## What happened to the Hubble mirror?

#### 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.

## What is the difference between object distance and focal length?

#### 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 is the difference between real images and virtual images?

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

#### 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.

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

#### 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.

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

#### 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.

## Why a spoon will allow one to “appear” up-side down on one side and right-side…

#### Why a spoon will allow one to “appear” up-side down on one side and right-side up on the other side?

A spoon forms an inverted real image of you when you look into the concave (hollow) side. This real image is located a few centimeters in front of the spoon, where you can touch it with your finger or insert a small piece of paper into it. Try it, you will see the pattern of light appear on the paper sliver. A spoon forms a right side up virtual image of you when you look into the convex (bowed outward) side. This virtual image is located a few centimeters behind the spoon. You appear very small because it is a small virtual image that you are looking at. You cannot touch this virtual image.

## Why do virtual images often look far away?

#### Why do virtual images often look far away?

A virtual image is always located behind the optic (lens or mirror) that creates it. Thus when you look into a magnifying glass, eyepiece, or a make-up mirror, you see light that appears to come from beyond the optic that creates the image. You can’t touch the virtual image or put your hand in the pattern of light that you seem to see. The virtual image can appear to come from just behind the optic or from a great distance behind that optic. It depends on how things are arranged. As you lift a magnifying glass off the surface of a newspaper, the virtual image of the newspaper starts just behind the glass and slowly moves back away from the glass. As the distance between the magnifying glass and newspaper approach the magnifying glass’s focal length, the virtual image moves away to an infinite distance behind the glass. After that, there is no longer a virtual image at all. Instead, a real image begins to appear on the other side of the magnifying glass.

## Why is it that images are right side up (instead of upside-down) when looking th…

#### Why is it that images are right side up (instead of upside-down) when looking through a magnifying glass?

In forming a real image, a camera lens behaves symmetrically, taking light reaching it from above its central axis and projecting that light onto a spot below its central axis. But in forming a virtual image, a magnifying lens merely redirects the light subtly to have it appear to come from a point nearer or farther than the original object. You still see the object as it was (right-side up) but moved toward you or away from you.