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Optics Calculator — Thin Lens
This optics calculator helps you quickly find the image distance, magnification, and image type (real or virtual) for a thin lens. Simply enter the focal length and object distance to instantly get results based on the standard thin lens equation used in physics and optics.
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How to Use the Calculator
Enter the focal length in mm (positive = converging lens, negative = diverging lens) and the object distance from the lens in mm. The calculator will instantly compute the image distance, magnification, and describe the image type.
Calculation Example
Converging lens with focal length f = 50 mm, object distance do = 200 mm. Calculation: 1/di = 1/50 − 1/200 = 0.02 − 0.005 = 0.015, so di = 66.67 mm. Magnification: M = −66.67/200 = −0.33 (real, inverted, diminished image).
Frequently Asked Questions
What is the thin lens equation?
The thin lens equation is 1/f = 1/do + 1/di, where f is focal length, do is object distance, and di is image distance from the lens.
What does a negative focal length mean?
A negative focal length indicates a diverging (concave) lens, which always produces a virtual, upright, and diminished image on the same side as the object.
When is the image real?
The image is real when the image distance di is positive — light rays actually converge on the opposite side of the lens from the object.
When is the image virtual?
The image is virtual when di is negative — light rays diverge and the image forms on the same side as the object (e.g., in a magnifying glass).
What is optical magnification?
Magnification M = -di/do. A negative value means an inverted image; absolute value greater than 1 means the image is enlarged.
What happens when the object is at the focal point?
When the object distance equals the focal length (do = f), rays emerge parallel after the lens and the image forms at infinity.
What are applications of the thin lens equation?
The equation is used in designing eyeglasses, cameras, microscopes, telescopes, and all optical systems involving lenses.
Does the calculator account for optical aberrations?
No — the calculator uses the thin lens model, which ignores spherical, chromatic, and other aberrations of real lenses.
What does an inverted image mean?
An inverted image (negative magnification) forms when the object is beyond the focal point of a converging lens. The image is rotated 180° relative to the object.
How does the image change as the object moves closer to the lens?
As the object approaches the focal point of a converging lens, the real image moves farther away and becomes more magnified. Inside the focal length, the image becomes virtual and upright.
Results are approximate and based on the thin lens model. The calculator does not account for optical aberrations or lens thickness.
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