(1) In order to make the light of the candle flame pass through the convex lens, the surface of the convex lens should face the candle flame, so the surface should be perpendicular to the track of the optical bench. At the same time, in order to make the image on the screen, the surface of the screen should face the convex lens, so the screen should be perpendicular to the track of optical bench.

(2) During the experiment, adjust the heights of the convex lens and the light screen so that the centers of the candle flame, the convex lens and the light screen are approximately at the same height, so that the image of the candle flame is at the center of the light screen.

Extended data:

The optical axis is a line that is rotationally symmetric along a certain angle in an optical system such as a camera lens or a microscope.

The optical axis is a dotted line, which defines the path of light propagation in the system. For a system consisting of a simple lens and a mirror, the axis passes through the curvature center of each surface and coincides with the rotational symmetry axis. The optical axis usually coincides with the mechanical axis of the system, but not always with the optical axis of the off-axis optical system.

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