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Light Energy - Laws of Reflection

Grade 7ICSE

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

🔑Concepts

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Reflection of Light: The phenomenon of bouncing back of light into the same medium after striking a polished surface is known as the reflection of light.

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

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Reflected Ray: The ray of light which is sent back by the reflecting surface is called the reflected ray.

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Normal: It is an imaginary line drawn perpendicular (90∘90^\circ) to the reflecting surface at the point of incidence.

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

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Angle of Reflection (∠r\angle r): The angle formed 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, expressed as ∠i=∠r\angle i = \angle r.

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

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Diffused (Irregular) Reflection: Occurs when light falls on a rough or uneven surface, causing reflected rays to scatter in different directions.

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Lateral Inversion: The phenomenon in which the left side of an object appears as the right side of the image and vice versa in a plane mirror.

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Characteristics of Image in Plane Mirror: The image is virtual, erect, of the same size as the object, at the same distance behind the mirror as the object is in front of it, and laterally inverted.

📐Formulae

∠i=∠r\angle i = \angle r

∠i+Glancing Angle(θg)=90∘\angle i + \text{Glancing Angle} (\theta_g) = 90^\circ

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

💡Examples

Problem 1:

A ray of light strikes a plane mirror such that the angle between the incident ray and the mirror surface is 35∘35^\circ. Calculate the angle of reflection.

Solution:

  1. The angle between the mirror and the normal is 90∘90^\circ.
  2. The angle of incidence ∠i=90∘−35∘=55∘\angle i = 90^\circ - 35^\circ = 55^\circ.
  3. According to the Law of Reflection, ∠i=∠r\angle i = \angle r.
  4. Therefore, ∠r=55∘\angle r = 55^\circ.

Explanation:

The glancing angle is given as 35∘35^\circ. Since the normal is perpendicular to the surface, the angle of incidence is the complement of the glancing angle.

Problem 2:

If the angle between the incident ray and the reflected ray is 80∘80^\circ, find the angle of incidence.

Solution:

  1. Total angle = ∠i+∠r=80∘\angle i + \angle r = 80^\circ.
  2. Since ∠i=∠r\angle i = \angle r (Law of Reflection), we can write 2∠i=80∘2\angle i = 80^\circ.
  3. ∠i=80∘2=40∘\angle i = \frac{80^\circ}{2} = 40^\circ.

Explanation:

The total angle between the two rays is the sum of the angle of incidence and the angle of reflection. Because they are equal, the angle of incidence is exactly half of the total angle.

Problem 3:

A ray of light is incident normally on a plane mirror. What is the angle of reflection?

Solution:

  1. Normal incidence means the incident ray coincides with the normal.
  2. Therefore, the angle of incidence ∠i=0∘\angle i = 0^\circ.
  3. According to the Law of Reflection, ∠r=∠i\angle r = \angle i.
  4. Thus, ∠r=0∘\angle r = 0^\circ.

Explanation:

When light hits a mirror 'normally' (perpendicularly), it does not deviate from the normal, meaning the angle it makes with the normal is zero. Consequently, it reflects back along the same path.

Laws of Reflection - Revision Notes & Key Formulas | ICSE Class 7 Science