Class 8 Science NCERT Notes – Chapter 10: Light: Mirrors and Lenses (PDF, MindMap, Q&A, Quizzes)

Study Guide: Chapter 10 – Light: Mirrors and Lenses

I. Detailed Study Notes

1. Introduction to Spherical Mirrors

Unlike plane mirrors which form erect images of the same size as the object, spherical mirrors are curved and produce varied images. A common metallic spoon serves as a good initial example: the inner, inwardly-curved surface shows an inverted image, while the outer, outwardly-bulging surface shows an erect but smaller image. Spherical mirrors are a type of curved mirror shaped as if they were part of a hollow glass sphere.

2. Types of Spherical Mirrors

There are two primary types of spherical mirrors, distinguished by the direction of their reflective curve.

• Concave Mirror: This mirror has a reflecting surface that curves inwards. When represented in a diagram, the non-reflecting (coated) surface is shaded on the outer curve. They are formed by applying a reflective coating to the outer curved surface of a piece of ground and polished glass.

• Convex Mirror: This mirror has a reflecting surface that curves outwards, or bulges out. In diagrams, the inner curve is shaded to represent the non-reflecting surface. They are formed by applying a reflective coating to the inner curved surface of the glass piece.

3. Image Characteristics of Spherical Mirrors

The images formed by spherical mirrors change based on the type of mirror and the distance of the object from the mirror.

Concave Mirror:

• Object is close: The image is erect and enlarged (larger than the object).
• Object moves farther away: The image becomes inverted. As it moves away, the inverted image is initially enlarged and then gets progressively smaller.

Convex Mirror:

• The image is always erect and diminished (smaller than the object).
• As the object moves away from the mirror, the image size decreases slightly but remains erect.
• Lateral Inversion: This phenomenon, where the left and right sides of an image are swapped, is observed in all three types of mirrors: plane, concave, and convex.

4. Applications of Spherical Mirrors

The unique properties of concave and convex mirrors lead to their use in various practical applications.

Concave Mirrors:

• Reflectors: Used in torches and the headlights of cars and scooters due to their ability to converge light.
• Dentistry: Dentists use them as dental mirrors to get an enlarged view of teeth when held close.
• Telescopes: Modern reflecting telescopes use a large concave mirror as their main mirror.
• Solar Concentrators: Devices like solar furnaces use concave mirrors to concentrate sunlight into a small area, generating enough heat to melt steel or produce steam for electricity.

Convex Mirrors:

• Vehicle Side-View Mirrors: These mirrors are convex because they provide an erect, diminished image and a much wider field of view of the road and traffic behind. A warning, “Objects in mirror are closer than they appear,” is often included because the diminished image makes objects seem farther away than they are.
• Road Safety: Installed at sharp bends and intersections to give drivers visibility of the other side and prevent collisions.
• Surveillance: Used in large stores to monitor a wide area and deter theft.

5. The Laws of Reflection

The formation of images in all mirrors is governed by two fundamental laws of reflection. These laws apply to plane, concave, and convex mirrors alike.

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

Key Terms for Reflection:

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

When a light ray is incident along the normal, both the angle of incidence and the angle of reflection are zero.

6. Lenses: Transparent Curved Surfaces

A lens is a piece of transparent material, such as glass or plastic, with curved surfaces that allows light to pass through it. Unlike mirrors, we see objects through a lens.

7. Types of Lenses

Similar to mirrors, lenses are categorized as convex or concave based on their shape.

• Convex Lens: Thicker at the middle and thinner at the edges. A magnifying glass is a common example of a convex lens.

• Concave Lens: Thicker at the edges and thinner at the middle.

8. Image Characteristics of Lenses

The appearance of an object viewed through a lens depends on the lens type and the object’s distance.

Convex Lens:

• Object is close: The object appears erect and enlarged.
• Object moves farther away: The object appears inverted. The inverted image is initially enlarged and then diminishes in size as the distance increases.

Concave Lens:

• The object always appears erect and diminished in size, regardless of its distance from the lens.

9. Light Interaction with Mirrors and Lenses

The curvature of mirrors and lenses causes them to interact with parallel beams of light in distinct ways.

Device	Action on Parallel Light Beams	Nickname
Concave Mirror	Converges (brings rays closer together)	Converging Mirror
Convex Mirror	Diverges (spreads rays apart)	Diverging Mirror
Convex Lens	Converges (brings rays closer together)	Converging Lens
Concave Lens	Diverges (spreads rays apart)	Diverging Lens

This converging property allows both a concave mirror and a convex lens to focus sunlight onto a single point, creating enough heat to burn paper.

10. Applications of Lenses

Lenses are used in a vast array of optical instruments.

• Eyeglasses: Correct vision issues.
• Cameras: Including smartphone cameras, use lenses to focus images.
• Telescopes & Microscopes: Use lenses to magnify distant or tiny objects.
• Human Eye: Contains a natural convex lens that can change its shape to focus on objects at different distances.

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II. Short-Answer Quiz

Answer each question in 2-3 sentences.

