Method of measuring focal length

1. sunlight focusing method (coarse focal length measurement method): put the convex lens directly at the sunlight, then put a piece of paper on its other side, and change the distance between the lens and the paper until the light spot on the paper becomes the smallest and brightest, and this light spot is the focus of the convex lens. Measuring the distance from the smallest and brightest spot to the convex lens with a scale is the focal length! The physical knowledge used is: "the light parallel to the main optical axis will be refracted into focal light after passing through the convex lens." Sunlight is parallel light, which passes through the convex lens and focuses on the focal point. This method has a large error and can only be used as a method of coarse focal length measurement)

2. Double focal length method: place the candle, convex lens and light screen on the light table once, then light the candle so that the center of the candle flame, the optical center of the convex lens and the center of the light screen are at the same height, and then adjust the distance from the candle flame to the convex lens and the distance from the light screen to the convex lens until the image on the light screen is equal to the candle flame. At this point, measure the object distance with a scale. Half the object distance is equal to the focal length. The physical knowledge used is that when an object is at twice the focal length, its image is at twice the focal length, which is inverted image, arithmetic and real image. )

3. Parallel light source method: shoot a beam of parallel light on the convex lens along the main optical axis direction, and the refracted light obtained on the light screen intersects at a point, namely the focal point, and measure the distance from the point to the optical center of the convex lens, namely the focal length. This method is similar to the first method, except that the parallel light source of sunlight is no longer used, but artificial parallel light is used. )

4. Focus incidence method: place a luminous point light source, such as a small bulb, on one side of the convex lens, place a light screen on the other side of the convex lens, move the small bulb along the main optical axis until a prototype spot with the same diameter as the convex lens is obtained on the light screen, and measure the distance from the small bulb to the convex lens as the focal length. (This method is actually the reverse measurement of the third method "parallel light source method". Physical knowledge used: "The light passing through the convex lens will be refracted into light parallel to the main optical axis". )

5. Focused non-imaging method: look at the object through a convex lens. Adjust the distance from the object to the convex lens when observing the enlarged image of the object. When the object is just invisible from the visible, the distance from the object to the convex lens is measured as the focal length. The physical knowledge used in this method is that the object does not image when it is in focus. )