Nureen and Kafiya: 13.3 Images in Lenses - Converging

Images in Lenses - Converging

The location and type of lens affect the image that is formed. In lenses, you can determine what the image will look like by using ray diagrams similar to the ones used for converging and diverging mirrors. Previously, we used reflected rays, however for determining images in lenses we use refracted rays. The incident rays enter the lens and emergent rays exit it, emergent rays are light rays that leave a lens after refraction. To learn more about converging lenses, click HERE.

   

To understand  how refraction works in a lens, we can look at a rectangular prism. In the image below, you will see that the incident ray is refracted twice, once between the air and glass which causes the light ray to bend towards the normal because air is less dense than glass. The second refraction occurs between the glass and air, causing the refracted ray from the first refraction to refract a second time, this time the light ray bends away from the normal, because glass is more dense than air, this causes the emergent ray that has just left the prism to be almost identical to the incident ray. This ray is slightly displaced, causing it to be parallel to the incident ray. The thinner the prism, the less the displacement is. To learn more about the refraction in converging lenses, click HERE.

How to Locate the Image in a Converging Lens

Terminology for Ray Diagrams:

2F’: twice the distance from the optical centre to the  secondary principal               focus

F’: secondary principal focus, this is on the side of the incident rays

O: point at the exact centre of the lens

2F : twice the distance of optical centre to principal focus

F: The point where all the refracted rays or emerging rays converge known as focal point or principal focus

There are 3 rules for locating images in converging lenses:

1. A ray parallel to the principal axis is refracted through the principal focus (F)

2. A ray through the secondary principal focus (F’) is refracted parallel to the principal axis

3. A ray through the optical centre (O) continues straight ahead without being refracted.

A ray going through the optical centre (O) does not refract because it is similar to a small rectangular prism in the centre of the lense with no noticeable displacement because the width of the lens is so small.

When the object is located beyond 2F’ it is smaller than when the object is placed between 2F and F,

as the object comes closer to 2F’, it gets larger until the object is at 2F’, then the image formed by the emergent rays is the same size as the original object and it is at 2F on the other side (but still inverted).

If you continue moving the object forward, it will be between 2F’ and F’ and the image will be larger and at 2F.

all of the mentioned positions are real and inverted. The image is only virtual when the object is inside F’, the refracted rays either diverge or spread apart, but our human brain traces these incident rays coming from the object straight backwards, forming a larger upright virtual image. 

No clear image is formed at F’ because the emergent rays are parallel to each other.

Location	Size	Attitude	Location	Type
Beyond 2F’	smaller	inverted	Between 2F & F	real
At 2F’	same size	inverted	At 2F	real
Beyond 2F’ and F’	larger	inverted	Beyond 2F	real
At F’	no clear image
Inside F’	larger	upright	Same size as object	virtual