Lenses

Before the first telescopes were invented at the beginning of the 17th century, people looked up at the stars with their naked eyes. The first refracting telescope that Galileo developed in 1609 was likely about as powerful as a pair of binoculars today, but he eventually became the first person to see the rings of Saturn. Sir Isaac Newton developed the reflecting telescope, allowing scientists to see exponentially more stars. Telescopes became increasingly more powerful and sophisticated throughout the centuries, but the launch of the Hubble Space Telescope was a giant step forward. Without Earth’s atmosphere to hinder viewing, the Hubble Space Telescope sights far into deep space, capturing images of galaxies in all stages of evolution, black holes, planets being created, and stars being born and dying. In the works is the James Webb Space Telescope, which is scheduled to launch in October 2018. It will be able to discern even more about our universe by looking through dust that blocks visible light. What wonders will we be able to discover about the past and future of our universe? We can only imagine…

Galileo Drawing of Isaac Newton’s reflecting telescope

Hubble Space Telescope

View of deep space via telescope

A common student misconception is that light passes through transparent objects without changing direction. In truth, light always travels in a straight line, but it travels at different speeds through different materials. Generally, the denser the material, the slower light travels. When light travels through air and then hits a glass window, the speed of light changes while traveling through the window so that it is refracted (bent). It travels a constant speed through the window and then is refracted again as it enters the substance on the other side of the window. So light always travels in straight lines, but not the same straight line when it encounters different materials.

Characteristics of LightBefore investigating lenses, you must understand a few characteristics of light. According to current scientific knowledge, light travels faster than anyother known entity. It travels through space at approximately 186,000 miles a second. It could travel around the equator seven times in a second! Light travels in a straight line.

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LensesWhat happens when light hits an object? It either passes through, is reflected, or is absorbed. When it passes through something, it may be bent (refracted) and will then continue in a new straight line. The angle of refraction is the change in direction of a ray of light as it passes from one material into another. Every material bends light differently. The index of refraction is a value that describes the ability of a material to bend light. Each material has its own index of refraction. Look at the table below to see the index of refraction for some common materials.

Index of Refraction for Common Materials

Material Index of Refraction

Vacuum 1.0

Air 1.0003

Water 1.33

Glass 1.5

Vegetable oil 1.57

Although there is a broad range of “light,” when we speak of “light,” we are usually referring to white visible light. White light is composed of a spectrum of colors—red, orange, yellow, green, blue, indigo, and violet. Interestingly, the index of refraction varies very slightly depending upon the color of the light. Dispersion describes how the refractive index varies slightly depending upon the color of the light.

How does a prism work? Look at the image below and try to explain how it breaks white light into its spectrum of colors.

LensesA lens is made of a transparent material through which you can view images. Because lenses are made of different materials than air, they refract (or bend) light. Variously shaped lenses bend light in different ways and are used for specific purposes.

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LensesA convex lens is curved on both sides. It is thicker in the middle and thinner at the edges. This means the light rays coming out of it come together at a point—they converge.

A focal point is an important aspect of lenses that needs to be understood. It is the point at which light rays meet before or after refraction through a lens. The focal points are in different places depending upon whether a lens is convex or concave. When light enters a converging lens, the focal point occurs after the light has entered and exited the lens. When light enters a diverging lens, the focal point is actually located before the light enters the lens and is scattered. In other words, a convex lens bends light in toward a focal point while a concave lens refracts light away from a focal point. Look at the images to contrast the focal points of the two lenses.

Lenses are used to make many tools that help people to see distant objects as if they were closer or microscopic objects as if they were larger. Convex lenses are used in cameras, telescopes, magnifying glasses, and binoculars. Concave lenses are used in flashlights, peepholes, and glasses for nearsightedness. How do these tools use lenses to assist people? TelescopesIn 1608, Hans Lippershey applied for the first patent for a refracting telescope. Galileo improved upon that model and viewed craters on the Moon the very next year. One century later, Isaac Newton invented the reflecting telescope. Since then, telescopes have gotten larger and more sophisticated to help us see far into the heavens. All telescopes use lenses in some capacity, but reflecting telescopes also use mirrors. Telescopes use the light from distant objects and bring it to a concentrated point called the focal point. The telescope then enhances the light from the focal point using a lens that magnifies the light and image on your eye, which makes the object appear much larger.

