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Light - Sources of Light

Grade 6ICSE

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

🔑Concepts

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Light is a form of energy that produces the sensation of vision. It travels at a very high speed of approximately 3imes108 m/s3 imes 10^8 \text{ m/s} in vacuum.

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Luminous objects are those that emit their own light, such as the SunSun, stars, and a glowing electric bulb.

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Non-luminous objects do not emit their own light but reflect light falling on them, making them visible. Examples include the MoonMoon, planets, and furniture.

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Natural sources of light exist in nature (e.g., the SunSun), while artificial or man-made sources are created by humans (e.g., candles, LEDLED lamps).

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Rectilinear propagation of light is the property of light by which it travels in straight lines in a homogeneous transparent medium.

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A point source of light is a source of negligible size, while an extended source has a finite size (e.g., a 40 W40 \text{ W} tube light).

📐Formulae

Speed of light (c)≈3×108 m/s\text{Speed of light (c)} \approx 3 \times 10^8 \text{ m/s}

Speed(v)=Distance(d)Time(t)\text{Speed} (v) = \frac{\text{Distance} (d)}{\text{Time} (t)}

💡Examples

Problem 1:

If the distance between the SunSun and the EarthEarth is approximately 1.5×108 km1.5 \times 10^8 \text{ km}, calculate the time taken by light to reach the EarthEarth. Use speed of light c=3×105 km/sc = 3 \times 10^5 \text{ km/s}.

Solution:

t=dc=1.5×108 km3×105 km/s=0.5×103 s=500 secondst = \frac{d}{c} = \frac{1.5 \times 10^8 \text{ km}}{3 \times 10^5 \text{ km/s}} = 0.5 \times 10^3 \text{ s} = 500 \text{ seconds}

Explanation:

By using the formula for time, t=dvt = \frac{d}{v}, we divide the total distance by the speed of light. 500 seconds500 \text{ seconds} is approximately 8 minutes8 \text{ minutes} and 20 seconds20 \text{ seconds}.

Problem 2:

Classify the following as Luminous or Non-luminous: SunSun, MoonMoon, VenusVenus, FireflyFirefly.

Solution:

Luminous: SunSun, FireflyFirefly. Non-luminous: MoonMoon, VenusVenus.

Explanation:

The SunSun and FireflyFirefly generate their own light (through nuclear fusion and bioluminescence respectively). The MoonMoon and the planet VenusVenus only reflect the light received from the SunSun.

Sources of Light - Revision Notes & Key Formulas | ICSE Class 6 Science