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Light: Shadows and Reflections - Reflection of Light

Grade 7CBSE

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

🔑Concepts

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Reflection of light is the process where a ray of light falls on a polished or shiny surface (like a mirror) and bounces back into the same medium.

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A plane mirror forms an image that is virtual (cannot be obtained on a screen) and erect (upright).

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The size of the image formed by a plane mirror is exactly the same as the size of the object: hobject=himageh_{object} = h_{image}.

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The distance of the image from the mirror is equal to the distance of the object from the mirror. If the object is at a distance dd from the mirror, the image is also at a distance dd behind the mirror.

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Lateral Inversion: The right side of the object appears to be the left side of the image, and the left side of the object appears to be the right side of the image.

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Regular Reflection: Occurs when light reflects off a smooth, polished surface (like a mirror) in a definite direction.

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Diffused (Irregular) Reflection: Occurs when light reflects off a rough or uneven surface (like a piece of paper or a wall), causing light to scatter in different directions.

📐Formulae

∠i=∠r\angle i = \angle r

Distance between Object and Image=Distance of Object from Mirror+Distance of Image from Mirror\text{Distance between Object and Image} = \text{Distance of Object from Mirror} + \text{Distance of Image from Mirror}

Object Distance(u)=Image Distance(v)\text{Object Distance} (u) = \text{Image Distance} (v)

💡Examples

Problem 1:

An object is placed at a distance of 4 m4\text{ m} in front of a plane mirror. If the object is moved 1 m1\text{ m} towards the mirror, what is the new distance between the object and its image?

Solution:

4 m−1 m=3 m4\text{ m} - 1\text{ m} = 3\text{ m} Total distance=3 m+3 m=6 m\text{Total distance} = 3\text{ m} + 3\text{ m} = 6\text{ m}

Explanation:

Initially, the object was at 4 m4\text{ m}. After moving 1 m1\text{ m} closer, the new object distance is 3 m3\text{ m}. In a plane mirror, the image distance equals the object distance. Therefore, the image is also 3 m3\text{ m} behind the mirror. The total distance between the object and its image is 3+3=6 m3 + 3 = 6\text{ m}.

Problem 2:

If a ray of light strikes a plane mirror such that the angle of incidence is 35∘35^{\circ}, what is the angle of reflection?

Solution:

∠i=35∘\angle i = 35^{\circ} ∠r=∠i=35∘\angle r = \angle i = 35^{\circ}

Explanation:

According to the laws of reflection, the angle of incidence (∠i\angle i) is always equal to the angle of reflection (∠r\angle r). Since the angle of incidence is given as 35∘35^{\circ}, the angle of reflection must also be 35∘35^{\circ}.