Thin lens action

A thin lens changes the direction of light passing through it by refraction. The way the light bends depends on the type of lens.

The principal axis is the straight line through the lens’s optical centre, perpendicular to the lens. For rays initially parallel to this axis, the point towards which the lens directs them, or from which they appear to spread, is called the principal focus or focal point. The focal length is the distance from the optical centre to the principal focus.

Converging lens

A converging lens bends rays initially parallel to the principal axis towards one another. The rays meet at the principal focus on the opposite side of the lens from the incoming light.

Diverging lens

A diverging lens bends rays initially parallel to the principal axis away from one another. The emerging rays do not meet at the focus. Instead, if they are traced backwards as straight lines, they appear to come from the principal focus on the same side of the lens as the incoming light.

Real-image ray diagrams for converging lenses

A converging lens bends light rays so that rays from a point on an object can meet again on the other side of the lens. A real image forms where the rays themselves meet. A ray diagram locates this image by tracing selected rays from the object through the lens.

The principal axis is the straight line through the optical centre of the lens. The lens has a principal focus on each side, at a distance from the lens called the focal length. A ray travelling parallel to the principal axis is refracted through the principal focus on the far side of the lens.

Drawing the rays and locating the image

For an object farther from the lens than its focal length, draw the diagram to scale:

1. Draw the lens, the principal axis and the two focal points. Draw the object as an arrow perpendicular to the axis, with its base on the axis.
2. From the tip of the object, draw a ray parallel to the principal axis to the lens. From the lens, draw the refracted ray through the principal focus on the far side.
3. From the same object tip, draw a second ray through the optical centre of the lens. Continue it approximately straight through the lens.
4. Mark the point where the two rays meet on the far side. This is the image of the object’s tip. Draw an arrow from the principal axis to this point to represent the image.

The image arrow points in the opposite direction to the object arrow, so the real image is inverted. Its distance from the lens and its height on the scale drawing give the image’s position and size.

The rays used in the diagram start at one point: the tip of the object. Rays from each other point on the object also converge at a corresponding image point. Together, these image points form the complete real image.

Image characteristics

An image can be described by comparing it with the object and by considering how the light rays form it. The main characteristics are size, orientation and whether the image is real or virtual.

Image size and orientation

An image is enlarged if it is larger than the object, the same size if it is equal in size, and diminished if it is smaller.

Its orientation describes which way up it is relative to the object. An upright image has the same orientation as the object; an inverted image is upside down relative to it.

Real and virtual images

A real image forms where light rays actually converge, or meet. A screen placed at the image position intercepts the rays, producing a visible projection of the image on the screen.

A virtual image forms at a position from which diverging rays appear to come. If the rays are extended backwards as imaginary lines, those lines meet at the virtual image position. The actual rays do not meet there, so the virtual image cannot be projected visibly onto a screen.

Correction of long- and short-sightedness

For an object to appear sharp, light from it must be focused on the retina, the light-sensitive surface at the back of the eye. Corrective lenses change the direction of light before it enters the eye, so that the eye can focus it on the retina.

Correcting long-sightedness

A long-sighted person has difficulty focusing clearly on nearby objects. Light from a near object would come to a focus behind the retina, so it has not yet been brought to a sharp focus when it reaches the retina.

A converging lens bends the incoming light towards the principal axis before it enters the eye. This helps the eye bring the light to a focus sooner, on the retina rather than behind it.

Correcting short-sightedness

A short-sighted person has difficulty focusing clearly on distant objects. Light from a distant object is brought to a focus in front of the retina and then spreads out again before reaching it.

A diverging lens spreads the incoming light before it enters the eye. The eye then brings the light to a focus farther back, on the retina rather than in front of it.