Question

Can someone clear up these confusions re the eye and vision for me?

- When a virtual image is formed through the use of artificial lens (glasses) does this means that simultaneously a real image is formed on the retina? Or is it that it is possible to

see some images with the aid of lens by bypassing the retina?

- On the principal axis, why is the 'far point' of a person suffering from hyperopia considered negative?

Answer 1

retina is a light sensitive screen on the back of the eye ball, which recieves the light from the surroundings after it has been focussed by the eye lens. To be able to see an object the light from it must reach the retina because it is in the retina where the incoming light is converted into electrical signals and sent to the brain. There is simply no way , we can see an object artificially by bypassing the retina. The eye lens is a biconvex lens whose focal length can be adjusted variably with the help of ciliary muscles, this biconvex lens captures all the light from the surroundings and forms a real and inverted image on the retina. it is important to note that only a real image can be captured on any kind of screen.

Now the question arises, when and why we need artificial lenses. the power of accomodation of the human eye allows a human to see distinctly with an average near point of 25 cm and far point at infinity. This means a normal human eye can form fully focused clear images of objects situated anywhere between 25 cm from the eyes and infinity. At a distance less than 25 cm, the image formed on the retina starts getting hazy, we can still see, the light is still reaching the retina but it isnt well focused for us to see the object clearly. sometimes the near point and far point shifts from their usual positions due to age or other medical conditions. take for example, hypermetropia(far sightedness). in hypermetropia the near point of the eye shifts farther, and nearby objects appear hazy. to fix this condition we need a converging len which can make the objects present at the near point of normal human eye virtually appear at the near point of the hypermetropic eye. So the lens acts as an aid to vision , and in no way is bypassing the retina.

As per the cartesian sign convention for lenses, the distances of objects or images on the left side of the lens are taken to be negative, while the distances of objects or images on the right side are taken to be positive. in hyperopia, the near point shifts farther or towards the left, hence becomes more negative. while solving numericals to find the focal length of corrective lenses, the distance of object and its virtual image formed on the right side of the eye lens is taken to be negative.