Q1 An object is placed 30 cm in front of a converging lens of focal length 20 cm. Find the position, nature and magnification of the image.
Q2 An object of height 2 cm is placed 10 cm in front of a converging lens of focal length 15 cm.What is the image height?
Q3 An object is placed 10 cm in front of a diverging lens of focal length 30 cm. Find the position, magnification and nature of the image.
Q4 The height of an image is double that of an object when viewed through a converging lens of focal length 20 cm. What are the two possible locations of the object?
Q5 The cross threads of a ray box are positioned 32 cm from a screen.What are the two locations for a converging lens, of focal length 7.5 cm, in order that a sharp image of the cross threads can be seen on the screen?
Ray box with cross threads Lens Screen
u 32 cm Fig 13.25
MANDATORY EXPERIMENT 18
Measurement of the Focal Length of a Converging Lens Apparatus Converging lens, screen, ray box with cross threads, metre stick
Procedure ■ Find the approximate focal length of the lens by focusing the image of a distant object (e.g. a window at the far end of the room) on a piece of paper. The distance from the lens to the paper is the focal length.
■ Set up apparatus as illustrated in Fig 13.26, with the ray box outside the focal length. Measure the object distance (u) from the cross threads to the lens using a metre stick.
Ray box with cross threads Lens Screen
uv Fig 13.26
■ Move the screen back and forth until the image of the cross threads comes into clear focus. Measure the image distance (v) from the screen to the lens using a metre stick.
■ Repeat the process using different object distances so as to get six or seven readings.
