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Light: Shadows and Reflections - Images Formed in a Plane Mirror

Grade 7CBSE

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

🔑Concepts

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A plane mirror is a flat, polished surface that reflects light to form an image of an object placed in front of it.

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The image formed by a plane mirror is always virtual, meaning it cannot be obtained on a screen because the light rays do not actually meet at the image point.

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The image is always erect (upright) and of the same size as the object being reflected.

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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: This is the phenomenon where the left side of the object appears as the right side of the image, and the right side of the object appears as the left side of the image.

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The Law of Reflection states that the angle of incidence is always equal to the angle of reflection: ∠i=∠r\angle i = \angle r

📐Formulae

Distance of Object from Mirror=Distance of Image from Mirror\text{Distance of Object from Mirror} = \text{Distance of Image from Mirror}

Height of Object=Height of Image\text{Height of Object} = \text{Height of Image}

∠i=∠r\angle i = \angle r

💡Examples

Problem 1:

An object is placed at a distance of 15 cm15\text{ cm} in front of a plane mirror. What is the distance between the object and its image?

Solution:

The distance of the image from the mirror is equal to the distance of the object from the mirror, which is 15 cm15\text{ cm}. Therefore, the total distance between the object and the image is: 15 cm+15 cm=30 cm15\text{ cm} + 15\text{ cm} = 30\text{ cm}

Explanation:

In a plane mirror, the image is formed as far behind the mirror as the object is in front of it. To find the total distance from object to image, we add the object-to-mirror distance and the mirror-to-image distance.

Problem 2:

If a person stands 2 m2\text{ m} away from a plane mirror and then moves 0.5 m0.5\text{ m} closer to the mirror, what will be the new distance between the person and their image?

Solution:

Initial distance = 2 m2\text{ m}. After moving 0.5 m0.5\text{ m} closer, the new object distance is: 2.0−0.51.5\begin{array}{r} 2.0 \\ -0.5 \\ \hline 1.5 \end{array} New object distance = 1.5 m1.5\text{ m}. Since image distance equals object distance, the image is also 1.5 m1.5\text{ m} away from the mirror. Total distance = 1.5 m+1.5 m=3.0 m1.5\text{ m} + 1.5\text{ m} = 3.0\text{ m}.

Explanation:

When the object moves, the image moves an equal distance. The new distance between the person and the mirror is 1.5 m1.5\text{ m}, so the distance to the image (which is 1.5 m1.5\text{ m} behind the mirror) is 3 m3\text{ m}.

Problem 3:

Why is the word 'AMBULANCE' written as 'existsONforall1capgWforall\\exists ON\\forall 1\\cap gW\\forall' (laterally inverted) on the front of ambulance vehicles?

Solution:

The word is written in a laterally inverted manner so that the driver of a vehicle ahead of the ambulance can read it correctly in their rear-view mirror.

Explanation:

Due to lateral inversion in plane mirrors (or side mirrors), the left and right sides are interchanged. By writing the word inverted on the vehicle, the reflection in the mirror appears correctly as 'AMBULANCE', allowing drivers to give way quickly.