How to roughly measure the focal length of a convex lens What instruments are needed, physical quant

1. The one on the first floor, take the convex lens to the sun, get a minimum and brightest spot on the ground or paper, and measure the distance from the center of the convex lens to the spot with a scale. This distance is the focal length.

2. It's more troublesome. Instruments: light base, light screen, candle, convex lens, scale (that is, the set that studies the imaging law of convex lens).

A moving candle is a candle that looks exactly as big as a candle on the light screen. Measure the distance from the light screen to the convex lens or the candle to the convex lens. At this point, these two are equal, which is the focal length.

3. Distant imaging. Instruments: light screen, convex lens.

Adjust the position of the light screen so that distant objects, such as those in a classroom window or outside the window, are imaged on the light screen, and the distance between the light screen and the lens is approximately the focal length of the lens. (The object must be farther away).

1 Formula method.

The optical apparatus base was used to make a convex lens into a real image, the object distance U and image distance V during imaging were measured and recorded, and the focal length f of the lens was calculated according to the lens imaging formula, and the average value was taken after multiple measurements.

2 Conjugation.

Use the light fixture base to fix the position of the light source and the light screen, and measure their spacing l. Place the convex lens with the focal length to be measured in the middle and move the convex lens along the main axis to make the image of the light source inverted twice on the light screen. The position of the lens during the two imaging is recorded, and the distance d d of the lens movement during the two imaging processes is obtained, and the focal length f of the convex lens can be calculated according to the formula, which is called the conjugate method. This is one of the methods commonly used in the laboratory to measure the focal length of convex lenses.

3 Parallel light focusing method.

According to the characteristics of the convex lens, let the parallel light (such as sunlight) incident on the convex lens along the main axis direction, place a light screen parallel to the lens on the other side, adjust the position of the light screen to make the spot on the light screen the smallest and brightest, and the distance between the lens and the light screen is the focal length of the convex lens. This is an easy way to roughly measure the focal length of a convex lens.

In the laboratory, the focal length of the convex lens can also be estimated by the distant object imaging method instead of the parallel light focusing method, which is similar to the parallel light method. Adjust the position of the light screen so that distant objects, such as those in a classroom window or outside the window, are imaged on the light screen, and the distance between the light screen and the lens is approximately the focal length of the lens.

Scale, directional light source, white paper.

Method; The light is parallel to the convex lens, and the parallel light will be in focus after passing through the convex lens, and a white paper will be placed behind the convex lens; Adjust the distance from the lens to the paper until the brightest and largest spot appears on it, at which point the distance from the white paper to the lens is the focal length of the lens; There is a scale to measure out.

Instruments: convex lens, scale.

Physical quantity: distance l

Relationship: f=l

Facing the convex lens with the sun, adjust the convex lens until the smallest and brightest spot is obtained on the ground, and measure the distance l between the convex lens and the spot at this time, which is approximately the focal length f of the convex lens

There are two easy ways to do this:

First, the parallel ray focusing method:

When the smallest and brightest spot appears on the white screen, measure the distance between the spot and the center of the convex lens with a scale, which is the focal length of the convex lens.

Second, infinity imaging method:

On the paper on the desktop, use the convex lens to move up and down against the paper, and pay attention to the words on the paper through the convex lens. When you can't see either an upright or upside-down word, measure the distance between the paper and the optical center of the convex lens with a scale, which is the focal length of the convex lens.

Scheme 1: On the light table, arrange candles, convex lenses and light screens in turn. Adjust the distance between them so that the candle forms an inverted image on the light screen, in which case the image distance is twice the focal length. f=image distance 2

Solution 2: In sunlight, let the sunlight shine vertically through the convex lens on the light screen (or ground), and adjust the distance between the convex lens and the light screen, you can get a minimum and brightest point on the light screen, that is, the focal point. Use a scale to measure the distance from the focal point to the center of the convex lens, which is the focal length of the convex lens.

Take a convex lens facing the sunlight, and then place a piece of paper on the other side of it, changing the distance between the lens and the paper, knowing that the spot on the paper becomes the smallest and brightest. Measure the distance from this smallest, brightest spot to the convex lens. The measured length is approximately equal to the focal length of the convex lens.

