame will no longer be visible, proving that light travels in straight lines.

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Explain the concept of luminous and non-luminous objects with examples.

Luminous objects are those that produce or emit light of their own, such as the Sun, candles, or electric bulbs. In contrast, non-luminous objects do not emit their own light; instead, they become visible to us only when they reflect light from a luminous source. Examples of non-luminous objects include the Moon, chairs, tables, and books. Our eyes perceive these objects when light from a luminous source bounces off their surfaces and enters our eyes.

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Explain the construction and working of a pinhole camera.

A pinhole camera consists of a hollow rectangular box with a small hole (pinhole) made in the center of one face. The inside of the box is blackened to prevent internal reflections. The opposite face is left open and covered with a translucent screen, like tracing paper. When an object is placed in front of the pinhole, light rays from each point of the object travel in straight lines through the pinhole. Due to this straight-line travel and the crossing of rays at the pinhole, an inverted and real image of the object is formed on the tracing paper screen.

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Explain the occurrence of a lunar eclipse.

A lunar eclipse occurs when the Sun, the Earth, and the Moon align in a straight line, with the Earth positioned directly between the Sun and the Moon. In this configuration, the Earth casts its shadow onto the Moon, preventing sunlight from reaching it. As the Moon moves through Earth’s shadow, it can either be completely covered by the dark central part (umbra), leading to a total lunar eclipse, or partially covered by the umbra and the lighter outer part (penumbra), resulting in a partial lunar eclipse. This event only happens on a full moon night.

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How are shadows formed? What are the necessary conditions for shadow formation?

Shadows are formed when an opaque object blocks the path of light, preventing light rays from reaching the area behind it. This creates a dark region, or patch, on a surface beyond the object. This phenomenon is a direct result of light’s rectilinear propagation. For a shadow to form, three necessary conditions must be met: there must be a source of light, an opaque object to obstruct the light’s path, and a screen or surface to receive the shadow, as shadows cannot form in thin air.

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Discuss the factors on which the size of the image in a pinhole camera depends.

The size of the image formed by a pinhole camera primarily depends on two factors: the distance between the screen and the pinhole (camera length), and the distance between the object and the pinhole. If the screen is moved closer to the pinhole, the image becomes smaller but brighter. Conversely, if the screen-to-pinhole distance increases, the image becomes larger but less bright. Additionally, as the object is moved farther away from the pinhole, the image size decreases, and moving the object closer increases the image size.

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