Question

You can buy a pair of 3.5 Diopter reading glasses off the rack at the local pharmacy. What is the focal length of these glasses in centimeters ?
2.86×10¹  cm

Computer's answer now shown above. You are correct. Your receipt no. is 163-8554

Previous Tries

The prescription for your corrective lenses is -6.5 D. What will their focal length be?
cm

You are trying to burn a leaf using a pair of you grandmother's old reading glasses. If the reading glasses are rated with a refractive power of 4.25 D, how far must you hold the glasses from the leaf in order to burn through the leaf as quickly as possible?
cm

Answer 1

Part 1.

power, P = 3.5D ( positive power means the lens are converging lens or convex lens)

so focal length,f = 1/P in meter = 1/3.5 =0.2857 m = 28.57 cm

answer: 28.57 cm

Part2.

Power, P = -6.5 D (negative power means the lesns are diverging or concave lens)

Focal length, f = 1/P = 1/(-6.5) in meter = - 0.1538 m = - 15.38 cm

Answer: -15.38 cm

Part 3.

focal length of grandmother's lens = f = 1/ P =1/4.25 meter = 0.2352 m = 23.52 cm  

in order to burn the leaf quickly we have to place the leaf at the focal length of the glass .

so you must hold the glass at 23.53 cm above the leaf in order to burn through the leaf as quickly as possible.

Answer: 23.52 cm

thank you.