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Light: Shadows and Reflections - Sources of Light

Grade 7CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Light is a form of energy that enables us to see objects around us. We see an object when light from a source (or reflected from the object) reaches our eyes.

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Luminous Objects: Objects that emit light of their own are called luminous objects. Examples include the Sun, stars, a burning candle, and a glowing electric bulb.

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Non-luminous Objects: Objects that do not emit their own light but become visible when light from a luminous source falls on them and is reflected towards our eyes. Examples include the Moon, a chair, and a book.

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Natural Sources of Light: Sources found in nature, such as the Sun (the primary source of light for Earth) and stars.

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Artificial (Man-made) Sources of Light: Sources created by humans, such as electric lamps, torches, and candles.

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Rectilinear Propagation of Light: Light always travels in a straight line. This property is responsible for the formation of shadows.

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Speed of Light: Light travels at an incredibly high speed of approximately 3×108 m/s3 \times 10^8 \text{ m/s} in a vacuum or air.

📐Formulae

Speed of Light (c)≈300,000,000 m/s=3×108 m/s\text{Speed of Light (c)} \approx 300,000,000 \text{ m/s} = 3 \times 10^8 \text{ m/s}

Distance=Speed×Time\text{Distance} = \text{Speed} \times \text{Time}

💡Examples

Problem 1:

Why is the Moon considered a non-luminous object even though it appears bright in the night sky?

Solution:

The Moon does not produce its own light.

Explanation:

The Moon is a non-luminous body. It appears bright because it reflects the sunlight that falls on its surface towards the Earth. Therefore, it is seen due to the reflection of light from a luminous source (the Sun).

Problem 2:

If light from the Sun takes approximately 500500 seconds to reach the Earth, and the speed of light is 3×108 m/s3 \times 10^8 \text{ m/s}, calculate the approximate distance between the Sun and the Earth in kilometers.

Solution:

1.5×108 km1.5 \times 10^8 \text{ km}

Explanation:

Using the formula Distance=Speed×Time\text{Distance} = \text{Speed} \times \text{Time}: Distance=(3×108 m/s)×500 s\text{Distance} = (3 \times 10^8 \text{ m/s}) \times 500 \text{ s} Distance=1500×108 meters\text{Distance} = 1500 \times 10^8 \text{ meters} Distance=1.5×1011 meters\text{Distance} = 1.5 \times 10^{11} \text{ meters} To convert to kilometers, divide by 10001000 (10310^3): Distance=1.5×1011103=1.5×108 km\text{Distance} = \frac{1.5 \times 10^{11}}{10^3} = 1.5 \times 10^8 \text{ km}

Problem 3:

Classify the following as Luminous or Non-luminous: Sun, Mirror, Firefly, Planets, Burning Torch.

Solution:

Luminous: Sun, Firefly, Burning Torch; Non-luminous: Mirror, Planets.

Explanation:

The Sun, fireflies (bioluminescence), and a burning torch emit their own light. Mirrors and planets only reflect light from other sources.