Light: Mirrors and LensesClass 8 Science Notes

What Are Spherical Mirrors?

Imagine you're at a science center like Meena, looking at those funny curved mirrors. You see yourself larger than life, or upside down, or tiny! These aren't your everyday flat mirrors. They're spherical mirrors, and they can change how you look based on their shape.

Spherical mirrors are shaped like a part of a hollow glass sphere. It's important to note that they aren't made by slicing a glass sphere. Instead, they are created by grinding and polishing a flat piece of glass into a curved surface. These mirrors have reflecting surfaces that can be curved inwards or outwards, leading to different types of images.

• Concave Mirror: A spherical mirror that curves inwards.
• Convex Mirror: A spherical mirror that curves outwards.

The non-reflecting surface of the mirror is often shaded in diagrams. If a reflective coating, like a thin layer of aluminum, is applied on the outer curved surface, it forms a concave mirror. If the coating is applied on the inner curved surface, it forms a convex mirror.

How Can We Distinguish Between Concave and Convex Mirrors?

You can distinguish between concave and convex mirrors by simply looking at them from the side. Place the mirrors on a table with the reflecting surfaces facing upwards and view them at eye level. If the reflecting surface curves inwards, it's a concave mirror. If it curves outwards, it's a convex mirror.

What Are the Characteristics of Images Formed by Spherical Mirrors?

Spherical mirrors create images that are different from those made by plane mirrors. The image characteristics depend on the type of spherical mirror and the distance of the object from the mirror.

• Plane Mirror: Always forms an erect (upright) image of the same size as the object.
• Concave Mirror:
  • When an object is close to the mirror, the image is erect and enlarged (larger than the object).
  • When an object is farther away, the image becomes inverted (upside down).
  • The image size initially increases and then decreases as the object moves away.
• Convex Mirror:
  • Always forms an erect and diminished (smaller than the object) image.
  • The size of the image decreases slightly as the object moves away.
• Lateral Inversion: This is seen in all three types of mirrors (plane, concave and convex).

Concave and convex mirrors are used in many everyday applications:

• Concave Mirrors:
  • Reflectors in torches, headlights of cars and scooters.
  • Dental mirrors to provide an enlarged view of teeth.
  • Reflecting telescopes.
• Convex Mirrors:
  • Side-view mirrors in vehicles, providing an erect and smaller image of the traffic behind, along with a wider field of view.
  • Road safety mirrors at intersections and sharp bends to prevent collisions.
  • Surveillance mirrors in stores to monitor a large area and deter thefts.

What Are the Laws of Reflection?

When light bounces off a surface, it follows specific rules known as the laws of reflection.

1. Angle of Incidence = Angle of Reflection: The angle at which light strikes a surface (angle of incidence) is equal to the angle at which it bounces off (angle of reflection).
2. The Incident Ray, Normal, and Reflected Ray Lie in the Same Plane: The incoming light ray (incident ray), the line perpendicular to the surface at the point of contact (normal), and the outgoing light ray (reflected ray) all lie in the same plane.

These laws are valid for all types of mirrors: plane, concave, and convex.

To understand these laws better, consider the following terms:

• Incident Ray: The ray of light that falls on the mirror.
• Reflected Ray: The ray of light that comes back from the mirror.
• Normal: A line making an angle of 90° to the mirror at the point where the incident ray strikes the mirror.
• Angle of Incidence (i): The angle between the normal and the incident ray.
• Angle of Reflection (r): The angle between the normal and the reflected ray.

When multiple parallel beams of light fall upon different types of mirrors:

• Plane Mirror: The reflected beams are also parallel.
• Concave Mirror: The reflected beams converge (get closer together).
• Convex Mirror: The reflected beams diverge (spread out).

Concave mirrors can concentrate sunlight into a small area. This is why they are used in solar concentrators, which heat a liquid to produce steam for electricity or heat.

What Is a Lens?

A lens is a piece of transparent material, usually made of glass or plastic, which has curved surfaces. Unlike mirrors, lenses allow light to pass through them. Lenses can be convex or concave.

• Convex Lens: Thicker at the middle compared to the edges.
• Concave Lens: Thicker at the edges compared to the middle.

When an object is placed behind a convex lens at a small distance and seen through the lens, the object appears erect and enlarged in size. As the distance between the object and the convex lens increases, the object appears inverted, initially enlarged and then diminishing in size. An object placed behind a concave lens and seen through the lens, always appears erect and diminished in size. Its size changes, as its distance from the lens increases.

Lenses can also converge or diverge light:

• Convex Lens: Converges the light falling on it and is also called a converging lens.
• Concave Lens: Diverges the light falling on it and is also called a diverging lens.

Convex lenses can be used to focus sunlight and burn paper, similar to how concave mirrors work.

Lenses are used in eyeglasses, cameras, telescopes, and microscopes. Even the human eye has a convex lens inside it!