Ray Diagrams - Three Rays and Lens Image Formation

Oct 26, 2026

Uploaded by Aran

Ray diagrams explain how light rays form images through a lens using three characteristic rays from an object point: one parallel to the principal axis through the focal point, one through the focal point emerging parallel, and one through the lens center continuing undeflected. The notes define object distance, image distance, and focal length, including sign conventions for real/inverted versus virtual/upright images. Multiple-choice problems apply these rules to convex and diverging lenses.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages15

Reading Time38 min

AccessFree to download

Ray diagrams

Three principal rays

Object, image distance, and focal length sign conventions

Lens image formation and magnification questions

Ray diagrams • Three rays leave one point on an “object”: 1) A ray parallel to the principal axis (aka optic axis), will (seem to) pass through the focal point F. 2) A ray that (seems to) pass(es) through F’, will end up parallel to the principal axis. 3) A ray that passes through the center ofthe lens, will continue virtually undeflected. • Where these three rays converge (or seem to converge), is the corresponding point on the image. Figure 33.6c Object and Image distances • d0 = “object distance” = distance of object from lens. • di = “image distance” = distance of image from lens. • f = “focal length” = distance of F from lens. • d0 is positive if the object is on the same side of the lens as the incident rays. • di is positive if the image is on the same side of the lens as the transmitted rays. -di positive means image is “real and inverted”. -di negative means image is “virtual and upright”. • f is positive if incident rays parallel to the optic axis actually converge at F (or F’) . f is negative if the rays only appear to converge at F (or F’) . 1 1 1 ~~ ~~ 􀀎 􀀠 ~~ ~~do di f m 􀀠 h~~i~~ho~~ ~~ 􀀠 􀀐 ~~ ~~dd~~i~~o~~ ~~ Figure 33.9 A convex lens has a focal length f. The only way to get a magnification of–1 is to 1) place a real object at the focal point. 2) place a real object at a distance 2f from the lens. 3) place a real object at a distance 3f from the lens. 4) Magnifications from a positive lens can never be negative. 5) None of these is correct. The image produced by the converging lens is at which point? (F marks the two focal points.) A converging lens and a screen are so arranged that an image of the sun falls on the screen. The distance from the lens to the screen is 1) the focal length. 2) the object distance. 3) the magnifying power. 4) one-half the radius of curvature of one of the lens faces. 5) the average radius of curvature of the two lens faces. When a real object is placed just inside the focal point F ofa diverging lens, the image is 1) virtual, upright, and diminished. 2) real, inverted, and enlarged. 3) real, inverted, and diminished. 4) virtual, upright, and enlarged. 5) virtual, inverted, and diminished. A real image is formed by a converging lens. Ifa weak diverging lens is placed between the converging lens and the image, where is the new image located? 1) farther from the converging lens than the original image. 2) closer to the converging lens than the original image. 3) at the original image position. Figure 33.25 Figure 33.26 Figure 33.27a

FAQs

What are the three characteristic rays in a ray diagram for a lens?

One ray parallel to the principal (optic) axis passes through the focal point F; a ray passing through F emerges parallel to the principal axis; a ray through the lens center continues virtually undeflected.

How are object distance, image distance, and focal length defined and signed?

Object distance d0 measures the object’s distance from the lens, image distance di measures the image’s distance from the lens, and f is the focal length measured from F. d0 is positive when the object is on the incident-ray side; di is positive when the image is on the transmitted-ray side; positive di indicates real and inverted, while negative di indicates virtual and upright.

When does a converging lens produce a virtual, upright image?

A virtual, upright image corresponds to a negative image distance di in the sign convention given. One example question states that placing a real object just inside the focal point F of a diverging lens yields a virtual, upright image (option 1).

Which object distances can give magnification −1 for a convex lens?

The material lists options for obtaining magnification −1: placing a real object at the focal point, or at distances 2f and 3f, while also noting that magnifications from a positive lens can never be negative. The correct choice must be determined from the question’s options.

Oct 26, 2026

Uploaded by Aran