An object is placed 30 cm from a convex lens of focal length 20 cm. How far from the lens is the image?
Answer and explanation
Correct answer: C. 60 cm
- Step 1: Lens formula: 1/f = 1/u + 1/v (real-is-positive). Step 2. 1/v = 1/20 − 1/30, so v = 60 cm. Answer: 60 cm. Why not the others: 12 cm is what you get if you use 1/v = 1/f + 1/u. 10 cm is what you get if you subtract the distances. 50 cm is what you get if you add the distances.
Practise more Waves, Heat and Optics.
Free questions with an explanation for every one, and full timed ECAT Pakistan mock tests.
More Waves, Heat and Optics questions
- A body of volume 0.01 m³ is fully immersed in a liquid of density 800 kg/m³. What is the upthrust? (take g = 10 m/s²)
- A body moving at 2 m/s accelerates uniformly at 6 m/s² for 5 s. What is its final speed?
- What force gives a mass of 12 kg an acceleration of 2 m/s²?
- What is the kinetic energy of a 10 kg body moving at 14 m/s?
- How much gravitational potential energy does a 2 kg mass gain when lifted 6 m? (take g = 10 m/s²)
- A force of 40 N acts at 60° to the horizontal and moves a body 13 m along the horizontal. How much work does it do?