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Light: Shadows and Reflections - Light through Transparent, Translucent, and Opaque Materials

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 travels in a straight line, a property known as the rectilinear propagation of light.

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Transparent materials: These materials allow light to pass through them completely. As a result, we can see through them clearly (e.g., clear glass, water, and air).

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Translucent materials: These materials allow light to pass through them only partially. Objects seen through them appear blurred or hazy (e.g., butter paper, frosted glass, and thin tissue).

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Opaque materials: These materials do not allow any light to pass through them. We cannot see through them at all (e.g., wood, metal, and cardboard).

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Shadow Formation: A shadow is a dark patch formed when an opaque object blocks the path of light. A shadow requires a source of light, an opaque object, and a screen.

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The size of a shadow changes depending on the distance between the light source and the object. If the distance dd between the source and the object decreases, the shadow size increases.

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Reflection: This is the process where a surface (like a mirror) bounces back the light that falls on it, changing its direction.

📐Formulae

v=c≈3×108 m/sv = c \approx 3 \times 10^8 \text{ m/s}

Shadow Size∝1Distance between Source and Object\text{Shadow Size} \propto \frac{1}{\text{Distance between Source and Object}}

💡Examples

Problem 1:

An opaque object of height ho=10 cmh_o = 10 \text{ cm} is placed between a point light source and a screen. If the shadow formed on the screen is twice the size of the object, calculate the height of the shadow hsh_s.

Solution:

Given that the shadow is twice the size of the object: hs=2×hoh_s = 2 \times h_o Substituting the value: hs=2×10 cm=20 cmh_s = 2 \times 10 \text{ cm} = 20 \text{ cm}

Explanation:

The size of a shadow formed by a point source is proportional to the distance of the screen from the source relative to the object's position. In this case, the magnification factor is 22.

Problem 2:

Classify the following materials as Transparent, Translucent, or Opaque: (i) A sheet of plane glass, (ii) A piece of rock, (iii) A sheet of cellophane.

Solution:

(i) Plane glass: Transparent, (ii) Piece of rock: Opaque, (iii) Cellophane sheet: Translucent (or Transparent depending on color/thickness).

Explanation:

Transparent materials allow light to pass (100%100\% transmission), Opaque materials block all light (0%0\% transmission), and Translucent materials allow partial transmission.