krit.club logo

Light Energy - Formation of Image in a Plane Mirror

Grade 7ICSE

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

🔑Concepts

•

Reflection is the phenomenon where a ray of light, after striking a smooth polished surface (like a plane mirror), returns to the same medium.

•

The first law of reflection states that the incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.

•

The second law of reflection states that the angle of incidence ∠i\angle i is always equal to the angle of reflection ∠r\angle r.

•

An image formed by a plane mirror is always virtual, meaning it cannot be obtained on a screen, and it is always erect (upright).

•

The size of the image is exactly equal to the size of the object.

•

The distance of the object from the mirror uu is equal to the distance of the image from the mirror vv. Therefore, u=vu = v.

•

Lateral Inversion is a characteristic of plane mirrors where the left side of the object appears as the right side of the image and vice-versa.

•

The number of images nn formed by two plane mirrors kept at an angle θ\theta can be calculated using the relation between 360∘360^\circ and θ\theta.

📐Formulae

∠i=∠r\angle i = \angle r

u=vu = v

n=360∘θ−1n = \frac{360^\circ}{\theta} - 1

Distance between object and image=u+v=2u\text{Distance between object and image} = u + v = 2u

💡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:

30 cm30\text{ cm}

Explanation:

In a plane mirror, the object distance uu is equal to the image distance vv. Given u=15 cmu = 15\text{ cm}, therefore v=15 cmv = 15\text{ cm}. The total distance between the object and the image is u+v=15 cm+15 cm=30 cmu + v = 15\text{ cm} + 15\text{ cm} = 30\text{ cm}.

Problem 2:

A ray of light is incident on a plane mirror such that the angle between the incident ray and the mirror surface is 40∘40^\circ. Calculate the angle of reflection ∠r\angle r.

Solution:

50∘50^\circ

Explanation:

The normal is at 90∘90^\circ to the mirror surface. The angle of incidence ∠i=90∘−40∘=50∘\angle i = 90^\circ - 40^\circ = 50^\circ. According to the law of reflection, ∠i=∠r\angle i = \angle r, so ∠r=50∘\angle r = 50^\circ.

Problem 3:

How many images will be formed if two plane mirrors are placed parallel to each other (i.e., θ=0∘\theta = 0^\circ)?

Solution:

Infinite

Explanation:

When mirrors are parallel, θ=0∘\theta = 0^\circ. Using the logic of the formula n=360∘θ−1n = \frac{360^\circ}{\theta} - 1, as θ\theta approaches 00, the number of images nn becomes infinite due to multiple successive reflections.