Light - Reflection and Refraction — Question 9
Back to all questionsQuestion 9
One-half of a convex lens is covered with black paper. Will this lens produce a complete image of the object? Verify your answer experimentally. Explain your observations.
Yes, it will produce a complete image of the object. However, the intensity or brightness of the image will be reduced.
Experiment : To show that a half-covered convex lens can produce a complete image of the object.
Apparatus needed : A convex lens, a candle, and two black sheets of paper.
Procedure :
Place a convex lens on the table. Light a candle with a match stick and place it near one side of the lens on the table (turn off the lights of the room in order to get dark).
Take the sheet of paper and place it on the other side of the lens as the candle.
Now focus the image formed on the sheet of paper by moving the sheet only.
Observe the image carefully.
Mark the positions of candle, lens and screen.
Now, turn off the light. Take the convex lens and cover half of it with the second sheet of black paper.
Mount the lens on the table in the original position and repeat the above experiment.
Observations : A complete image of the object is obtained in both the cases. However, the image formed with half of the convex lens covered is less bright.
Conclusion : The half covered lens will also produce a complete image of the object. However, the image formed will be less bright.
Chapter 9: Light: Reflection and Refraction — Quick Revision Guide
Introduction
Light behaves predictably when it reflects off mirrors and refracts through lenses. This chapter covers the laws of reflection and refraction, image formation by spherical mirrors and lenses, and numerical problem-solving using mirror and lens formulae.
Key Points at a Glance
- Laws of reflection: angle of incidence = angle of reflection; incident ray, reflected ray, normal in same plane
- Concave mirror: converging; image depends on object position (6 cases from ∞ to between P and F)
- Convex mirror: always virtual, erect, diminished image; used as rear-view mirror (wider field of view)
- Mirror formula: 1/v + 1/u = 1/f; magnification m = −v/u = h'/h
- Sign convention: object on left → u negative; concave f negative; convex f positive
- Refraction: bending of light at interface; Snell's law: sin i / sin r = n21; denser medium bends towards normal
- Convex lens: converging; 6 image positions; concave lens: always virtual, erect, diminished
- Lens formula: 1/v − 1/u = 1/f; m = v/u; Power P = 1/f(m); unit: dioptre (D)
- Convex lens: +f, +P; Concave lens: −f, −P; combination: P = P1 + P2
Real-World Connections
Concave mirrors in solar cookers and headlights; convex mirrors for vehicle rear-view; magnifying glass is a convex lens; spectacles correct vision defects; cameras and projectors use lenses.
Quick Self-Test (5 Questions)
- What is the most important concept you learned from this chapter?
- Can you write three key equations/formulae from this chapter from memory?
- Draw a labelled diagram relevant to this chapter without looking at your notes.
- Explain one real-world application of a concept from this chapter.
- What is one common mistake students make in this chapter, and how can you avoid it?
Further Study
- NCERT Textbook Chapter 9
- NCERT Exemplar Problems
- Bright Tutorials Detailed Notes: ch09-light-reflection-refraction.html
- Bright Tutorials Practice Questions: ch09-light-reflection-refraction.html
- Previous Year CBSE Board Papers