What happens when you move the lens away from the picture?

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First, it is important to realize that lenses only focus at one distance clearly. That is why, when you use a pair of binoculars, you have to ajdust the focus as you look from near to far. In the eye, we need to be able tofoxue as well; we something that can ajust its focus.

The Lens (also called the Crystalline Lens) is the adjustable focusing element of the eye. It is located just behind the iris. This process of adjusting the focus for different distances by changing the shape of the lens is called accommodation. Accommodation is the process of adjusting the lens of the eye so that you can see both near and far objects clearly. This process is very rapid although changing accommodation from a near object to a far object is faster than going from a far object to a near object (Kirchhof, 1950). Accommodation is controlled by muscles connected to the lens, called ciliary muscles. The ciliary muscles work automatically without conscious control. The ciliary muscles can contract and increase the curvature of the lens so that the lens thickens. The increased curvature of the lens allows the eye to focus on a close object. When the person then has to look at a faraway object, the muscles relax and the focus of the lens changes to an object further away. Look at an object close to you, such as the text of the book. Then look up and look out the window and across the street. As you do, the process of accommodation automatically adjusts your focus.

Use this activity to explore how accommodation helps us keep objects at different distances in focus.

To see the illustration in full screen, which is recommended, press the Full Screen button, which appears at the top of the page.

Illustration Tab

Settings

Below is a list of the ways that you can alter the illustration. The settings include the following:

Light Position: moves the light closer or farther from the eye.
Turn Light On: pressing this button will start the light. Pressing the button again removes the light.
Eye Accommodates: when checked, the lens of the eye accommodates, or adjusts size, to keep the light in focus. When not selected, the eye does not accommodate and only one distance from the eye will be in focus.

Reset

Pressing this button restores the settings to their default values.

Introduction

The power of an unknown lens can be determined by neutralising it with another lens of known power.  Neutralisation is based on the fact that if you look at an object through a convex or concave lens and move the lens from side to side (right and left or up and down), the image that you see through the lens will also move.  If the lens has no power - or has been neutralised by placing a lens of equal power but opposite sign against the unknown lens - then there is no image movement.

Movements

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1. A plus lens moves the image in the opposite direction to the lens:

• If you move the lens to the right, the image will move to the left.
• If you move the lens up, the image will move down, and vice versa.

2.  A minus lens moves the image in the same direction to the lens:

• If you move the lens to the right, the image moves right.
• If you move the lens up, the image moves up, etc.

3. A sphero-cylindrical lens causes scissoring movements on rotation:

• If you rotate the lens about its optical centre the image of a cross will appear to scissor.
• The principle meridia are determined by marking the orientation at which the cross lines are perpendicular.

4.  A lens with no power causes no movement

• Regardless of lens movement, the image remains stationary.

5. A lens with a ground prism will displace the cross image as follows:

• Base up: displaces image downwards
• Base down: displaces image upwards
• Base in: displaces image outwards
• Base out: displaces image inwards

Steps in Neutralisation

1. Draw a cross.  On a piece of paper draw a cross with lines perpendicular to each other and at least 15cm long.

2. Determine principle meridia.  Look at the cross through the lens - the further away the cross is from you during this task the better e.g the other end of the room.  Rotate the lens to determine if their is astigmatism by observing for scissoring movements of the cross image.  If there are no scissoring movements, the lens is spherical (there are no principle meridia).  If scissoring movements are present, then rotate the lens such that the cross lines of the image are exactly perpendicular - that is, they line up with the cross lines of the object cross.  This is the orientation of the principle meridia of the lens. You can now mark the principle meridia on the lens with spots using a felt-tip pen.  Keep the lens in this orientation for the remainder of the task.

3. Determine the optical centre.  Hold the lens as in step 2 above: that is, with the cross lines of the image perpendicular to each other.  Now move the lens up-and-down or side-to-side to ensure that the lines of the cross image exactly overlap with the lines of the object cross. Held in this position, the point at which the cross lines intersect on the lens is the optical centre of the lens.  Mark the optical centre with a felt tip pen. Neutralisation of the lens should occur at the optical centre.  Note: if, despite up-and-down or side-to-side movements of the lens, it is not possible for the image and object lines of the cross to be aligned, then there is a ground prism in the lens.   Neutralise the ground prism using a prism of equal power and opposite direction, to bring the crosses into alignment. 

