The big ideaEvery ray diagram in this lesson uses the same two moves: one ray drawn parallel to the axis that bends through the focal point, and one ray drawn straight through the lens's center (or the mirror's center of curvature) undeviated. Where those rays cross, or where their backward extensions cross, is the image.
Two rays are all you ever need
The ray diagrams look intimidating because there's a different rule for lenses, mirrors, inside F, and outside F. There are really only two moves, reused four ways.
Not sure about the basics underneath this lesson?
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Move 1: parallel, then through F
Draw a ray from the top of the object straight across, parallel to the axis, until it hits the lens or mirror. Then bend it through the focal point (for a lens, the far one; for a mirror, the same-side one).
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Move 2: through the center, undeviated
For a lens, draw a ray from the top of the object straight through the exact center, it doesn't bend at all. For a mirror, aim a ray at the center of curvature (C) instead, since a ray hitting a curved mirror perpendicular to its surface reflects straight back the way it came.
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Where they cross is the image
If the two rays actually cross after the lens or in front of the mirror, that's a real image right there. If instead they spread apart, extend both backward with dashed lines until they cross behind, that crossing point is a virtual image.
Optional interactive:PhET Geometric Optics lets you move the object across F and 2F. Draw two rays first, then use the model to check them rather than copying its answer.
Watch someone else explain it
Physics Classroom: Converging-Lens Ray Diagrams — Optional worked visual for the principal-ray rules. Pause before each ray and predict its path yourself.
Got it now? Scroll back up and re-read the section — it should land differently.
Shared vocabulary for lenses and mirrors
The principal axis is the horizontal reference line running through the center of a lens or mirror. The focal point (F) is where parallel rays converge after passing through a converging lens or reflecting off a converging mirror. The object is drawn as a vertical arrow so you can compare its orientation to the image's. A convex (converging) lens is thickest in the middle and bends parallel rays together; a concave (converging) mirror curves away from the object and reflects parallel rays together. This lesson only traces converging lenses and converging mirrors.
Solid principal rays meet at real-image tips; dashed backward constructions meet at virtual-image tips.
Ray diagram: converging lens, object beyond 2F
Two rays leave the top of the object. One travels parallel to the axis, then bends through the far focal point. One passes straight through the lens's center, undeviated. Where they cross on the far side locates the image.
Ray diagram: converging lens, object inside F
When the object sits closer to the lens than the focal point, the two rays diverge on the far side instead of crossing there. Trace them backward (dashed) on the object's own side, and that is where they cross, giving a virtual image. This is exactly how a magnifying glass works.
Not sure about this bit?
This is the one that trips people up, because the rays don't meet where you'd first look for them.
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Why they diverge
When the object is closer than the focal length, the rays leaving the lens are already spreading apart, not converging, no matter how far you extend them forward.
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The fix: look backward
Draw both rays as dashed lines extending backward, behind the lens, back toward the object's own side. That's the trick behind every magnifying glass: you're looking at a virtual image that appears bigger and farther away than the real object.
Got it now? Scroll back up and re-read the section — it should land differently.
Ray diagram: converging mirror, object beyond F
The same two-ray method works for mirrors: one ray parallel to the axis reflects through the focal point; one ray aimed at the center of curvature (C, at twice the focal length) reflects straight back along itself. Real images for a mirror form on the same side as the object.
Ray diagram: converging mirror, object inside F
With the object closer to the mirror than F, the reflected rays diverge outward instead of crossing in front of the mirror. Their backward extensions cross behind the mirror, giving a virtual image, the same way a shaving or makeup mirror magnifies your face when held close.
Real vs. virtual, upright vs. inverted, size
Three questions to ask about every image you locate. Real or virtual? A real image forms where light rays actually cross and can be projected on a screen; for a lens it's on the opposite side from the object, for a mirror it's on the same side. A virtual image forms where rays only appear to cross when extended backward; it cannot be projected, but you can still see it by looking through the lens or into the mirror. Upright or inverted? Rays crossing above the axis give an upright image; below, an inverted one. What size? Compare image height to object height directly from the diagram: smaller, same size, or larger.
Virtual images are not blurry
A common misconception is that a virtual image is somehow fuzzy or fake compared to a real one. It isn't. Look at your reflection in a bathroom mirror or through a magnifying glass: both are ordinary, sharp, virtual images. The only real difference is whether the image could be projected onto a screen (real) or only seen by looking through the optical device (virtual). Neither is "more real" to your eyes.
Task-fit practice
Work through each ray diagram in your head (or on paper) before checking the answer.
