CBSE Class 10 Science 2019 · Paper 31/1/1 · Question 27

Precautions (any four): (1) The lens should be held vertical, with its faces parallel to the screen. (2) The position of the lens (or screen) should be adjusted to obtain the clearest, sharpest image on the screen. (3) The base of the lens stand, the screen and the measuring scale should be in a straight line, and the distance measured along the scale. (4) At least three observations should be taken and their mean found. Other correct precautions also earn credit, for example: the object should be very far away, the lens should be clean, and the scale should be read without parallax.