With movement:  Minus lens
Against movement: Plus lens

4. Neutralise each meridian.  Holding the lens in orientation with its principle meridia horizontal and vertical as in step 3, move the lens from side to side.   If there is with movement, the lens is minus along this meridian, and you require a plus lens to neutralise.  If there is against movement, the lens is plus along this meridian and you require a minus lens to neutralise.  Using a bracketing technique, find the lens that achieves neutralisation along this meridian (ie no movement) and remember that the power of the unknown lens is the same size but opposite in sign to the neutralising lens.   Once the horizontal meridian is neutralised, you can move the lens up and down to neutralise the vertical merdian or vice versa.

5. Draw a power cross.  The power of the unknown lens along the merdian tested is the same as the power of the neutralising lens but opposite in sign. Thus, for a lens with merdia at 90 degrees and 180 degrees, if a +2.00D lens was required to neutralise the vertical meridian, then the power in the vertical merdian (90 degrees) is -2.00D.   And if a +3.00 lens was required to neutralise the horizontal merdian then the power of the lens in the horizontal merdian is -3.00 (at 180 degrees). 

6. Convert to sphero-cylindrical formula.  Convert your power cross into a spherocylindrical formula in the standard manner (see tip below).  Thus for the power cross in the example above (-2.00D at 90 and -3.00D at 180), the spherocylindrical formula is: -2.00D/-1.00x 90

Hints & Tips: Conversion of Power Cross to Spherocylindrical formula: 
(the power of the more positive meridian) / - (difference between meridia) x (angle of more positive meridian)

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A convex lens is thicker at the centre than at the edges.

Convex lenses are thicker at the middle. Rays of light that pass through the lens are brought closer together (they converge). A convex lens is a converging lens. When parallel rays of light pass through a convex lens the refracted rays converge at one point called the principal focus. The distance between the principal focus and the centre of the lens is called the focal length.

Use of Convex Lenses – The Camera

A camera consists of three main parts.

1. The body which is light tight and contains all the mechanical parts.
2. The lens which is a convex (converging) lens).
3. The film or a charged couple device in the case of a digital camera.

The rays of light from the person are converged by the convex lens forming an image on the film or charged couple device in the case of a digital camera. The angle at which the light enters the lens depends on the distance of the object from the lens. If the object is close to the lens the light rays enter at a sharper angled. This results in the rays converging away from the lens. As the lens can only bend the light to a certain agree the image needs to be focussed in order to form on the film. This is achieved by moving the lens away from the film. Similarly, if the object is away from the lens the rays enter at a wider angle. This results in the rays being refracted at a sharper angle and the image forming closer to the lens. In this case the lens needs to be positioned closer to the film to get a focused image. Thus the real image of a closer object forms further away from the lens than the real image of a distant object and the action of focusing is the moving of the lens to get the real image to fall on the film. The image formed is said to be real because the rays of lighted from the object pass through the film and inverted (upside down).

The Magnifying Glass

A magnifying glass is a convex lens which produces a magnified (larger) image of an object.

A magnifying glass produces an upright, magnified virtual image. The virtual image produced is on the same side of the lens as the object. For a magnified image to be observed the distance between the object and the lens must be shorter than the focal length of the lens.

A magnifying glass is a convex lens which produces a magnified image of an object.

Magnification

The magnification of a lens can be calculated using the following formula;

As this is a ratio between heights it has no units. A magnification of 2 means the image is twice the size of the object and a magnification of 1 indicates an image size being the same as the object size.

Concave Lens

A concave lens is thinner at the centre than at the edges.

Concave lenses are thinner at the middle. Rays of light that pass through the lens are spread out (they diverge). A concave lens is a diverging lens. When parallel rays of light pass through a concave lens the refracted rays diverge so that they appear to come from one point called the principal focus. The distance between the principal focus and the centre of the lens is called the focal length. The image formed is virtual and diminished (smaller)