# Ray Optics: Image formation

1. Oct 21, 2015

### Prannoy Mehta

In most (perhaps all) books it was mentioned that intersection of the light from the object when reflected by the mirror or refracted by a lens intersect at a particular point. That point when traced till the principal axis gives the image. (Provided object lies on the axis). I do not understand why is there a image formed when two (refracted/reflected) rays intersect.

2. Oct 21, 2015

### Drakkith

Staff Emeritus
In order to form an image, the light emitted from any single point on an object needs to be focused down to a single point on the focal plane (note that the focal point is actually an entire focal plane, with points from off the axis being focused down to their own focal points also off the axis). If the light rays are spread out when they should be intersecting, as is the case with poor quality optics or aberrations in your eye, the resulting image becomes blurred because light from one point on an object intersects with light rays from a different point on the object when the light reaches the focal plane. If one of these two points was supposed to be dark, then the light from the other point has the effect of lightening the first point, perhaps so much so that the contrast between the two points is completely erased. If one point was a different color than the other, then you have multiple colors overlapping and you lose color detail. Mix both of these together and you get a blurry image with little to no contrast. If bad enough, you have failed to form a recognizable image.

3. Oct 22, 2015

### mukundpa

I think you want a lower level explanation.
The rays from an object are diverging and when they enter our eye we can see the object. When the rays from a point object after reflection or reflection meets at a point or appear to come from a point other then object and enters eye that give an illusion that the object is at that point. This illusion is called an image. Actually all the rays after reflection or refraction are passing through the image point but as only two intersecting lines are sufficient to define the position of a point, we trace only two rays to get the position of image. All other rays will also pass through the same point.

4. Oct 22, 2015

### Prannoy Mehta

I understood Drakkith's explanation Mukundpa. Thank you for the assistance :)

Drakkith, your explanation implies that there are infinite number of images formed. Is it right ?

5. Oct 22, 2015

### Drakkith

Staff Emeritus
I'm not sure what the technical definition of an image is, but I like to think of there being only a single image that is formed when rays from many different points in front of the optical system are focused down to their appropriate spots on the focal plane.