Grade 8 Chapter 16: Reflection of Light

Introduction

Light is a form of energy that enables us to see the world around us. It travels in straight lines and interacts with surfaces in various ways. One of the most common and important phenomena involving light is reflection, which is how mirrors work and why we see objects. This chapter will delve into the laws governing the reflection of light, how different types of mirrors form images, and their practical applications in our daily lives.

Light: Properties and Reflection

• Light: A form of electromagnetic radiation that travels in straight lines (rectilinear propagation). It does not require a medium to travel (can travel through vacuum).
• Reflection of Light: The phenomenon of bouncing back of light rays from a surface after striking it.

Types of Reflection

• Regular Reflection (Specular Reflection): Occurs when light rays strike a smooth, polished surface (like a plane mirror). Parallel incident rays reflect as parallel reflected rays. Forms clear, sharp images.
• Diffused Reflection (Irregular Reflection): Occurs when light rays strike a rough or irregular surface (like a wall, paper). Parallel incident rays reflect in different directions. Does not form clear images, but allows us to see objects from various angles.

Laws of Reflection

Reflection of light follows two fundamental laws:

1. First Law of Reflection: The incident ray, the reflected ray, and the normal to the surface at the point of incidence all lie in the same plane.
2. Second Law of Reflection: The angle of incidence (∠i) is always equal to the angle of reflection (∠r).
   ∠i = ∠r

Image Formation by a Plane Mirror

A plane mirror is a flat, smooth, and highly polished reflecting surface.

• Characteristics of Image formed by a Plane Mirror:
  • Virtual: The image cannot be obtained on a screen. It appears to be formed behind the mirror.
  • Erect (Upright): The image is upright, not inverted.
  • Laterally Inverted: The left side of the object appears as the right side of the image, and vice-versa.
  • Same Size: The size of the image is the same as the size of the object.
  • Same Distance: The distance of the image behind the mirror is equal to the distance of the object in front of the mirror.

Spherical Mirrors

Spherical mirrors are mirrors whose reflecting surface is a part of a hollow sphere. They are of two types:

1. Concave Mirror

• Definition: A spherical mirror whose inner (concave) surface is the reflecting surface. It converges parallel rays of light after reflection.
• Appearance: Reflecting surface bulges inwards, like the inner surface of a spoon.
• Nature: Converging mirror.

2. Convex Mirror

• Definition: A spherical mirror whose outer (convex) surface is the reflecting surface. It diverges parallel rays of light after reflection.
• Appearance: Reflecting surface bulges outwards, like the outer surface of a spoon.
• Nature: Diverging mirror.

Terminology Related to Spherical Mirrors

• Pole (P): The center of the reflecting surface of a spherical mirror.
• Center of Curvature (C): The center of the hollow sphere of which the mirror is a part.
• Radius of Curvature (R): The radius of the hollow sphere of which the mirror is a part. It is the distance between the Pole and the Center of Curvature (R = PC).
• Principal Axis: The straight line passing through the Pole and the Center of Curvature.
• Principal Focus (F):
  • Concave Mirror: A point on the principal axis where all parallel rays of light converge after reflection.
  • Convex Mirror: A point on the principal axis from which parallel rays of light appear to diverge after reflection.
• Focal Length (f): The distance between the Pole and the Principal Focus.
  f = R/2 (Focal length is half of the radius of curvature).

Rules for Drawing Ray Diagrams for Spherical Mirrors

To determine the position and nature of the image formed by spherical mirrors, we use ray diagrams based on these rules:

1. A ray parallel to the principal axis, after reflection, passes through the principal focus (F) in a concave mirror or appears to diverge from the principal focus (F) in a convex mirror.
2. A ray passing through the principal focus (F) of a concave mirror or directed towards the principal focus (F) of a convex mirror, after reflection, becomes parallel to the principal axis.
3. A ray passing through the center of curvature (C) of a concave mirror or directed towards the center of curvature (C) of a convex mirror, after reflection, is reflected back along the same path.
4. A ray incident obliquely to the principal axis, towards the pole (P) of the mirror, is reflected obliquely, making equal angles with the principal axis (following the laws of reflection).

Image Formation by Concave Mirror

The nature, position, and size of the image formed by a concave mirror depend on the position of the object:

• Object at Infinity: Image at F, real, inverted, highly diminished (point size).
• Object Beyond C: Image between F and C, real, inverted, diminished.
• Object at C: Image at C, real, inverted, same size as object.
• Object Between C and F: Image beyond C, real, inverted, magnified.
• Object at F: Image at infinity, real, inverted, highly magnified.
• Object Between F and P: Image behind the mirror, virtual, erect, magnified. (This is the only case where a concave mirror forms a virtual image).

Image Formation by Convex Mirror

A convex mirror always forms a virtual, erect, and diminished image, regardless of the object's position.

• Object at Infinity: Image at F (behind the mirror), virtual, erect, highly diminished (point size).
• Object Between Infinity and P: Image between P and F (behind the mirror), virtual, erect, diminished.
• Nature: Always forms virtual, erect, and diminished images.

Uses of Mirrors

Uses of Concave Mirrors

• Shaving mirrors: To see a magnified, erect image of the face.
• Dentists' mirrors: To see magnified images of teeth.
• Headlights of cars/scooters, searchlights, torchlights: To get a powerful parallel beam of light (light source placed at focus).
• Solar furnaces/concentrators: To concentrate sunlight at the focus to generate heat.

Uses of Convex Mirrors

• Rear-view mirrors in vehicles: To see traffic behind. They provide a wider field of view and always form erect, diminished images.
• Shop security mirrors: To monitor a large area of the shop.
• Street light reflectors: To spread light over a wider area.

Summary

• Light: Travels in straight lines, reflects off surfaces.
• Reflection: Bouncing back of light. Types: Regular (smooth surface, clear image), Diffused (rough surface, no clear image).
• Laws of Reflection: Incident ray, reflected ray, normal are in same plane; Angle of incidence = Angle of reflection (∠i = ∠r).
• Plane Mirror: Forms virtual, erect, laterally inverted, same size, same distance image.
• Spherical Mirrors:
  • Concave Mirror: Converging, inner surface reflecting. Forms real/virtual, inverted/erect, magnified/diminished images depending on object position. Used in shaving mirrors, headlights.
  • Convex Mirror: Diverging, outer surface reflecting. Always forms virtual, erect, diminished images. Used as rear-view mirrors, security mirrors.
• Key terms: Pole (P), Center of Curvature (C), Radius of Curvature (R), Principal Axis, Principal Focus (F), Focal Length (f = R/2).
• Ray diagrams help in understanding image formation.

References

1. Maharashtra State Board Science and Technology Standard Eight Textbook (Specific Edition/Year) - Chapter 16: Reflection of Light.
2. Maharashtra State Board 8th Standard Science Syllabus.
3. Balbharati Science and Technology Textbook.