Complete Solutions and Summary of Light: Mirrors and Lenses – Curiosity Class 8 Science Chapter 10 – Summary, Questions, Answers, Extra Questions
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Light: Mirrors and Lenses
Chapter 10: Curiosity — Textbook of Science for Grade 8
Complete Study Guide with Interactive Learning
Chapter Overview
2
Laws of Reflection
Concave
Converging Mirror/Lens
Convex
Diverging Mirror/Lens
3
Types of Mirrors
What You'll Learn
Spherical Mirrors
Understanding concave and convex mirrors and their image formation characteristics.
Laws of Reflection
Exploring the fundamental laws governing light reflection in mirrors.
Lenses
Learning about convex and concave lenses and how they affect light.
Applications
Discovering real-world uses of mirrors and lenses in everyday devices.
Scientific Context
This chapter explores how curved mirrors and lenses manipulate light differently from plane surfaces. It begins with observations of spherical mirrors forming enlarged or diminished images and extends to lenses in devices like magnifying glasses. Activities demonstrate convergence and divergence of light, with historical notes on ancient Indian astronomers using reflective surfaces for stargazing.
Key Highlights
Spherical mirrors and lenses enable diverse applications, from dental tools to solar concentrators. The laws of reflection apply universally, while curved surfaces allow light concentration for practical uses like burning paper or telescopes.
Comprehensive Chapter Summary
1. Introduction to Spherical Mirrors
The chapter starts with Meena's experience at a science center observing distorted images in curved mirrors. Unlike plane mirrors that form same-size erect images, spherical mirrors—parts of hollow spheres—create enlarged or diminished images. Concave mirrors curve inward, convex outward.
2. Characteristics of Images in Spherical Mirrors
Concave Mirrors
Form enlarged erect images close up, inverted when distant. Used in torches, dental mirrors, and telescopes.
Convex Mirrors
Always form diminished erect images, providing wider views. Used in vehicle side mirrors and surveillance.
Image Formation
Size and orientation change with object distance; lateral inversion occurs in all mirrors.
3. Laws of Reflection
Angle Equality
Angle of incidence equals angle of reflection, validated through experiments with plane mirrors.
Planar Alignment
Incident ray, normal, and reflected ray lie in the same plane, demonstrated by bending paper experiments.
Application to Spherical Mirrors
Laws hold, but curvature causes convergence (concave) or divergence (convex) of parallel rays.
4. Light Behavior in Mirrors
Convergence and Applications
Concave mirrors concentrate light, enabling burning paper or solar furnaces; convex diverge for safety views.
5. Introduction to Lenses
Convex and Concave Lenses
Convex lenses converge light, forming enlarged/inverted images; concave diverge, always diminished erect.
Real-World Uses
Lenses in eyeglasses, cameras, microscopes; eye lens adjusts for focus.
6. Light Through Lenses
Convex lenses burn paper like concave mirrors; laws of refraction implied in behavior, with convergence/divergence.
Key Concepts and Definitions
Spherical Mirror
Curved mirror shaped like part of a sphere; concave (inward) or convex (outward).
Laws of Reflection
1. Angle i = angle r; 2. Rays and normal in same plane.
Lens
Transparent material with curved surfaces; convex (thicker middle) or concave (thicker edges).
Convergence
Parallel rays come together after reflection/refraction (concave mirror/convex lens).
Divergence
Parallel rays spread apart (convex mirror/concave lens).
Image Characteristics
Erect/inverted, enlarged/diminished based on distance and type.
Important Facts and Figures
Concave
Used in Telescopes
Convex
Side-View Mirrors
Lateral
Inversion in All
Solar
Concentrators
Magnifying
Glass: Convex Lens
Questions and Answers from Chapter
Short Questions
Q1. The angle made by the incident ray with the normal to the mirror is 40°. What is the angle made by the reflected ray with the mirror?
Answer: 50°.
Q2. What is the angle of reflection when light falls along the normal?
Answer: 0°.
Q3. Which mirror forms an enlarged image?
Answer: Concave mirror.
Q4. Which type of mirror is used in side-view mirrors?
Answer: Convex mirror.
Q5. When the light is incident along the normal on the mirror, what is the angle of incidence?
Answer: 0°.
Q6. Which mirror always forms a diminished erect image?
Answer: Convex mirror.
Q7. What happens to the image in a convex mirror as a woman walks towards it?
Answer: Her erect image keeps increasing in size.
Q8. Which type of lens is a magnifying glass?
Answer: Convex lens.
Q9. What is a concave mirror?
Answer: Spherical mirror with reflecting surface curving inwards.
Q10. Why are convex mirrors preferred for observing traffic?
Answer: They provide a larger view area.
Q11. In Figure 10.27(a), what type of mirror is indicated?
