Lenses & the EyeOxford AQA IGCSE Physics: Revision notes

A lens forms an image by refracting light as it passes through it.

There are two types of lens, which affect light in opposite ways:

• Convex (converging) lens — bulges outwards; parallel rays passing through are brought together to a single point, the principal focus
• Concave (diverging) lens — curves inwards; parallel rays passing through spread out (diverge) as if they came from the principal focus on the same side as the incoming rays

The focal length is the distance from the centre of the lens to the principal focus.

Images are described using three properties:

• Size — relative to the object (magnified, same size, or diminished)
• Orientation — upright or inverted
• Type — real (light rays actually meet, can be projected on a screen) or virtual (rays only appear to meet, cannot be projected)

For a convex lens, the image formed depends on the object's distance from the lens:

• Object beyond 2 × focal length: real, inverted, diminished image
• Object at 2 × focal length: real, inverted, same size image
• Object between focal length and 2 × focal length: real, inverted, magnified image
• Object within the focal length: virtual, upright, magnified image (this is how a convex lens works as a magnifying glass)

A concave lens always forms a virtual, upright, diminished image, regardless of object distance.

Ray diagrams for lenses use two or three standard construction rays, drawn on graph paper:

1. A ray from the top of the object, travelling parallel to the axis, which refracts through the principal focus on the far side (convex) or appears to come from the principal focus on the near side (concave)
2. A ray from the top of the object passing straight through the centre of the lens, undeviated
3. Where these rays meet (or appear to meet) marks the position of the top of the image

Magnification is calculated using:

magnification = image height ÷ object height

A magnification greater than 1 means the image is enlarged; less than 1 means it is diminished.

The eye contains several key structures, each with a specific function:

• Cornea — refracts light as it enters the eye
• Pupil/iris — the iris controls the size of the pupil, the opening that controls how much light enters
• Variable focus lens — fine-tunes the focusing of light onto the retina
• Ciliary muscle and suspensory ligaments — work together to change the shape of the lens
• Retina — the light-sensitive surface at the back of the eye where the image forms

When the ciliary muscle contracts, the suspensory ligaments slacken, allowing the lens to become fatter (more curved) to focus on near objects. When the ciliary muscle relaxes, the suspensory ligaments tighten, pulling the lens thinner (flatter) to focus on distant objects.

The near point is the closest distance at which the eye can focus clearly, approximately 25 cm. The far point is the furthest distance the eye can focus, at infinity. Together these define the normal range of vision.

• Long sight — occurs when the eyeball is too short, or the lens is unable to focus enough; close objects cannot be focused. Corrected using a convex lens, which adds extra converging power
• Short sight — occurs when the eyeball is too long, or the lens is unable to focus enough; distant objects cannot be focused. Corrected using a concave lens, which adds extra diverging power to spread the light out before it enters the eye

Both the eye and a camera use a lens to focus light onto a light-sensitive surface, but they focus in different ways:

• In the eye, focusing is achieved by changing the shape of the lens (via the ciliary muscle)
• In a camera, focusing is achieved by varying the distance between the lens and the film or CCD sensor

Lasers are also used in applications involving focused light, including cutting, cauterising (sealing tissue with heat), and eye surgery. (The detailed physics of how lasers work is not required.)