To start a fire by concentrating sunlight with a converging lens, you must hold

Question

To start a fire by concentrating sunlight with a converging lens, you must hold the lens _____ one focal length above the kindling. a. just over b. exactly c. less than In the projector in the back of our classroom, the liquid crystal panel that produces the image is placed _____ one focal length from the lens and is... a. just over b. exactly c. less than oriented ______ a. right-side up. b. upside down. A converging lens held 2.0 m (200 cm) from a window forms an image on a wall 22.2 cm past the lens. The focal length of the lens must equal a. 9.0 cm. b. 20 cm. c. 25 cm. d. 44.4 cm.

Explanation / Answer

27) exactly we have to place the lens at its focal length then all the sunlight focusses at its focal length leads to more concentration of light and fire will catch so answer is exactly.

28) just over is the correct answer

29) right side up.

30) 1/u+1/v=1/f here u=200cm and v=22.2cm then the focal length is 20cm

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