krit.club logo

Light - Laws of Reflection and Regular/Diffused Reflection

Grade 8CBSE

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

🔑Concepts

•

Reflection of Light: The phenomenon where a light ray, upon striking a surface, bounces back into the same medium.

•

Incident Ray: The ray of light that falls on the reflecting surface. It is denoted as the incoming path.

•

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

•

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

•

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

•

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

•

First Law of Reflection: The angle of incidence is always equal to the angle of reflection. This is expressed as ∠i=∠r\angle i = \angle r.

•

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

•

Regular Reflection: Reflection from a smooth and polished surface (like a mirror) where all parallel incident rays are reflected as parallel rays. This produces a clear image.

•

Diffused (Irregular) Reflection: Reflection from a rough or uneven surface where parallel incident rays are reflected in different directions. Note: The laws of reflection still hold at every point; the 'diffusion' is caused by surface irregularities, not a failure of the law.

📐Formulae

∠i=∠r\angle i = \angle r

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

💡Examples

Problem 1:

A light ray strikes a plane mirror such that the angle between the mirror and the incident ray is 40∘40^\circ. Find the angle of reflection.

Solution:

The angle of reflection is 50∘50^\circ.

Explanation:

The normal is perpendicular to the mirror (90∘90^\circ). The angle of incidence ∠i\angle i is the angle between the incident ray and the normal. Thus, ∠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 2:

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

Solution:

45∘45^\circ

Explanation:

The total angle between the incident ray and the reflected ray is the sum ∠i+∠r\angle i + \angle r. Since ∠i=∠r\angle i = \angle r, we can write this as 2∠i=80∘2\angle i = 80^\circ. Therefore, ∠i=80∘2=40∘\angle i = \frac{80^\circ}{2} = 40^\circ.

Problem 3:

What happens to the reflected rays when parallel light hits a piece of crumpled aluminum foil?

Solution:

Diffused reflection occurs.

Explanation:

Because the surface of crumpled aluminum foil is rough and irregular, the parallel incident rays strike different parts of the surface at different angles. While ∠i=∠r\angle i = \angle r at each individual point, the normals are not parallel, causing the reflected rays to scatter in various directions.