Answer: Plane mirror.
Q12. How does a pencil appear when viewed through water in a tumbler?
Answer: Bent or changed in shape.
Q13. What mirror is used by dentists?
Answer: Concave mirror.
Q14. What lens forms an always diminished erect image?
Answer: Concave lens.
Q15. What is the angle of reflection if angle of incidence is 40°?
Answer: 40°.
Medium Questions
Q1. The angle made by the incident ray with the normal to the mirror is 40°. What is the angle made by the reflected ray with the mirror?
Answer: The angle of incidence is 40°, so the angle of reflection is also 40°. The reflected ray makes an angle of 50° with the mirror since the total from normal to mirror is 90°. (3 marks)
Q2. Draw the reflected ray when light falls along the normal.
Answer: The reflected ray goes back along the same path as the incident ray. Angle of incidence and reflection are both 0°. This follows the laws of reflection. (3 marks)
Q3. Match the images of a sketch pen cap to the correct mirror types.
Answer: Enlarged erect: Concave; Diminished erect: Convex; Same size: Plane. Based on distance-dependent characteristics. (3 marks)
Q4. Match the images of a sketch pen cap to lens or glass types.
Answer: Enlarged: Convex lens; Diminished: Concave lens; Same: Flat glass. Lenses curve light differently. (3 marks)
Q5. When light is incident along the normal, which statement is true?
Answer: Angle of incidence is 0°. Reflection occurs back along the path. No deviation happens. (3 marks)
Q6. Identify mirrors from graph sheet images.
Answer: Straight lines: Plane; Curved inward: Concave; Curved outward: Convex. Based on image distortion. (3 marks)
Q7. What will a woman see walking towards a large convex mirror?
Answer: Her erect image keeps increasing in size. Convex always erect and diminished, but larger closer. No inversion. (3 marks)
Q8. Hold a magnifying glass over text and observe changes.
Answer: Close: Enlarged erect; Far: Inverted. Magnifying glass is convex lens. Image inverts at certain distance. (3 marks)
Q9. Match Column I (types) with Column II (descriptions).
Answer: Concave mirror: Curves inwards; Convex mirror: Erect diminished; Convex lens: Inverted at distance; Concave lens: Diminished. (3 marks)
Q10. Assertion: Convex mirrors preferred for traffic. Reason: Larger view area.
Answer: Both correct, reason explains assertion. Convex diverge for wider field. Safer for vehicles. (3 marks)
Q11. Which statement is true for Figure 10.27?
Answer: (a) Plane mirror; (b) Concave mirror. Based on image size and inversion. Plane same, concave enlarged/inverted. (3 marks)
Q12. How does the pencil appear through water? Explain.
Answer: Bent due to refraction. Light bends at air-water interface. Shape changes at water level. (3 marks)
Q13. What mirror is used in dental instruments?
Answer: Concave for enlarged view. Provides clear inspection. Curved inward focuses light. (3 marks)
Q14. Draw reflected rays for tilted mirror at 20° from normal.
Answer: Reflected at 20° to normal. Follows i = r. Use ruler for accuracy. (3 marks)
Q15. Match images to mirrors in Figure 10.23.
Answer: (i) Plane; (ii) Convex; (iii) Concave. Based on erect/diminished/enlarged. (3 marks)
Long Questions
Q1. The angle made by the incident ray with the normal to the mirror is 40°. What is the angle made by the reflected ray with the mirror?
Answer: According to the first law of reflection, the angle of incidence equals the angle of reflection. Here, the angle of incidence is 40° to the normal, so the angle of reflection is also 40° to the normal. The mirror surface is perpendicular to the normal, forming 90° angles. Thus, the angle between the reflected ray and the mirror is 90° - 40° = 50°. This can be verified by drawing the normal and measuring angles, ensuring the rays lie in the same plane as per the second law.
Q2. Fig. 10.22 shows three different situations where a light ray falls on a mirror. Draw the reflected ray in each case and find the angle of reflection.
Answer: In case (i), light along normal: reflected back, angle 0°. In (ii), tilted mirror but along normal: same, reflected back, angle 0°. In (iii), 20° from normal: reflected at 20° to normal, total deviation 40°. Use protractor for drawing. All follow i = r and planar alignment. This demonstrates laws apply regardless of mirror orientation if incidence is measured to normal.
Q3. In Fig. 10.23, the cap of a sketch pen is placed in front of three types of mirrors. Match each image with the correct mirror.
Answer: Image (i): Plane mirror (same size erect). Image (ii): Convex mirror (diminished erect). Image (iii): Concave mirror (enlarged inverted). Concave forms enlarged close, inverted far; convex always diminished erect for wide view; plane unchanged. Lateral inversion in all. This distinguishes based on curvature effects.
