There are four common methods:

1. Convergence method

Let the light gathering seat be parallel to the sun, put the light screen and the convex lens on the light gathering seat in turn, fix the light screen (or convex lens), and move the convex lens (or light screen) until the light spot on the light screen is the smallest and brightest, and the distance from the bright spot to the convex lens is the focal length.

2. observing the virtual image method

put a light screen and a convex lens on the light gathering seat in turn, paste a larger character on the light screen, and the convex lens looks at the characters on the light screen. When the virtual image of the word appears, gradually increase the distance between the convex lens and the light screen until the virtual image of the word just disappears, and the distance from the light screen to the convex lens is the focal length.

3. The method of making objects and images equal

Put a light screen, a convex lens and a burning candle on the light gathering seat in turn, keep the convex lens motionless, and move the lit candle and the light screen at the same time until an inverted image with the same size as the candle flame appears on the light screen, then the focal distance is half of the object distance or image distance.

4. One-shot imaging method

Put a light screen, a convex lens and a burning candle on the light gathering seat in turn, move the candle and the light screen until a clear inverted enlarged (or reduced) image appears on the light screen, measure the object distance u and the image distance v, and substitute it into the formula 1/f=1/U+1/V to calculate the focal length.

the distance from the optical center of the lens to the focus of light gathering when parallel light is incident. An optical system with a short focal length has a better ability to collect light than an optical system with a long focal length. Simply speaking, the focal length is the distance from the focal point to the center point of the mirror. The focal length f <: Image distance < 2f to image.

Extended information:

For a thin lens in air, the focal length is the distance from the center of the lens to the main focus. For a converging lens (such as a convex lens), the focal length is positive, and a beam of parallel light will converge on a point. For a divergent lens (such as a concave lens), the focal length is negative, and a parallel beam of light will spread out after passing through the lens.

according to the most common marking habit, if the lens on the first surface is convex, the value of R1 is positive, and if it is concave, it is negative. If the second surface is a concave lens, the value of R2 is positive, and if it is a convex lens, it is negative. It should be noted that even so, different authors may have different labeling habits.

for a mirror with spherical curvature, the focal length is equal to half the radius of curvature of the mirror. The focal length of convex mirror is positive, and the focal length of concave mirror is negative.

neither convex lens nor concave lens has a certain focus, and only rays parallel to the main optical axis and at the same distance from the main optical axis will completely intersect on the main optical axis. The reason why many rays passing through the convex lens parallel to the main optical axis but with unequal distances from the main optical axis have a "focus" is that the curvature radius of the convex lens mirror is large, and the difference in deflection of light degree is not obvious.

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