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Light: Mirrors and Lenses - What Are the Laws of Reflection?

Grade 8CBSE

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

🔑Concepts

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Reflection of light is the phenomenon of bouncing back of light rays into the same medium when they strike a polished or shiny surface.

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Incident Ray: The ray of light that falls on the reflecting surface.

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Reflected Ray: The ray of light that is sent back by the reflecting surface after reflection.

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Normal: An imaginary line drawn perpendicular to the reflecting surface at the point where the incident ray strikes (point of incidence).

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Angle of Incidence (anglei\\angle i): The angle formed between the incident ray and the normal.

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Angle of Reflection (angler\\angle r): The angle formed between the reflected ray and the normal.

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First Law of Reflection: The angle of incidence is always equal to the angle of reflection (anglei=angler\\angle i = \\angle r).

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Second 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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Regular Reflection: Occurs when a parallel beam of light falls on a smooth and polished surface (like a plane mirror), and the reflected rays are also parallel to each other.

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Diffused (Irregular) Reflection: Occurs when a parallel beam of light falls on a rough or uneven surface, and the reflected rays are scattered in different directions. Note that the laws of reflection are still obeyed at every single point of the surface.

📐Formulae

∠i=∠r\angle i = \angle r

Angle with mirror surface (Glancing angle)+∠i=90∘\text{Angle with mirror surface (Glancing angle)} + \angle i = 90^\circ

💡Examples

Problem 1:

A ray of light strikes a plane mirror such that the angle between the incident ray and the reflected ray is 60∘60^\circ. Calculate the angle of incidence.

Solution:

30∘30^\circ

Explanation:

According to the first law of reflection, ∠i=∠r\angle i = \angle r. The problem states that the total angle between the incident ray and the reflected ray is 60∘60^\circ. Therefore, ∠i+∠r=60∘\angle i + \angle r = 60^\circ. Substituting ∠r\angle r with ∠i\angle i, we get 2∠i=60∘2\angle i = 60^\circ, which means ∠i=60∘2=30∘\angle i = \frac{60^\circ}{2} = 30^\circ.

Problem 2:

If a ray of light falls normally (perpendicularly) on a plane mirror, what will be the angle of reflection?

Solution:

0∘0^\circ

Explanation:

When a ray falls normally, it is along the normal itself. Thus, the angle of incidence ∠i=0∘\angle i = 0^\circ. By the law of reflection, ∠r=∠i\angle r = \angle i, so the angle of reflection is also 0∘0^\circ. The ray retraces its path.

Problem 3:

A ray of light makes an angle of 40∘40^\circ with the surface of a plane mirror. Find the angle of reflection.

Solution:

50∘50^\circ

Explanation:

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