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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 3×108 m/s3 \times 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.