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Light, Shadows and Reflections - Luminous and Non-luminous Objects

Grade 6CBSE

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 the world around us. It makes things visible by reflecting off their surfaces and entering our eyes.

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

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Non-luminous Objects: These are objects that do not emit light of their own but become visible when light from a luminous source falls on them and is reflected. Examples include the Moon, chairs, books, and mirrors.

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The Moon is a non-luminous object because it does not have its own light; it reflects the light from the Sun. Its distance from Earth is approximately 3.84×105 km3.84 \times 10^5 \text{ km}.

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

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The speed of light in vacuum or air is extremely high, denoted by the symbol cc, and is approximately 3×108 m/s3 \times 10^8 \text{ m/s}.

📐Formulae

c≈3×108 m/sc \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:

Why do we see a non-luminous object like a wooden table in a room?

Solution:

We see the wooden table because light from a luminous source (like a bulb or the Sun) falls on the table and gets reflected into our eyes.

Explanation:

Non-luminous objects do not possess their own light energy. They act as reflecting surfaces for the light emitted by luminous objects.

Problem 2:

If light travels from the Sun to the Earth in approximately 500 seconds500 \text{ seconds}, 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.

Solution:

d=v×t=(3×108 m/s)×500 s=1.5×1011 md = v \times t = (3 \times 10^8 \text{ m/s}) \times 500 \text{ s} = 1.5 \times 10^{11} \text{ m}

Explanation:

By using the formula for distance where dd is distance, vv is the speed of light, and tt is time, we find the distance is approximately 150 million kilometers150 \text{ million kilometers}.

Problem 3:

Classify the following as Luminous or Non-luminous: (a) Stars (b) Planets (c) Firefly (d) Metal Sheet.

Solution:

(a) Luminous, (b) Non-luminous, (c) Luminous, (d) Non-luminous.

Explanation:

Stars and fireflies produce their own light through nuclear fusion or bioluminescence respectively. Planets and metal sheets only reflect light from external sources.

Luminous and Non-luminous Objects - Revision Notes & Key Formulas | CBSE Class 6 Science