The “mirage” that appears on the calm and windless sea in summer: looking into the distance, sometimes you can see ships, mountains, pavilions, temples, etc. appearing in the air in the distance. After the rain, a beautiful rainbow appears in the sky, and the scenery on the shore is reflected on the calm lake, which is beautiful. Why is the sky blue on a sunny day, a shadow play that is loved by children and has a long history? Why are the morning glow and sunset glow red and yellow? Why are there flickering and jumping light spots under the trees in the clear sky? These are all related to the magical phenomenon of light.
The study of light has a long history. In the early Warring States Period, Mozi discovered the law of reflection of light. The light phenomenon in life is mainly caused by the linear propagation of light, the reflection of light, the refraction of light, the scattering of light, the dispersion of light and so on.
Light propagates in a straight line in the same homogeneous medium, which is the straight line propagation of light. The formation of shadows, the principle of shadow puppetry, the imaging of small holes, solar eclipses, lunar eclipses, etc. are all phenomena of straight-line propagation of light. The light emitted by the sun passes through the small holes between the gaps of the leaves and forms a circular flickering spot on the ground.
A solar eclipse is when the moon moves between the sun and the earth, blocking part or all of the light from the sun to the earth, and it looks like part or all of the sun disappears, resulting in partial or total solar eclipses. A lunar eclipse is when the moon moves to the shadow of the earth, and the earth is between the moon and the sun at this time, and the three are in a line. The area between the moon and the earth will be shaded by the earth because of the sunlight, and the moon cannot reflect it. It seems that the moon is missing a piece of light, resulting in a partial lunar eclipse or a total lunar eclipse.
With a history of about 1,000 years, the representative of human intangible cultural heritage – Chinese shadow puppetry, let the audience watch the lantern shadow performed by a flat puppet through a white curtain to achieve an artistic effect. It uses leather materials, and mainly uses transparent pigments of five pure colors such as red, yellow, cyan, green, and black when coloring. With the help of the straight line transmission of light, the shadows of shadow puppet figures and props projected on the screen under the illumination of the back light appear. Magnificent and clear, with a unique beauty.
The scenery on the shore is reflected on the calm lake, which belongs to the plane mirror imaging, which is formed by the reflection of light.
The reflection of light refers to the phenomenon that when light propagates to different substances, it changes the propagation direction on the interface and returns to the original substance. The imaging principle of various mirrors is the reflection of light, but the application in life is different. Objects with a flat and smooth surface that can be imaged are called flat mirrors. Calm water surfaces, polished metal surfaces, glass plates, etc. are all equivalent to flat mirrors. Full-length mirrors for home use, mirrors around the walls in dance studios or gyms, and small mirrors that dentists put in their mouths when examining teeth are all flat mirrors; submarine periscopes use flat mirrors to change the light path; store decorations use flat mirrors to expand the visual space. Convex mirrors have a divergent effect, also known as wide-angle mirrors, reflectors, and turning mirrors. It is mainly used for various curves and intersections, which can expand the driver’s vision and detect vehicles on the opposite side of the curve as soon as possible, thereby reducing the occurrence of traffic accidents. It is also used in supermarkets to prevent theft and monitor dead spots. The rear view mirror outside the car cab is a convex mirror, which makes use of the divergence effect of the convex mirror on the light and the characteristics of erecting, shrinking and virtual images to make the observation range larger and ensure driving safety. Concave mirrors have a converging effect on light, and solar cookers, desk lamps, TV satellite dishes, and radars use concave mirrors to condense light. Searchlights, flashlights and the headlight shades of various motor vehicles use the concave mirror to pass the focal light and become parallel light parallel to the main axis after reflection. The movie screen is made of rough cloth, the purpose is to make the light hit it diffusely reflect, reflect various colors of light, so that the reflected light shoots in all directions, so that all the audience can enjoy the content of the screen from different angles.
The old man wearing reading glasses can see things close by, and the short-sighted student wearing glasses can see the words on the blackboard in the distance, the mirage, these are all due to the refraction of light. Refraction of light refers to the phenomenon that when light is incident from one medium obliquely into another medium, the direction of propagation changes, causing the light to be deflected at the junction of different media. When people see things in the water from the shore, they will be shallower than they actually are, because the light is refracted when it enters the air from the water, and what we see is a virtual image. The fisherman has to fork down when he is fishing, and when you are swimming, the trees on the shore when you are in the water tend to be taller than they actually are. Using the refraction of light, people use the camera to leave good memories. A slide projector and a projector were made to facilitate academic reports. A mirage is an optical illusion, the result of the combined action of light refraction and total reflection, caused by uneven air density. Light has a different refractive index in air of different densities. In summer, the air temperature in the lower layer on the sea surface is lower than that in the upper layer, the density is higher than that in the upper layer, and the refractive index is also higher than that in the upper layer. When the light emitted by distant peaks, buildings, etc. shoots into the air, due to continuous refraction, it deviates more and more from the normal direction, and the incident angle entering the upper air continues to increase, so that total reflection occurs, and the light is reflected back to the ground. Looking at the light, you will see the distant scene hanging in the air, this is the mirage. Mirages can be seen not only in the sea and in the desert, but also occasionally on asphalt roads.
Blue sunny days and red-orange-yellow morning sunsets are caused by sunlight scattered by the atmosphere. Sunlight contains seven colors of red, orange, yellow, green, blue, indigo, and violet, with wavelengths decreasing from red to violet. Sunlight can only reach the ground after passing through the earth’s atmosphere. The air and water vapor in the atmosphere will emit sunlight scattered in all directions, which is called light scattering. Air scatters light with shorter wavelengths more easily, so when sunlight passes through the atmosphere, blue, indigo, and violet light with short wavelengths is scattered, and people see blue sky. Red, orange, and yellow light have longer wavelengths and are not easily scattered. When they pass through the thicker atmosphere, people can see the gorgeous morning glow of fiery red or golden yellow. When the sun is blocked by clouds, we can still see the light around us because there is a lot of scattered light around. The beautiful rainbow that appears in the sky after rain is formed by the dispersion of light caused by the refraction of light. When it rains and the sky is fine, a large number of small water droplets are suspended in the air. When the sunlight shines on the small water droplets, the light is refracted. After the white light is refracted by the water droplets, it is divided into seven colors of light. This phenomenon is called dispersion of light. Newton was the first to observe the dispersion of light using a prism.
[equipment preparation]
Candles, small mirrors, cell phones, cardboard with arrows drawn, clear glasses, straws, clear glasses, tea cans, lighters, disposable plastic cups.
[Experiment 1]
Hand shadow game:
Turn off the lighting in your home at night, turn on the flashlight of your mobile phone, and observe the shadow of your hand on the wall.
[Experiment 2]
Plane mirror imaging:
Use two small mirrors to observe the image of the red pattern on the lid of the tea can, adjust the angle of the two mirrors, and see how many patterns you can see at most.
[Experiment 3]
Bent straw: The
straw is inserted obliquely into a transparent glass filled with water, and it is found that the straw is bent at the water surface, and the position of the straw under the water surface rises (shallows).
[Experiment 4]
Changed arrow:
Put the cardboard with the right arrow behind the transparent glass filled with water, change the distance between the cardboard and the glass, find the change, and it will become larger or smaller.
