Chapter 18 Mirrors and Lenses

Curved Mirrors Concave shaped mirrors cause parallel light rays to converge. Convex shaped mirrors cause parallel light rays to diverge.

Concave Mirrors Centre of Curvature (C): geometric centre of “sphere” Vertex (V): centre of a reflective surface Principal Axis (P): straight line passing between the vertex and the centre of curvature.

Principal Focus (F): the point where a group of parallel light rays striking a converging mirror will reflect through. Focal Length (f): distance along the PA between the vertex and the principal focus Finding Images in Convex Mirrors

Finding Images in Concave (Converging) Mirrors: Draw at least two of the following rays: 1. A ray drawn parallel to the PA is reflected through F. 2. A ray drawn through F is refected parallel to the PA. 3. A ray that passes through C is reflected straight back along the same path. FC

Real vs. Virtual Images An image is REAL if rays actually converge and pass through the image. A real image can be seen on a screen. An image is VIRTUAL if rays only appear to be diverging from a point.

Images in Concave Mirrors

Spherical Aberration spherical mirrors are unable to focus parallel light rays to a single point a perfectly made mirror is slightly parabolic, and has no spherical aberration.

Concave Mirror Examples

Concave Mirrors Focus and Centre of Curvature are both found behind the mirror.

Finding Images in Convex (Diverging) Mirrors 1.A ray directed to F reflects back parallel to the PA 2.A ray parallel to PA reflects back as though it has gone through F. 3.A ray appearing to pass through C will bounce back along the same path.

Images in Convex Mirrors Always virtual (diverging light rays) Reduced in size Erect located between V and F NOTE: Because image is reduced in size, there is an ENLARGED FIELD OF VIEW (useful in rearview mirrors, security mirrors, etc.)

Locate I.

Locate O.

Locate F and C OO FC

Lenses Converging (Convex) Lens Diverging (Concave) Lens Optical Centre (O): geometric centre of the lens Focal Point (F): parallel rays converge at focal point. Principal Axis (PA): perpendicular to mid-plane of lens. Focal Length (f): distance between F and O, measured along the PA.

Converging Lenses Converging lenses have 2 focal points, located at the same focal length on either side of the lens. There is some lateral displacement that occurs when rays pass through the lens; however, it is insignificant in thin lenses and therefore ignored in our diagrams

Images Formed by Converging Lenses 1.A ray that is parallel to the PA is refracted through F. 2.A ray that passes through F is refracted parallel to the PA. 3.A ray that passes through O goes straight through, without bending.

Diverging Lenses Parallel rays are refracted so that they radiate out from a virtual focus. Image is always virtual, erect, smaller, and located between F and O.

Images formed by Diverging Lenses 1.A ray that is parallel to the PA is refracted as if it originated from the F on the object’s side of the lens. 2.A ray that is drawn as if it would pass through the opposite F will refract parallel to the principal axis 3.A ray that passes through O goes straight through the lens without bending.

o o F F