Lenses & Mirrors

What this investigation asks

A lens does not form different pieces of an image with independent halves of its surface. This investigation follows ray paths and screen images to connect object position, focal length, and magnification. Begin by identifying the optical element and sign convention. A real image can be received on a screen because rays from a point meet again; a virtual image is inferred from the rays' apparent origins. A bright patch or a larger-looking object is not enough to determine which kind of image the apparatus has formed.

Design and measure the comparison

Move the object relative to a converging lens while keeping its focal length and object size fixed. Find the screen position giving a sharp image and record object distance, image distance, orientation, and size. Compare positions outside the focal distance, then investigate an inside-focal case separately. The screen does not receive a finite real image in every configuration. Cover part of the lens in a controlled comparison: retain the complete object and inspect both brightness and the presence of its different features rather than assuming the covered lens region removes a matching half of the image.

Connect the record to the model

Use the thin-lens relationship to connect the distance measurements and use magnification to infer relative image size and orientation. The course's ideal ray constructions use the selected focal length and optical sign convention. Rays from each object point pass through multiple parts of the lens, explaining why partial covering can leave a complete but dimmer image. Compare the corresponding mirror cases through their reflected paths. A negative image-distance result describes a location under the convention, not a negative physical amount of light or an automatically inverted image.

Interpret the evidence and its limits

Keep geometrical optics separate from the wave spreading omitted by this ideal construction. Finite apertures, imperfect focus, and the model's selected lens approximation limit the inference. A virtual image cannot be made sharp on a downstream screen just by moving that screen farther. If a calculation predicts a divergent image distance at the focal setting, interpret the limiting ray geometry rather than replacing it with an arbitrary large measured distance. Report the element, reference distances, orientation, and method used to determine the image, so a numerical answer can be checked against the ray account.

Reference conditions and quantities

For the converging lens with focal length +20 cm and object distance 40 cm, the reference image distance is 40.00000000 cm and magnification is −1. The sign indicates inversion under the course convention. These matched-distance values describe that ideal lens and object placement; another focal length or object position requires a new image calculation.

Use the record to test your explanation

Write a prediction before collecting the comparison, naming the quantity that should change and the condition you will alter. Keep a reference run and vary one factor at a time when the question calls for a controlled test. Record the selected apparatus settings, units, and measurement interval. Where an activity supplies Ideal and Real modes or seeded variation, retain that identity with each record rather than mixing incompatible runs in one calculation.

Separate an instrument reading or counted observation from the model relationship used to explain it. Keep raw values alongside derived results so another student can reconstruct the argument. A graph needs labeled axes and a stated comparison range. If a result differs from a reference, inspect the conditions, rounding, and measurement procedure before changing the explanation.

For a classroom discussion, compare claims supported by the same evidence and identify an additional measurement that would distinguish competing accounts. Finish with a claim, the relevant observations, and reasoning tied to this course's model. State a specific limitation instead of assigning a general error label to every disagreement.

Related investigations

Review boundary bending in Refraction, then compare aperture effects in Double Slits & Gratings. Identify whether a question requires the course's ray construction or the wave model before transferring an optical explanation.