(50-155) Optical Box

Transcription

1 (50-155) Optical Box Your optical box should have the following items: 1 Optics Box 3 color filters (one of each): red, green, and blue. 1 curved mirror 1 right angle prism 1 equilateral prism 1 semicircular lens 1 convergent lens 1 divergent lens 3 ray cards Note: your Optics Box will require a 12V power supply. The field of optics is a very old one. The strange properties of light have been noticed since ancient times. Greek philosophers tried to understand it, and the Islamic Empire explored it in great detail. Light was studied by the Chinese and Maya. It has fascinated mankind for since immemorial. The study of light as a Science didn't really get its start until the age of enlightenment. Isaac Newton, though most famous for his theories on gravity, spent a large part of his career studying light. He was the first scientist to learn that white light is actually composed of many colors, which was the first step in understanding something called a wavelength. Gradually, other scientists built upon this foundation, and today optics is a well understood field. However, what exactly is light? In the most fundamental sense, we still really don't know. However, there are some generally accepted theories that describe the behavior of light quite well. The most commonly help and rigorous view is that light is composed of tiny packets of energy called photons. These are massless and travel at immense speed, approximately 300 million meters per second. In fact, they are so fast that it took mankind centuries to realize they moved at all. Since they are massless and remove energy from an atom when they are created, they do not violate the conservation of energy law, regardless of how many there are. The total number of photons in the universe is impossible to guess. Some physicists think photons do not actually exist at all, but are simply wrinkles on the surface of the universe. While this is a fascinating idea, it goes beyond the scope of this manual. Photons are emitted when an atom is excited. It will give off a little bit of energy, and some of this energy will be in the form of an electromagnetic wave. Light is nothing more or less that a manifestation of electromagnetism. All electromagnetic waves, from radio to gamma radiation, can be thought of as different forms of light. It is unusual to think of a radio wave as light, but in essence it is. The type of light is determined by the wavelength. Light rays oscillate at different frequencies, which changes their properties. For example, blue light oscillates much faster than red light, making the two distinct from one another. There is a portion of the electromagnetic spectrum called visible light. Obviously, this is the portion we can see. It contains all the colors we are familiar with. This kit will be focused on the visible portion.

2 This kit will teach some of the basic concepts in optics, including but not limited to: Color mixing Lens properties Reflection Refraction Internal reflection For the first experiment, we will explore simple reflection. You will need a simple plane mirror and the following items: Insert the ray card into the holder so that a single ray emerges. Adjust the internal lens as needed to get the sharpest ray possible. You will explore how a light ray behaves when it strikes a flat reflective surface. \ When the light ray is as sharp as possible, shine it against the plane mirror. Pay special attention to the angle at which it strikes the mirror. You will find that no matter what angle the light ray strikes the mirror, it will reflect at the same angle. This is true for any angle, although for a 90 degree angle it is impossible to see! Try using two or three light rays against the plane mirror. You will find that the effect is the same. Have your students measure several angles and the resulting reflection. This property has several uses. Your students will probably think of a periscope, which uses two mirrors at 45 degrees to bend a light ray around a corner. Quality cameras often uses this effect to get light from the lens onto the film or image sensor. Despite being seemingly simple, knowing the exact nature of this effect is essential for any understanding of optics. Once your students understand that light can be understood and predicted, they are able to learn all about it. Light ray from Box Mirror Reflected Light Ray

3 For the next experiment, we will explore reflection off of a curved mirror. For this, you will need the following items: Insert the ray card into the holder so that a three rays emerge. Adjust the internal lens as needed to get the sharpest ray possible. You will explore how a light ray behaves when it strikes a curved reflective surface. Our mirror is a concave design, meaning that it bulges away from incoming light. When you shine the parallel rays onto the mirror, each one reflects at a slightly different angle. When this angle is just right, the light rays will converge on a single point. For this reason concave mirrors are sometimes called converging mirrors. Try to position the optics box so that the light becomes concentrated. Try using different numbers or rays and noting the change in position that must occur, if any. These types of mirrors are very useful, because they can be used to focus light. Some solar heating devices use them to concentrate sunlight onto a small point. More practically, they are used in nearly all professional telescopes. These telescopes will have giant, dish shaped mirrors that pull in even the faintest light and magnify it. This is a useful demo, because a curved mirror behaves nothing like a flat mirror. As we learned before, with a flat mirror all the light rays reflect at the same angle. Curved mirrors behave differently because the surface constantly changes its angle as you move from one end to the other. Light ray from Box Reflected Light Ray

