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Ray diagrams

Ray diagrams can be used to work out the appearance of images created by lenses and whether they are real or virtual.

1) Draw the lens and the $$\Tgreen{\text{principal foci}}$$ on both sides of the lens, and mark the position of the $$\Tviolet{\text{object}}$$.

2) Draw $$\Tred{\text{one ray}}$$ from the object straight through the centre of the lens without it changing direction.

3) Draw a $$\Tblue{\text{second ray}}$$ parallel to the axis heading towards the lens. At the centre of the lens, it is refracted so that it passes through the principal focus.

How to construct a ray diagram
How to construct a ray diagram

A real $$\Torange{\text{image}}$$ is formed where the two lines meet. If the image is on the same side of the axis as the object it is upright. If the image is further from the axis than the object it is magnified.

If the two lines do not meet, a virtual image is formed. To draw it, extend the rays coming from the lens to the side of the lens where the object is found. This is how a magnifying glass works.