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Light - Reflection and the law of reflection

Grade 5Cambridge (IGCSE)

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

🔑Concepts

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Light travels in straight lines called rays.

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Reflection occurs when light hits a surface and bounces off it.

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The 'Normal' is an imaginary line drawn at a right angle (90∘90^\circ) to the surface where the light ray hits.

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The 'Angle of Incidence' (∠i\angle i) is the angle between the incident ray and the normal.

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The 'Angle of Reflection' (∠r\angle r) is the angle between the reflected ray and the normal.

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Smooth, shiny surfaces like mirrors produce clear reflections (specular reflection), whereas rough surfaces scatter light in many directions (diffuse reflection).

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In a plane mirror, the image is the same size as the object and appears to be the same distance behind the mirror as the object is in front of it.

📐Formulae

∠i=∠r\angle i = \angle r

Angle of Incidence=Angle of Reflection\text{Angle of Incidence} = \text{Angle of Reflection}

💡Examples

Problem 1:

A ray of light strikes a plane mirror at an angle of 35∘35^\circ to the normal. What is the angle of reflection?

Solution:

The angle of reflection is 35∘35^\circ.

Explanation:

According to the Law of Reflection, the angle of incidence (∠i\angle i) is always equal to the angle of reflection (∠r\angle r). Since ∠i=35∘\angle i = 35^\circ, then ∠r=35∘\angle r = 35^\circ.

Problem 2:

If the total angle between the incident ray and the reflected ray is 100∘100^\circ, calculate the angle of incidence.

Solution:

∠i=50∘\angle i = 50^\circ

Explanation:

The total angle is the sum of the angle of incidence and the angle of reflection (∠i+∠r=100∘\angle i + \angle r = 100^\circ). Since ∠i=∠r\angle i = \angle r, we can divide the total angle by 22. Therefore, 100∘÷2=50∘100^\circ \div 2 = 50^\circ.

Problem 3:

A light ray hits a mirror such that the angle between the ray and the mirror surface is 30∘30^\circ. What is the angle of incidence?

Solution:

∠i=60∘\angle i = 60^\circ

Explanation:

The normal is at 90∘90^\circ to the mirror surface. To find the angle of incidence (the angle between the ray and the normal), we subtract the angle with the surface from 90∘90^\circ: 90∘−30∘=60∘90^\circ - 30^\circ = 60^\circ.