Lens & Mirror Calculator
Solve the thin lens equation for focal length, image distance, object distance, and magnification. Supports converging and diverging lenses, and concave and convex mirrors. No signup, runs entirely in your browser.
Optics Tips
Real vs virtual images
Positive image distance = real image (can be projected on a screen). Negative image distance = virtual image (appears to be on the same side as the object).
Focal point check
When the object is exactly at the focal point (dₒ = f), the image is at infinity. The tool returns a very large value — this is physically correct, not an error.
Magnification sign
Negative magnification means the image is inverted relative to the object. Positive magnification means the image is upright. The magnitude indicates size ratio.
Diverging lenses
Diverging lenses and convex mirrors always produce virtual, upright, diminished images regardless of object position. Focal length is entered as positive — the tool applies the negative sign internally.
Frequently Asked Questions
What is the thin lens equation?
The thin lens equation is 1/f = 1/dₒ + 1/dᵢ, where f is the focal length, dₒ is the object distance, and dᵢ is the image distance. The same equation applies to curved mirrors using the mirror equation.
What does magnification mean?
Magnification m = -dᵢ/dₒ. A value of +2 means the image is twice the size and upright (virtual). A value of -0.5 means the image is half the size and inverted (real). m = 1 means the same size.
What is the sign convention?
For lenses: object distance is positive when the object is in front of the lens (real object). Image distance is positive for real images (on the far side) and negative for virtual images. Focal length is positive for converging lenses and negative for diverging ones.
What is the difference between a real and virtual image?
A real image forms where light rays actually converge — it can be projected onto a screen. A virtual image forms where rays appear to diverge from — it cannot be projected. Diverging lenses and convex mirrors always form virtual images.
How does a concave mirror differ from a converging lens?
Both can form real inverted images when the object is beyond the focal point. The key difference is that mirrors work by reflection so the image forms on the same side as the object, while lenses form images on the opposite side.
Is my data stored?
No. All calculations run locally in your browser. No data is sent to any server.