When the convergence of this point is very small, the distance from this point to the convex lens is the focal length with a ruler.

Put the convex lens in the sunlight, adjust the distance, make it a minimal shadow on the light screen, and measure the distance from the lens to the point.

(1) Parallel light focusing method:

Experimental equipment: convex lens, white paper, scale ruler.

Experimental method: Take out the convex lens to be measured, face the sunlight, and then place a piece of paper on the other side of it, move it back and forth, and observe the change of the light spot (bright spot). Adjust the position of the convex lens to make the spot smallest and brightest.

Use a scale to measure the distance from the spot to the center of the convex lens and record it. Repeat the above experiment 2 times to find the average of the 3 measured distances, that is, the focal length of this convex lens.

2) Double focal length method:

Experimental equipment: light fixture base, light bulb, convex lens, light screen, scale test method: put the center of the bulb, convex lens, and light screen at the same height, move the bulb and the light screen back and forth until an inverted, equal-size real image is formed on the light screen, and measure the distance u or v from the center of the bulb or light screen to the convex lens with a scale, then f=u 2=v 2.

Repeat the above experiment 2 times, and find the average of the 3 measured distances, that is, the focal length of this convex lens.

In the sunlight method, parallel beams of light are passed through a lens and converged into focus.

(l) Measure the focal length with the plane mirror-assisted method.

A schematic diagram of the experimental setup is shown in Figure 2, 5, 2. Fix the flat mirror on the optical base. Remove the collimated light source lens and replace it with a lampshade with a triangular hole about 4 mm in length.

Make the distance from the collimated light source to the plane mirror slightly larger than the focal length of the lens being measured. Turn on the power supply and adjust the position and height of the measured biconvex lens until the light reflected back from the plane mirror still shines on the position of the small hole and forms a clear triangular image. And make one side of the image coincide with one side of the hole, and the image and the hole form a diamond.

In this case, the distance from the lens to the aperture is the focal length of the lens.

2) Use object distance and image distance to find the focal length.

Remove the convex lens of the parallel light source and attach the "1" screen. Fix the convex lens in the middle of the light fixture base, place the light source at the left end of the guide rail, and then move the frosted glass screen from far to near until a clear "1" image appears on the frosted glass screen. Although the distance between the "1" screen and the lens to the lens is v.

You can use the formula to find the focal length f.

3) Measure the focal length with two imaging methods.

The device is the same as "Finding Focal Distance with Object Distance and Image Distance". First, fix the "1" screen and the ground glass screen on the light base, so that the distance between the two is greater than 4 times the focal length. Place the convex lens between the two and slowly move from the "1" screen to the ground glass screen.

When a clear "1" appears on the ground glass screen, note the location of the lens. Continue to move the convex lens, and when a clear image appears on the ground glass screen, note the position of the lens. Let the distance between the two positions be d, and the distance between the "1" screen and the ground glass room is l, and the focal length of the lens can be found by formula.

Sunlight focusing method (coarse focal length method): Place the convex lens directly facing the sunlight, and then place a piece of paper on the other side of it, changing the distance between the lens and the paper until the spot on the paper becomes the smallest and brightest, and the spot is the focal point of the convex lens. Use a scale to measure the distance from this smallest, brightest spot to the convex lens, which is the focal length!

The physics knowledge used is that "light rays parallel to the principal optical axis are refracted through a convex lens and become over-focused". Sunlight is a directional light that converges at the focal point after passing through a convex lens.

This method has a large error and can only be used as a method for coarse focal length).

Method 1: Let the sunlight shine vertically on the mirror surface, and adjust to the minimum point of light where the sunlight converges, that is, the focal point Use a scale to measure the distance from the focal point to the convex lens is the focal length

Method 2: Put the lit candle, convex lens, and light screen on the light fixture base in turn, adjust the distance between the light screen and the candle, until you get an image of an equal-sized candle on the light screen Measure the distance from the candle to the light screen at this time, and then divide by 2, that is, the focal length