Ray Diagrams & Images

A ray is a line with an arrow showing the direction light travels. You already know the law of reflection: both angles are measured from the normal and are equal. To find a plane mirror image, draw two rays from one object point and reflect them using this law. Extend the reflected rays backward with dashed lines. Their extensions meet behind the mirror. This is a virtual image: the reflected rays do not travel there.

A plane mirror image is upright and the same size as the object. It is as far behind the mirror as the object is in front, measured perpendicular to the mirror. For example, an object 1 metre in front has its image 1 metre behind.

The axis passes through the lens centre at a right angle to the lens line. For an ideal thin converging lens in air, rays parallel to the axis meet at the far focal point. Focal length, f, is the distance from the centre to either focus. Trace two rays from the same object point: one parallel to the axis, then through the far focus; one straight through the centre. This model treats the lens as one line where rays bend.

If the object is farther from a converging lens than its focal length, rays from each object point meet on the opposite side. The image is real and inverted, so it can form on a screen there. Between f and 2f, the image is larger; at 2f, it is the same size; beyond 2f, it is smaller. Measure these object distances from the lens along the axis.

Magnified means larger; diminished means smaller. To calculate image height, multiply object height by the stated size factor. For example, an image three times as tall as a 4 cm object has height 3 x 4 = 12 cm. Height here means size, whether the image is upright or inverted.

If the object is between a converging lens and its near focal point, the outgoing rays spread apart. Extend them backward to find a virtual image on the object's side. It is upright and magnified, as with a magnifying glass over a word. A screen cannot receive this image directly because the outgoing rays do not meet there. At exactly f, the outgoing rays are parallel and no image forms at a finite distance.

A camera can be modelled using a converging lens and a sensor. A distant tree, well beyond 2f, produces a real, inverted, diminished image on the opposite side of the lens. The sensor must be at the image position to record a sharp picture. For a very distant object, this position is close to the far focal point.