# Properties of concave mirror used for correcting myopia.

#### avito009

Properties of concave lens used for correcting myopia.

Lets try to apply physics practically. I got my new pair of glasses with concave lens used to correct myopia. So I observed something and was curious to know about it.

In the beginning for a couple of days when i wore the glasses, nearby objects like the floor or steps appeared to be closer when viewed through these glasses. So in the very beginning it was difficult to judge the actual position of the steps. They appeared closer. But now I have got used to it so it is not a problem.

So as per my knowledge when an image is moved closer to a concave lens, it appears closer and the object appears to be smaller (Virtual Property). Is this property of concave lens responsible for this situation where the floor or steps appeared to be closer?

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Mirror?

#### Woody

I assume you mean lens rather than mirror, however, regardless of that...

There are several clues which our brains use to determine how far away an object we can see is.

Parallax, (the difference between what the right and left eye sees) is the one used in 3D cinemas and 3D computer games.
However the level of tension required in the muscles used to reshape the lens in our eyes to bring an object correctly into focus is also used to give an indication of distance.
(It is the dichotomy between this focus signal and the parallax signal that gives some people headaches in 3D cinemas)

The revised prescription in your new spectacles will change the amount of tension your eye muscles require to focus at a particular distance,
and your brain will take some time to re-calibrate itself to correlate the new tension to the correct distance.

#### avito009

I assume you mean lens rather than mirror, however, regardless of that...

There are several clues which our brains use to determine how far away an object we can see is.

Parallax, (the difference between what the right and left eye sees) is the one used in 3D cinemas and 3D computer games.
However the level of tension required in the muscles used to reshape the lens in our eyes to bring an object correctly into focus is also used to give an indication of distance.
(It is the dichotomy between this focus signal and the parallax signal that gives some people headaches in 3D cinemas)

The revised prescription in your new spectacles will change the amount of tension your eye muscles require to focus at a particular distance,
and your brain will take some time to re-calibrate itself to correlate the new tension to the correct distance.
You said that a person judges the distance because of parallax, the difference between what the right and left eye sees. But what about a one eyed person. This person is also able to judge the distance isn't he?

#### Woody

In the land of the blind

A one eyed person has to rely on other depth perception cues
Like the focus perception
or the way distance changes the way an object moves in the field of view as you move your head.

parallax is an important cue to depth perception, but not the only cue.

A one eyed person would not be able to see the 3D effect in a 3D cinema.

#### HallsofIvy

You said that a person judges the distance because of parallax, the difference between what the right and left eye sees. But what about a one eyed person. This person is also able to judge the distance isn't he?
Woody said, in his first post, "there are several clues which our brains use to determine how far away an object we can see is." Parallax is one of them. Another is comparing the sizes of things we are familiar with.

Many years ago, I suffered a mild stroke which left me with "third nerve palsy". One of the nerves that controls the motion of my left eye does not work properly so that I cannot use my two eyes together- I see two images, one slightly above the other. One of the things that means is that I have difficulty judging distance, especially of things relatively close. I have to be extremely careful walking on rough ground and very very careful walking down steps!

#### avito009

Wellbeing

Woody said, in his first post, "there are several clues which our brains use to determine how far away an object we can see is." Parallax is one of them. Another is comparing the sizes of things we are familiar with.

Many years ago, I suffered a mild stroke which left me with "third nerve palsy". One of the nerves that controls the motion of my left eye does not work properly so that I cannot use my two eyes together- I see two images, one slightly above the other. One of the things that means is that I have difficulty judging distance, especially of things relatively close. I have to be extremely careful walking on rough ground and very very careful walking down steps!
HallsofIvy, Sorry to hear about that! Are you still suffering? Is there a cure for your illness? By the way thanks for the answer!

#### avito009

Parallax explanation.

Now that we have talked about parallax I think an illustration as to how distance is measured using the parallax should be mentioned here. So here goes:

When an object (Chair) is viewed from both eyes at the same time. So the distance between right eye and left eye is the baseline which is lets say 204.2 cm.

Now,

Angle between right eye and the centre of the chair is say 87.2 degrees which is Angle B.

Angle between left eye and the centre of the chair is say 85.4 degrees which is Angle C.

Now Compute Angle A

180 degrees= A + B+ C
180 degrees= A + 87.2+85.4
A= 7.4 degrees

Now you are prepared to find sine A and sine B. Use your calculator.
sine A = sine 7.4 degrees = 0.128796
sine B = sine 87.2 degrees = 0.9988

Distance= Sine B x Baseline/Sine A

Substitute

Distance = 0.9988 x 204.2 cm/ 0.128796

Distance= 1583.565 cms

Therefore the chair is 1583.565 cms away from the person observing the chair.

Woody, Is this correct?

#### Woody

You seem to be going a strange route to get to the answer
and my eyes are not about 200cm apart (more like 100mm).

For an object at a distance of 1.5 metres the angle will be
atan(50/1500) which is about 2 degrees.

This imagines a right angle triangle from one of my eyes to the bridge of my nose and out to the object.
The angle given is the angle of the point of the triangle closest to the object.

You can see the effect if you focus on an object close to you, but then note the appearance of an object further away (just behind your foreground object).
You should notice that (as well as being blurry & out of focus) the further object appears doubled (two blurry & out of focus images)
Allow your focus to move to the background object.
You should now notice that the foreground object is the one producing two blurry & out of focus images.

Try repeating the experiment with one eye closed.