When you take an eye test at the doctor’s office, they use many lenses to find y…

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When you take an eye test at the doctor’s office, they use many lenses to find your prescription. Are these lenses cut differently so that to your eye, some objects (on the Snellen chart) look like virtual image and others are real?

The lenses that they place over your eye create virtual images that are closer or farther from your eyes than the object itself. Some lenses are converging (bending rays of light together) and these “magnifying” lenses form virtual images that are located farther away from you than the object itself. If you are farsighted (seeing distant objects well) you will be able to see a nearby object well through such glasses because you will see that object as more distant. Other lenses are diverging (bending rays of light apart) and these “demagnifying” lenses form virtual images that are located nearer to you than the object itself. If you are nearsighted (seeing nearby objects well) you will be able to see a distant object well through such glasses because you will see that object as nearer. If you vision is particularly poor, the lenses you need may not form virtual images at all but will still correct your vision. In that case, you do better to think of these lenses as joining together with the lens of your eye to form a single, image-forming lens. That combined lens works to form a real image on your retina. The other complication with eyeglasses is cylindrical correction (correction for astigmatism). Some people have lenses in their eyes that are not symmetrical and focus light differently up and down or left and right. A water glass is a cylindrical lens, focusing light horizontally but not vertically. To compensate for this cylindrical character, some eyeglasses have the opposite cylindrical character cut into them and rotated into the proper position.

When you look through the outer side of your eyeglasses, sometimes out of the co…

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When you look through the outer side of your eyeglasses, sometimes out of the corner of your eye, you can see a light star like all the light came together and lit up. Why do we see this? Where does it come from? Is it reflected light?

I’m not sure what effect you are observing. I do not see it myself. However, different types of lenses behave differently, so my nearsighted correction may not behave the same way yours does. There is certainly a reflection problem in some glasses. Although the main beams of light passing through the lens are handled well, internal reflections or reflections from behind the lens are not handled properly. They can form strange patterns of light on your retina, such as the light star you mention.

What would happen if a magnifying glass is set at the end of a telescope? How wo…

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What would happen if a magnifying glass is set at the end of a telescope? How would the stars appear?

You could place the magnifying glass at one of two spots: at the entrance to the telescope or at the eyepiece of the telescope. If you put it at the entrance, it would bend the light before it had a chance to reach the main optic for the telescope. The effect would be to increase the light bending ability of the main optic and reduce the lens’s focal length. This change would make it difficult to focus the telescope on distant objects, such as stars. The images of these distant objects would form too close to the main optic and you would have trouble observing them through the telescope’s eyepiece. But very nearby objects form real images farther from the main optic. The magnifying glass would help the main optic form real images of very nearby objects. It would act as a close-up lens. That is what close-up lens attachments for cameras or even cheap reading glasses do: they help the camera lens or your eye form an image of very nearby objects. On the other hand, a magnifying glass held over the eyepiece of a telescope would increase the power of the telescope. You would have to adjust the focus of the telescope because the added magnifying glass will reduce the effective focal length of the eyepiece. The new super eyepiece will have to be placed closer to the real image formed by the main optic of the telescope. When it is place properly, it will give you a very highly magnified view of that real image, so you will see a highly magnified view of the stars.

What is the difference between real images and virtual images?

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What is the difference between real images and virtual images?

A real image is a pattern of light in space that you can touch or put a piece of paper in. When you insert the paper in this light pattern, it appears just like the scene that created it, although it is typically flipped upside-down. A virtual image is an image that you cannot touch. As you look into the optic that creates this virtual image, you can see the image as though it were a pattern of light in space, but that pattern of light is located on the opposite side of the optic, where you cannot touch it. Subsequent optical devices (including the lens of your eye) can study this virtual image and form new images of it, but you can’t put a piece of film in the virtual image itself.

What is the difference between object distance and focal length?

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What is the difference between object distance and focal length?

The object distance is simply a measure of the distance between the object and the lens. The image distance is a measure of the distance between the lens and the image that it forms. A positive number for the image distance means that a real image forms. A negative number for the image distance means that a virtual image forms (on the same side of the lens as the object). The image distance depends on both the object distance and the focal length of the lens. The focal length of the lens is a characteristic of the lens itself and doesn’t change as the object and image distances change. The focal length is equal to the image distance when the object is very, very distant (e.g. a star). A positive focal length lens (a converging lens) forms a real image of the star at a distance from the lens equal to its focal length. A negative focal length lens (a diverging lens) forms a virtual image of the star at a distance from the lens equal to its focal length.

What happened to the Hubble mirror?

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What happened to the Hubble mirror?

The mirror of the Hubble space telescope was ground with the aid of a flawed measuring device. Although the mirror was perfectly ground, it was given the wrong curvature and thus did not form a clear image at its focus. Light from one star that hit different points on the mirror did not converge to a single point on the imaging chip. To correct for this problem, the astronauts inserted a corrective optic into the path of the light. This refractive lens compensates for the incorrect convergence of the light so that it reaches a single point on the imaging chip. However, because it is a refractive optic, it cannot pass all wavelengths of light. Any light that is absorbed by the refractive optic is no longer measurable with the telescope.

Diffraction: I would have thought that the waves wouldn’t go through the screen …

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Diffraction: I would have thought that the waves wouldn’t go through the screen because the wave was too long to recognize the holes in it. How did the light go through the screen?

When I sent laser light through a fine screen, it formed an interesting diffraction pattern on a distant wall. The holes in the screen were small, but not nearly as small as a wavelength of light. The light had no trouble going through these holes, but it did suffer diffraction effects. Because the wave passed through many separate holes, these waves interfered with one another and created the complicated pattern on the wall.

Why is film ruined when it is exposed to light?

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Why is film ruined when it is exposed to light?

Photographic film chemically records information about the light that it has absorbed. Normally, this light was projected on it by a lens and formed a clear, sharp pattern of the scene in front of the camera. However, if light strikes the film uniformly, the information recorded on the film will have nothing to do with an image. The entire sheet of film will record intense exposure to light and will have no structure on its chemical record.

Why do people in flash pictures have “red eye”? How do cameras try to solve th…

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Why do people in flash pictures have “red eye”? How do cameras try to solve that problem?

When light from the flash illuminates people’s eyes, that light focuses onto small spots on their retinas. Most of the light is absorbed, by a small amount of red light reflects. Because the lens focused light from the flash onto a particular spot on the retina, the returning light is focused directly back toward the flash. The camera records this returning red light and eyes appear bright red. To reduce the effect, some flashes emit an early pulse of light. People’s pupils shrink in response to this light and allow less light to go into and out of their eyes. Professional photographers often mount their flashes a foot or more from the lens so that the back-reflected red light that returns toward the flash misses the lens.

Why do camera flashes make eyes red and why do two flashes correct this problem?

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Why do camera flashes make eyes red and why do two flashes correct this problem?

The retinas of your eyes appear reddish when you look at them with white light. The red eye problem occurs because light from the flash passes through the lens of your eye, strikes the retina (which allows you to see the flash), and reflects back toward the camera. This reflection is mostly red light and it is directed very strongly back toward the camera. The camera captures this red reflection very effectively and so eyes appear red. The double flash is meant to get the irises of your eyes to contract (as they do whenever your eyes are exposed to bright light or you are startled or excited). The first flash causes your irises to contract so that less light from the second flash can pass into and out of your eyes. Unfortunately, this trick doesn’t work all that well.