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Light - Reflection of Light

Grade 6ICSE

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 objects. It travels in a straight line, which is known as the Rectilinear Propagation of Light.

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Reflection of light is the phenomenon of bouncing back of light rays into the same medium when it strikes a polished surface like a mirror.

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The Incident Ray is the light ray that strikes the reflecting surface. The Reflected Ray is the ray that bounces back from the surface.

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The Normal (NN) is an imaginary line drawn perpendicular to the reflecting surface at the point of incidence.

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The Angle of Incidence (anglei\\angle i) is the angle between the incident ray and the normal. The Angle of Reflection (angler\\angle r) is the angle between the reflected ray and the normal.

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First Law of Reflection: The incident ray, the reflected ray, and the normal at the point of incidence all lie in the same plane.

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Second Law of Reflection: The angle of incidence is always equal to the angle of reflection, i.e., anglei=angler\\angle i = \\angle r.

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Images formed by a plane mirror are: (1) Virtual and erect, (2) Of the same size as the object, (3) At the same distance behind the mirror as the object is in front of it, and (4) Laterally inverted.

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Lateral Inversion: The phenomenon where the left side of the object appears as the right side of the image and vice versa.

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Regular Reflection occurs from smooth surfaces like mirrors, where parallel incident rays remain parallel after reflection. Diffused (Irregular) Reflection occurs from rough surfaces, where reflected rays travel in different directions.

📐Formulae

∠i=∠r\angle i = \angle r

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

Angle between Incident ray and Reflected ray=∠i+∠r=2∠i\text{Angle between Incident ray and Reflected ray} = \angle i + \angle r = 2\angle i

💡Examples

Problem 1:

If a ray of light strikes a plane mirror such that the angle of incidence is 40∘40^\circ, what will be the angle of reflection?

Solution:

Given: ∠i=40∘\angle i = 40^\circ. According to the second law of reflection, ∠i=∠r\angle i = \angle r. Therefore, ∠r=40∘\angle r = 40^\circ.

Explanation:

The law of reflection states that the angle at which light hits a surface is equal to the angle at which it bounces off, relative to the normal.

Problem 2:

A light ray strikes a mirror and the angle between the incident ray and the mirror surface is 30∘30^\circ. Calculate the angle of incidence and the angle of reflection.

Solution:

The Normal is at 90∘90^\circ to the mirror. Angle with mirror + Angle of incidence =90∘= 90^\circ. So, ∠i=90∘−30∘=60∘\angle i = 90^\circ - 30^\circ = 60^\circ. Since ∠i=∠r\angle i = \angle r, then ∠r=60∘\angle r = 60^\circ.

Explanation:

The angle of incidence is measured from the normal, not the mirror surface. Subtracting the glancing angle from 90∘90^\circ gives the angle of incidence.

Problem 3:

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:

Object distance u=15 cmu = 15\text{ cm}. In a plane mirror, image distance v=uv = u, so v=15 cmv = 15\text{ cm}. Distance between object and image =u+v=15 cm+15 cm=30 cm= u + v = 15\text{ cm} + 15\text{ cm} = 30\text{ cm}.

Explanation:

The image is formed as far behind the mirror as the object is in front of it. The total distance is the sum of the object's distance from the mirror and the image's distance from the mirror.

Reflection of Light - Revision Notes & Key Formulas | ICSE Class 6 Science