Question

The sun is 150,000,000 km from earth; its diameter is 1,400,000 km. A student uses a 5.6-cm-diameter lens with f = 10 cm to cast an image of the sun on a piece of paper.Where should the paper be placed relative to the lens to get a sharp image?

Answer 1

Tan theta = 1,400,000 km / 150,000,000 = 1.4 / 150

For small angles tan theta = theta (in radians)

theta = .0093

Diameter of image = 10 cm * .0093 = .093 cm = .93 mm

The paper should be placed at 10 cm because the object is at infinity.