Q4. In Fig. 10.24 the cap of a sketch pen is placed behind a convex lens, a concave lens, and a flat transparent glass piece. Match each image with the correct type.
Answer: Image (i): Flat glass (same size). Image (ii): Convex lens (enlarged/inverted). Image (iii): Concave lens (diminished erect). Convex converges for magnification; concave diverges for reduction; flat no change. Size varies with distance in curved lenses.
Q5. When the light is incident along the normal on the mirror, which of the following statements is true?
Answer: Angle of incidence is 0°. Reflection occurs, ray retraces path. Not 90°, and reflection does take place. This is a special case where i = r = 0, rays in same plane. Experimentally, beam reflects back through slit.
Q6. In Fig. 10.25, identify the mirrors on the basis of the images of the graph sheet formed in the mirrors.
Answer: Straight grid: Plane mirror. Curved outward grid: Convex (diminished wide). Curved inward grid: Concave (enlarged focused). Graph distortion reveals curvature; plane no change, convex spreads, concave concentrates.
Q7. In a museum, a woman walks towards a large convex mirror. She will see that:
Answer: Her erect image keeps increasing in size. Convex always erect diminished, but closer objects appear larger though still smaller than actual. No inversion, unlike concave. This provides wider safety view.
Q8. Hold a magnifying glass over text and identify the distance where you can see the text bigger than they are written. Now move it away from the text. What do you notice? Which type of lens is a magnifying glass?
Answer: Close: Text enlarged erect. Away: Inverted, initially enlarged then diminished. Magnifying glass is convex lens, converging light. Image inverts beyond focal point, useful for reading but changes with distance like in microscopes.
Q9. Match the entries in Column I with those in Column II.
Answer: (i) Concave mirror: (a) Curves inwards. (ii) Convex mirror: (b) Always erect diminished. (iii) Convex lens: (c) Inverted at distance. (iv) Concave lens: (d) Always diminished. Matches based on definitions and image properties.
Q10. Assertion: Convex mirrors are preferred for observing the traffic behind us. Reason: Convex mirrors provide a significantly larger view area than plane mirrors.
Answer: Both correct, reason explains assertion. Convex diverge rays for wider field, erect diminished images. Safer than plane (limited view) or concave (inverted close). Used in vehicles with warning.
Q11. In Fig. 10.27, note that O stands for object, M for mirror, and I for image. Which of the following statements is true?
Answer: Figure (a) plane mirror (same size); (b) concave mirror (enlarged inverted). Not convex (diminished) or other combos. Based on ray diagrams and distance.
Q12. Place a pencil behind a transparent glass tumbler. Now fill the tumbler halfway with water. How does the pencil appear when viewed through the water? Explain why its shape appears changed.
Answer: Pencil appears bent at water level. Due to refraction: light bends entering water from air, different speeds. Part in water shifts apparent position. Demonstrates lens-like curved surface effect.
Q13. Visit a nearby hospital or the clinic of an ENT specialist, or a dentist, with your teacher or parents. Request the doctor to show you the mirrors used for examining ear, nose, throat, and teeth. Identify the kind of mirror used in these instruments.
Answer: Concave mirrors used for enlarged focused view. Curves inward to converge light, illuminate and magnify. Essential for detailed inspection in small areas like mouth or ear.
Q14. Harnessing sunlight is key to solving future energy challenges. In devices like solar cookers, mirrors are used to converge sunlight and generate heat. Think of a design for a solar cooker for your school or home and prepare a detailed proposal for it including the budget required.
Answer: Design: Parabolic concave mirror to focus sunrays on pot. Materials: Mirror sheets, stand, pot holder. Budget: Rs 5000 (mirrors Rs 3000, frame Rs 1500, misc Rs 500). Saves energy, uses convergence for cooking.
Q15. Use online tools or animation to do virtual experiments with spherical mirrors and lenses. Move objects in the simulation and observe how the image changes.
Answer: In simulations, concave mirror: close enlarged erect, far inverted diminished. Convex: always diminished erect. Convex lens: similar to concave mirror. Concave lens: diminished erect. Confirms distance effects on size/orientation.
Interactive Knowledge Quiz
Test your understanding of Light: Mirrors and Lenses
Quick Revision Notes
Spherical Mirrors
- Concave: Converges
- Convex: Diverges
- Images vary with distance
Laws of Reflection
- i = r
- Same plane
- Apply to all mirrors
Lenses
- Convex: Converges
- Concave: Diverges
- Used in optics
Applications
- Torches: Concave
- Magnifying: Convex lens
- Solar: Concave
Exam Strategy Tips
- Draw ray diagrams
- Understand convergence
- Match types to uses
- Practice activities
- Explain laws
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