1. Describe the primary physical difference between a concave mirror and a convex mirror.

2. What are the two defining characteristics of an image formed by a convex mirror?

3. State the first law of reflection concerning the angles of incidence and reflection.

4. Explain the phenomenon of convergence as it relates to a concave mirror.

5. Why are convex mirrors the preferred choice for vehicle side-view mirrors?

6. How does the image seen through a convex lens change as an object is moved from very close to farther away?

7. State the second law of reflection regarding the plane of incident and reflected rays.

8. What is the key difference in shape between a convex lens and a concave lens?

9. List two common applications of concave mirrors.

10. Define the terms “incident ray,” “reflected ray,” and “normal” in the context of light reflection.

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III. Answer Key

1. A concave mirror has a reflecting surface that curves inwards, like the inside of a spoon. A convex mirror has a reflecting surface that curves outwards, or bulges out, like the back of a spoon.

2. The image formed by a convex mirror is always erect (upright) and diminished (smaller than the object). The size of the image decreases slightly as the object moves away from the mirror.

3. The first law of reflection states that the angle of incidence is equal to the angle of reflection. This means the angle at which a light ray strikes a mirror’s surface relative to the normal is the same as the angle at which it reflects off the surface.

4. Convergence is the process where parallel beams of light that strike a concave mirror are reflected inwards to meet at a single point or area. This property allows the mirror to concentrate light and heat.

5. Convex mirrors are used for side-view mirrors because their outwardly curved surface provides a much wider field of view of the road behind the vehicle. Although the images are diminished, they are always erect, which is crucial for driver awareness.

6. When an object is close to a convex lens, it appears erect and enlarged. As the object is moved farther away, the image becomes inverted, and its size changes from enlarged to diminished.

7. The second law of reflection states that the incident ray, the normal to the mirror at the point of incidence, and the reflected ray all lie in the same plane. This means the reflection occurs on a flat, two-dimensional plane.

8. A convex lens is thicker in the middle and thinner at the edges. Conversely, a concave lens is thinner in the middle and thicker at the edges.

9. Two common applications of concave mirrors are as reflectors in torches and vehicle headlights to create a focused beam of light, and by dentists to get an enlarged view of teeth.

10. The “incident ray” is the ray of light that falls on the mirror. The “reflected ray” is the ray of light that comes back from the mirror. The “normal” is an imaginary line drawn perpendicular (at 90°) to the mirror’s surface at the point of incidence.

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IV. Essay Questions

Construct detailed responses to the following prompts, drawing upon the information from the study notes.

1. Compare and contrast the properties and behaviors of a concave mirror with those of a convex lens. Discuss their shapes, their effects on parallel light beams, and the types of images they can form at various object distances.

2. Elaborate on the two laws of reflection. Explain how these fundamental principles apply universally to plane mirrors, concave mirrors, and convex mirrors, despite the different images each type of mirror produces.

3. Provide a comprehensive overview of the real-world applications of spherical mirrors. For each application (e.g., dental mirrors, vehicle side-view mirrors, solar concentrators), explain precisely why the specific properties of either a concave or convex mirror make it uniquely suited for that purpose.

4. Describe a series of experiments using a comb and a torch to produce multiple parallel beams of light. Explain the observed results when these beams are directed at a plane mirror, a concave mirror, and a convex mirror, respectively.

5. Imagine you are given three unlabeled mirrors. Explain, step-by-step, how you could identify which mirror is plane, which is concave, and which is convex, using only a small object (like a pencil) and your observations of the image it forms at different distances.

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V. Glossary of Key Terms

• 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.
• Concave lens: A lens which is thicker at the edges as compared to the middle. It is a diverging lens.
• Concave mirror: A spherical mirror which has a reflecting surface that curves inwards. It is a converging mirror.
• Converge: To bring multiple beams of light closer together, concentrating them at a point or in a small area.
• Converging lens: An alternate name for a convex lens, due to its property of converging light rays.
• Convex lens: A lens which is thicker at the middle as compared to the edges. It is a converging lens.
• Convex mirror: A spherical mirror which has a reflecting surface that curves outwards. It is a diverging mirror.
• Diminished: An image that is smaller in size than the object.
• Diverge: To spread multiple beams of light apart.
• Diverging lens: An alternate name for a concave lens, due to its property of diverging light rays.
• Enlarged: An image that is larger in size than the object.
• Erect: An image that is upright and oriented in the same direction as the object.
• Incident ray: The ray of light that falls on a mirror or lens.
• Inverted: An image that is upside down relative to the object.
• Lateral inversion: The effect seen in mirrors where the left and right sides of an object appear to be swapped in its image.
• Lens: A piece of transparent material, usually made of glass or plastic, which has curved surfaces.
• Magnifying glass: A common example of a convex lens used to make small print appear bigger.
• Normal: A line drawn at a 90° angle (perpendicular) to a reflecting surface at the point of incidence.
• Plane mirror: A mirror with a flat reflecting surface that always forms an erect image of the same size as the object.
• Ray: A straight line with an arrow used to represent the path along which light travels.
• Reflected ray: The ray of light that comes back from a mirror after reflection.
• Solar concentrators: Devices that use mirrors (or lenses) to concentrate sunlight into a small area to generate high temperatures.
• Spherical mirrors: A type of curved mirror shaped like a part of a hollow sphere, including both concave and convex types.