Optics Bench
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flagWhat you'll discover
- arrow_forwardDraw the three principal rays for lenses and curved mirrors
- arrow_forwardPredict image position with the thin-lens/mirror equation 1/f = 1/v + 1/u
- arrow_forwardClassify images as real or virtual, upright or inverted, magnified or reduced
- arrow_forwardExplain how magnifying glasses, cameras and shaving mirrors work
Three rays find any image
To locate an image you only need to trace three special rays from the tip of the object. Ray one travels parallel to the principal axis and is bent through the focal point. Ray two passes through the optical centre of a lens (or strikes the pole of a mirror) and continues with its direction unchanged (mirrors reflect it symmetrically). Ray three goes through the focal point first and emerges parallel to the axis.
Wherever the rays — or their backward extensions — cross, that is the image tip. The simulation draws all three live as you drag the object, so the geometric construction becomes second nature.
The lens and mirror equation
Geometry compresses into one neat relation. Using the real-is-positive convention, 1/f = 1/v + 1/u, where u is the object distance, v the image distance and f the focal length. The magnification is m = v/u (with a sign telling you upright or inverted).
A converging lens with the object beyond f gives a positive v: a real, inverted image you could catch on a screen. Move the object inside f and v turns negative: the rays diverge and only their extensions meet, creating a virtual, upright, magnified image — the principle of the magnifying glass.
Real versus virtual images
A real image is formed where light rays actually converge. It can be projected on a screen — that is what happens on a cinema screen and on the retina of your eye. Real images from a single converging element are always inverted.
A virtual image forms where rays only appear to come from; no light passes through it, so no screen can catch it, yet your eye sees it perfectly well. Plane mirrors, diverging lenses and convex mirrors give only virtual, upright images. A concave mirror gives either type, depending on whether the object sits beyond or inside its focal point.
Optics in daily life
Every optical instrument is a story about focal lengths. A camera uses a converging lens with the object far beyond f, producing a small, real, inverted image on the sensor. A magnifying glass holds the object inside f for an enlarged virtual image. Your eye changes the focal length of its flexible lens to keep images sharp on the retina.
Convex mirrors on vehicle wing mirrors and at road bends give wide-angle, reduced virtual images — hence the warning that objects are closer than they appear. Dentists and make-up mirrors are concave, used inside f for upright magnification.