4 For the next experiment, we will explore prisms. You will learn about refraction and reflection. For this, you will need the following items: Insert the ray card into the holder so that one emerges. Adjust the internal lens as needed to get the sharpest ray possible. You will explore how a light ray behaves when it strikes a prism. When light goes from one medium to another, it changes speed. This can have a number of interesting effects. Using your light ray, try to duplicate the figures below. Figure 1 Figure 2 In Figure 1, the light ray enters the prism at a fairly steep angle. As it changes from air to acrylic, the light ray must slow down. However, all of the different wavelengths to not slow down equally. This causes them to become decoupled from each other, which makes the colors splay out. This display of colors is called a spectrum. Newton famously used a glass prism to split the spectrum for the first time in scientific history. He was greatly surprised by the result. In Figure 2, the angle is much shallower. Generally, this effect only takes place for angles less than 48 degrees. What happens here is that the light ray actually reflects off of the airacrylic interface, inside the prism. This phenomenon is aptly named total internal reflection. It is often used in situations where mirrors would be too fragile, such as in digital projectors or very high end cameras. Total internal reflection is also the mechanism that carries a light pulse along a fiber optic cable. It also occurs when lights are shown upward underwater, which can be very confusing for divers. Try to duplicate these effects with your right angle prism as well. You may find that different angles are needed.

5 For the next experiment, we will explore lenses. You will learn about convex and concave lenses. You will need the following items for this experiment: Insert the ray card into the holder so that three rays emerge. Adjust the internal lens as needed to get the sharpest ray possible. You will explore how a light ray behaves when it passes through a lens. When light goes from one medium to another, it changes speed. This can have a number of interesting effects. Using your light ray, try to duplicate the figures below:

6 Focal Point You probably noticed that different lens shapes have different properties. Convex lenses converge light onto something called a focal point. The focal point is the place where all the light rays meet. This distance between the lens and focal point is called the focal distance. Convex lenses are used to magnify images, like in a magnifying glass or most glasses. Older telescopes would use them as the primary optics element, but these have been largely replaced with mirrors. Concave lens spread light out. They are called divergent lenses for this reason. They are commonly used in applications where a good light spread is important, such as in flashlights. Lenses are extremely important. If you're reading this now, that is because you have a lens in your eye that focuses light, enabling you to see. Lenses always invert an image, so another lens or special software is required to re-orient the image for proper viewing. Some interesting experiments you can do with the lenses include: Try using one, two, three, and four rays. Do the lens properties hold up for varying number of light rays? Adjust the angle that the light rays strike the lens. Is there any change? Using the adjustment tab, move the internal lens in the optics box so that the rays are no loner parallel. Is there any change in the lens behavior? Place multiple lenses in a line. See how much you can adjust the light paths using more than one lens.

7 For the next experiment, we will explore color mixing. You will need the following items for this experiment: White light, while seemingly simple, is actually a very complex phenomenon. It is relatively uncommon in nature; our own sun is yellow, for example. Sunlight appears white to our eyes due to a fascinating twist: although the sun itself is yellow, it is able to produce a full spectrum. This means that instead of radiating just yellow light, as one might expect, it instead is able to produce all the colors of visible light, plus infrared and ultraviolet wavelengths. What we call visible light is only a narrow portion of the electromagnetic spectrum. In general, humans are able to see wavelengths between 400 and 700 nanometers. Some people have a slightly larger range, but this should be treated as the exception, rather than the rule. Color science is notoriously difficult to teach. Many students are skeptical when told that white light is actually made up of all the colors. How can something as pure as the color white be a blend of so many disparate colors, such as red, green, and blue? Below are the color mixing diagrams. You will want to install the color gets and position the mirrors as shown. You will need to use at least two different gel arrangements in order to get all of the colors. The colors that can be produced with this method are red, green, blue, cyan, magenta, yellow, and white.

8 Congratulations! You have learned about reflection, refraction, prisms, the spectrum, lenses, and color mixing. Your equipment is also capable of performing most of the experiments found in the optics chapter of your textbook. Use it to explore some of the more complex features of light, such as multi-element lenses, spectral analyses, and the way a human eye works. Your optics box is also equipped with magnetic feet, which allows you to mount it on a blackboard. This comes in handy for ray tracing exercises. Warranty and Parts: We replace all defective or missing parts free of charge. Additional replacement parts may be ordered toll-free. We accept MasterCard, Visa, checks and School P.O.s. All products warranted to be free from defect for 90 days. Does not apply to accident, misuse or normal wear and tear. Intended for children 13 years of age and up. This item is not a toy. It may contain small parts that can be choking hazards. Adult supervision is required.