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A concave lens is thinner in the middle and thicker at the edges. The light rays that come out of it diverge.

LensesRefracting TelescopesA refracting telescope uses two lenses to bend light and cause items to appear larger. They are made from a long cylinder and two separate lenses—an objective lens and an eyepiece lens. The function of the objective lens is to retrieve light from something and bend it to the focal point, which is located at the back of the tube near the eyepiece lens. The objective lens is convex so it can direct the gathered light to converge upon the focal point at the back of the telescope. The job of the eyepiece lens is to magnify the image so it appears larger to your eye. Eyepiece lenses can be either convex or concave.

Reflecting TelescopesA reflecting telescope is different from a refracting telescope in that it is constructed in such a way as to direct the flow of light onto a different path. It also uses two mirrors (primary and secondary) instead of an objective lens to gather the light and reflect it to a focal point. Instead of having an eyepiece at the opposite end of where the light enters the telescope, reflecting telescopes have an eyepiece lens on the side. Light enters and travels to the back of the telescope, where the primary mirror reflects it to the secondary mirror. The secondary mirror focuses the light to a focal point right in front of the eyepiece lens. The eyepiece lens magnifies the image onto the retina just like it does in a refracting telescope.

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LensesMicroscopesThe inventor of the optical microscope remains disputed, but the microscope appears to have been invented around the same time as the telescope in the early 17th century. Optical microscopes have one or more lenses and can work together to magnify objects up to 1250 times. They work in a manner similar to telescopes, with a few minor differences. Because telescopes collect light from dim objects that are far away, they require large objective lenses. Microscopes, on the other hand, only need to gather light from a small area that usually already has a light source, so it uses a very small objective lens. Then, just as with telescopes, an image is magnified by a second lens, called the eyepiece lens. Another variance from telescopes is that microscopes usually have a light source and a condenser, a tool to focus light on the exact area of the sample that will be magnified by the objective lens.

Have you ever gone to a restaurant and ordered a glass of water with a straw in it? When you look at the glass, the straw seems to be broken! Why does that happen? Write a paragraph and/or draw a diagram explaining what is happening in reference to the light. Hint: Think about the speed of light when it travels through different materials and what might happen when light moves from one material to another.

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Lenses

Since Hans Lippershey invented the first telescope in 1608, people have been making instruments that contain lenses for a variety of purposes—from telescopes to binoculars to spectacles. To help your child better understand the concept of lenses, create a lens together out of simple household materials. Materials: You will need a tennis ball, a serrated knife, vegetable oil, a small glass, distilled water, and a book. Procedure:• An adult should cut the tennis ball in half.• Pick one-half of the tennis ball and coat the inside with vegetable oil. • Place the half tennis ball in the glass (open side up) and make sure it is level.• Fill the tennis ball half to the top with distilled water.• Put the glass/tennis ball/water carefully in the freezer.• When the lens is frozen, pop it out of the tennis ball and run your warm hands over the flat part

of the lens to smooth it out.• Keep the curved side of the ice toward your eye and hold the lens over a page of the book.• Practice focusing the lens by moving it up and down, closer to and farther away from the page.• Try using both sides of the lens to view a variety of objects.• Hint: Try to get the lens out of the freezer as soon as it is thoroughly frozen because it may turn

cloudy and be hard to see through if it is left in the freezer for too long. Here are some questions to discuss with your child:

1. Why do you think it was necessary to smooth out the flat part of the lens?2. What did you notice when you held the ice lens over the book page?3. What happened when you “focused” the lens by moving it up and down over the book page?4. What did you observe when you tried both sides of the lens on a variety of objects around the

room?5. Why did the ice lens work?

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