Applications of convex lenses

Example 1 An old woman uses a magnifying glass to read the time. In order to see a larger and clearer image, she often does this ( )

A. Keep the newspaper and magnifying glass still, and keep your eyes farther away from the newspaper

B. Keep your eyes still and keep the magnifying glass farther away from the newspaper

C. Keep the newspaper and magnifying glass still, and keep your eyes closer to the newspaper

D. Keep your eyes still, and the magnifying glass should be closer to the camera

Analysis: The magnifying glass is a convex lens. From the convex lens imaging experiment, it can be seen that when the object is within one focal length, the greater the object distance, the longer the image distance. Big, the image is bigger. It can also be concluded from the principle of imaging that the light rays parallel to the main axis remain unchanged, and as the object moves away from the lens, the light passing through the optical center becomes smoother and smoother, so the intersection point of the reverse extension lines of the two rays is farther away from the lens. , the bigger the image is. That is, within one focal length, the closer the object is to the focus, the larger the image will be. So the answer is B.

Example 2 Xiao Ming holds a large-diameter experimental magnifying glass and stretches his arms to look at objects in the distance. He can see the image of the object. The correct statement below is ( )

A. The light entering the eye must be emitted by the image

B. The image must be a virtual image

C. The image must be upside down

D. The image must be magnified

Analysis: The magnifying glass is a convex lens. When you hold the convex lens and straighten your arm to look at a distant object, the object distance is much greater than twice the focal length, so the image will appear slightly on the inside of your hand. Greater than one focal length. The human eye is one arm away, so the light entering the human eye must be the light that is refracted and then separated into an image. We see it as if it comes from the image. This image must be an inverted and reduced real image. So the answer is C. In option A, not all the light entering the human eye is emitted from the image.

2. Application as a projector lens

Example 3 When screening a movie in rural areas, when testing the lens, I found that the image on the screen was a little smaller. How should I adjust the projector ( )

A. The projector should be farther away from the screen, and the film should be farther away from the lens

B. The projector is farther away from the screen and the film is closer to the lens

C. The projector is closer to the screen and the film is further away from the lens

D. The projector is closer to the screen, and the film is closer to the lens

Analysis: This is a question about convex lenses. The focal length of the movie projector lens (convex lens) is constant. According to the imaging rules of convex lenses, the distance between the film and the focus of the lens is The closer the distance, the larger the image on the screen, and at the same time, the farther the screen is from the lens.

Convex lens imaging, the closer the object is to the focus, the larger the image will be, and the farther the image is from the convex lens (this rule applies to both real and virtual images). Conversely, the farther the object is from the convex lens, the smaller the real image is and the closer the image is to the focus. Concave lens imaging, the farther the object is from the concave lens, the smaller the image formed, and the closer the image is to the virtual focus.

From the above analysis, we can see that the correct option for this question is B.

3. Application as a camera lens

Example 4: After taking a graduation group photo, a classmate wanted to take a single portrait. The method the photographer should take is ( )

A. Bring the camera closer to the classmate, and at the same time retract the lens closer to the film

B. Bring the camera closer to the classmate, and at the same time, extend the lens forward and away from the film

C. Move the camera away from the classmates, and at the same time retract the lens closer to the film

D. Move the camera away from the classmates, and at the same time extend the lens forward and away from the film

Analysis: The camera lens is equivalent to a convex lens. Behind the lens is a camera obscura, and the film is installed at the bottom of the camera obscura. The film acts as a light screen. After taking a group photo, and then taking a single portrait, the image will become larger. To make the image larger when imaging, the object distance should be reduced and the image distance should be increased at the same time, that is, by lengthening the camera obscura or extending the lens forward. From the above analysis, we can see that the correct option for this question is B.

Example 5 When using a camera to photograph objects at the bottom of a pool, if the camera position remains unchanged, compare the two situations with and without water in the pool (assuming that in the two situations, the objects seen by the human eye are equal to large), then there is water ( )

A. The camera obscura should be slightly shorter, and the resulting image will be slightly larger

B. The camera obscura should be slightly shorter, and the resulting image will be slightly smaller

C. The camera obscura should be slightly longer, and the resulting image will be slightly larger

D. The camera obscura should be slightly longer, and the image obtained will be slightly smaller

Analysis: According to the camera imaging principle, when the object distance decreases, the image distance becomes larger. To get a clear image of the object on the film, you should The camera obscura is appropriately elongated, and at the same time, the image on the film will be slightly larger than the original image.

In this example, the distance of the object at the bottom of the pool relative to the camera lens is different when there is water and when there is no water; due to the refraction of light, when there is water in the pool, it is equivalent to the distance between the object and the camera lens. The distance is reduced. According to the imaging principle, the correct option is C.

Example 6 When surveying and mapping personnel draw maps, they need to take pictures of the ground from an airplane in the air, which is called aerial photography. If the focal length of the aerial photography camera used is 50mm, the distance from the film to the lens is ( )

A. Beyond 10mm

B． Slightly less than 50mm

C. Slightly larger than 50mm

D. Equal to 50mm

Analysis: Aerial photography refers to taking pictures of the ground from an airplane. Since the object is very far away from the convex lens, it can be seen as light incident from infinite distance, and the image it forms should be slightly larger than the focal length. Therefore, the correct option for this question is C.

Commentary Solving problems related to the application of cameras, slide projectors and magnifying glasses cannot be separated from the imaging laws of convex lenses. The best way to master these rules is to draw pictures. Therefore, students should repeatedly draw the imaging light path diagrams of objects at different positions of the convex lens after class. On this basis, they should master the tables listed in the knowledge point analysis, and then do this The problem is readily available.