Chapter Four - Lenses - Thin lenses, focal points and focal lengths - Image formation

Oct 26, 2026

Uploaded by Elsa

Chapter Four focuses on the fundamentals of lenses, starting with the classification of converging and diverging thin lenses and their defining geometry. It explains lens concepts such as optical and principal axes, paraxial ray behavior, focal points, and focal lengths, including primary and secondary focal points for both convex and concave lenses. The chapter then builds to image formation, showing how object position relative to focal planes determines image distance, size, and magnification, supported by ray-diagram interpretations and related methods.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages36

Reading Time1 h 31 min

AccessFree to download

Introduction

Types of Lenses and Basic Definitions

Convex and Concave Lenses

Optical Center and Principal Axis

Focal Points and Focal Lengths

Primary and Secondary Focal Points

Paraxial Rays and Focal Plane

Image Formation

Converging Lens Image Position and Magnification

Ray Methods and Lens Formulas

Oblique-Ray Method and Pararelle Method

| Chapter Four ( Lenses) Thin lenses, focal points and focal lengths , Image formation, Conjugates points and planes, the parallel-Ray method | | --- | | The oblique-Ray method, Use of the lens formula, Lateral magnification, Virtual Images and lens maker formula. States of lens | | Thin-lens combinations, The power of a thin lens, Derivation of the lens Makers formula. Thick Lines, two spherical surfaces.
Focal points and principal points, General Thick-Lens Formula, cardinal points, problems | 1. * Introduction Shapes of several standard forms of thin lenses were shown in Fig (4-1) 1).These are examples of converging lenses. 2)They have positive focal lengths. 3)They are thickest in the middle. 1)These are examples of diverging lenses.2)•They have negative focal lengths. 3)They are thickest at the edges. Figure (4-1) explains converging and diverging lenses Define thin lens is one which thickness is small compared with the image object distances primary and Secondary focal lengths. Details of several of lenses according different of radii of curvature can be explained in fig(4-2) There are two types of lenses includes of (1) Convex or Converging ,(2) Concave or Diverging. Figure 4.2 types of lenses a) converging lenses b) diverging lenses. Definition term important 1. Convex lens : it’s the lens whose causes the rays of light to form a converging beam 2. A concave lens: it’s the lens which forms a diverging beam. 3. Optical center: center of lens , sometime its take the sample C, the direction through the optical center and perpendicular to the lens is called the principal axis. 4. principal axis: the axis of symmetry is called the principal axis ofthe lens. From now on we think of every lens as having rotational symmetry about its . A beam parallel to the principal axis will form a converging beam with a convex lens and a diverging beam with a concave lens see fig.(4-3) . Fig 4-3 explain ray treatment in the concave lens and focus in the primary focal points Ray treatment at pararelle method at the principle axis 1 in the convex lens At ray oblique method in the focal points 2 and refracted into covex lens then parallel . in breifly a convex lens is converging of any light ray pass through it. A ray oflight is refracted twice by a lens, once when it passes into the lens (air to glass) and once when it emerges from the lens (glass to air) . Convex lens causes the rays of light to form a converging beam and a concave lens forms a diverging beam. The centre ofthe lens is called the optical centre and the direction through the optical centre and perpendicular to the lens is called the principal axis. A beam parallel to the principal axis will form a converging beam with a convex lens and a diverging beam with a concave lens. (4-2) FOCAL POINTS AND FOCAL LENGTHS Diagrams showing the refraction of light by an equiconvex lens and by anequiconcave lens are given in figure 4.2. The axis in each case is a straight line through the geometrical center ofthe lens and perpendicular to the two faces at the points of intersection. For spherical lenses this line which joins the centers of curvature of the two surfaces. Ray diagrams shown in the figure 4.2 illustrates the primary and secondary focal points F and F' and the corresponding focal lengths f andf' of thin lenses. The primary focal point (F) of convex lens: an axial point having the property that any ray coming from it , travels parallel to the axis after refraction. The secondary focal point (F') of convex lens: an axial point having the property that any incident ray traveling parallel to the axis will, after refraction, proceed toward. The primary focal point (F) of concave lens: an axial point having the property that any ray proceeding toward it travels parallel to the axis after refraction. The secondary focal point (F') of concave lens: an axial point having the property that any incident ray traveling paral

FAQs

How do converging and diverging thin lenses differ by focal length and thickness shape?

Converging lenses have positive focal lengths and are thickest in the middle, while diverging lenses have negative focal lengths and are thickest at the edges.

What are the primary and secondary focal points of a convex lens?

The primary focal point is an axial point where rays from it emerge parallel after refraction, and the secondary focal point is where rays parallel to the axis proceed toward after refraction.

How does image distance and size change when the object moves relative to the focal plane of a converging lens?

If the object moves closer to the primary focal plane, the image forms farther from the secondary focal plane and becomes larger. If the object moves farther from F, the image forms closer to F' and becomes smaller.

Oct 26, 2026

Uploaded by Elsa