9TH GRADE · LIGHT PHENOMENA
Lenses, eyes and optical instruments
Focus, imaging, lens formula and power.
The entire theory of the lesson is collected here; you can read it without an account. In interactive mode, the same theory appears as cards, followed by questions. with checking the answer and task, and progress is saved.
Basic Concepts
Converging and diverging lenses
A converging lens turns a parallel beam into a converging one, and a diverging lens makes it divergent.
The type of lens in the medium is determined by the shape of the surfaces and the relative refractive index.
Constructing an image with a lens
The image is found by the intersection of the rays or their continuations after passing through the lens.
A converging lens can give a real or virtual image, while a diverging lens usually gives an imaginary reduced one.
Eye and optical instruments
The eye focuses the image on the retina, and devices control the path of the rays to magnify or register.
The glasses adjust the focus position, the magnifying glass increases the angular size, and the camera creates the actual image.
Physical quantities
Focal length
The distance from the optical center of the lens to the main focus.
Unit: meter ·
Optical power
The reciprocal of the focal length of a lens in meters.
Unit: diopter ·
Distance to object
The distance from the lens to the object along the main axis.
Unit: meter ·
Image distance
Distance from the lens to the actual image.
Unit: meter ·
Linear lens magnification
The ratio of image size to object size.
Unit: dimensionless quantity
Lesson formulas
Thin Lens Formula
The reciprocal focal length is equal to the sum of the reciprocal distances to the object and the image.
Lens power
Optical power in diopters is equal to the reciprocal of the focal length in meters.
Linear lens magnification
Linear magnification is equal to the ratio of the distance to the image to the distance to the object.
Related formulas
The same formulas in the reference, with symbols and units.
Tasks on the topic
Statements, answer options and the walkthrough.