Waves: Images and Ray Diagrams

Types of Images

An image produced by a mirror or lens can be either real or virtual.
A real image is formed where light from an object comes together (converges) to form a visible reproduction of an object. They can be projected onto a screen.
A virtual image is formed when light rays do not actually meet but appear to meet when traced back. This type of image cannot be projected onto a screen.

Ray diagrams are graphical representations used to illustrate the path of light and show the formation of images by mirrors and lenses.
Diagrams should always be drawn using a ruler and should be as accurate as possible to get a reliable result.

In a plane mirror, the image formed is virtual, upright, and the same size as the object.
To draw a ray diagram for a plane mirror, place the object in front of the mirror and draw the incident ray from the top of the object to the mirror. Then, draw the reflected ray from the mirror to the eye, making sure to show that the angles of incidence and reflection are equal.
The object and image distances from the mirror are equal.

Convex mirrors diverge light rays and form a smaller, upright, virtual image.
When creating a ray diagram for a convex mirror, two rays are typically drawn: one parallel to the axis (which gets reflected as if it has come from the focal point) and another one appearing to originate from the focal point (which gets reflected parallel to the axis).
The image appears to be located behind the mirror.

Concave mirrors converge light rays to form either a virtual or real image depending on the object’s position relative to the focal point.
To draw a ray diagram for a concave mirror, two rays are drawn: one parallel to the axis (which gets reflected through the focal point) and another one through the focal point (which gets reflected parallel to the axis).
If the object is within the focal length of the mirror, the image formed is virtual, upright and magnified. If the object is at or beyond the focal point, the created image is real, inverted, and can be magnified or reduced.

The same rules apply for ray diagrams with convex and concave lenses. A convex lens converges light and can form virtual or real images, whilst a concave lens only forms virtual, reduced, upright images.
One critical difference in the rules for drawing ray diagrams for lenses vs mirrors is that for lenses, rays that pass through the focal point before reaching the lens are refracted parallel to the axis on exit. And rays that arrive parallel to the axis are refracted so they pass through the focal point on exit.

Remember: Practice is key! Draw these diagrams multiple times until you’re comfortable with them.