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# TOPIC 3: LIGHT | PHYSICS FORM 3

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## Â â€‹â€‹â€‹LIGHT PART Iâ€‹â€‹Â

â€‹â€‹Â Reflection of Light from Curved Mirrors â€‹â€‹Â Types of curved mirrorsÂ Â â€‹â€‹â€‹â€‹Â

<> Convex Â â€‹â€‹ â€‹â€‹ â€‹â€‹ â€‹â€‹ â€‹â€‹ â€‹â€‹â€‹â€‹ (diverging mirror) â€‹â€‹Â

<> Concave Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹â€‹â€‹Â (converging mirror) â€‹â€‹Â

â€‹â€‹Â Terms used in curved mirrorsâ€‹â€‹Â

Consider the diagram below when two curved mirrors are joinedÂ â€‹â€‹

â€‹â€‹Â Whereby:â€‹â€‹Â â€‹â€‹Â

AB = Convex mirror Â â€‹â€‹â€‹â€‹Â whileÂ â€‹â€‹Â â€‹â€‹â€‹â€‹Â ST = Concave mirror â€‹â€‹Â

C = centre of curvatureÂ â€‹â€‹

L = pole of the Concave mirror Â â€‹â€‹â€‹â€‹Â whileÂ â€‹â€‹Â â€‹â€‹â€‹â€‹Â K = pole of the Convex mirror â€‹â€‹Â

CL and CK are radii of curvature of Concave mirror and convex mirror respectivelyâ€‹â€‹

CL and CK are principal axes of Concave and Convex mirror respectivelyâ€‹â€‹

Centre of Curvature: Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹â€‹â€‹Â Is the centre of the sphere in which the mirror is a part. â€‹â€‹Â Radius of Curvature: Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹â€‹â€‹Â Is the distance or length between the pole of the curved mirror and the centre of curvature. â€‹â€‹Â

Principal Axis: â€‹â€‹Â Is the line joining the pole of the curved mirror and the centre of curvature. â€‹â€‹Â

Consider when light is reflected in curved mirrors as shown in the diagrams below.Â â€‹â€‹

â€‹â€‹Â Principle Focus, F: â€‹â€‹Â Is the point in which the light is reflected in curved mirror â€‹â€‹Â

Focal Length, f Â â€‹â€‹ â€‹â€‹â€‹â€‹Â Is the distance between pole of the curved mirror and the principal focus. Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹â€‹â€‹Â NB: Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹â€‹â€‹Â Given that focal length is always half the radius of curvature.â€‹â€‹Â â€‹â€‹Â Location of Image using Ray diagramsâ€‹â€‹Â â€‹â€‹Â

The following are the rules used to locate image in the curved mirror.Â â€‹â€‹Â

A ray of light travelling to the mirror parallel to the principal axis, a ray is reflected through the principal focus â€‹â€‹Â

A ray of light travelling to the mirror through the centre of curvature is reflected along its own path â€‹â€‹Â

A ray of light travelling to the mirror through the principal focus is reflected parallel to the principal axis â€‹â€‹Â

Note:â€‹â€‹Â Any two of these rays are sufficient to locate the image. â€‹â€‹Â

â€‹â€‹Â Procedure to draw ray diagramsâ€‹â€‹Â â€‹â€‹Â oâ€‹â€‹Â Choose an appropriate scale so that the ray diagram fits on the available space. â€‹â€‹Â oâ€‹â€‹Â Draw a horizontal line to represent the principal axis of the mirror. Mark the focal point of the mirror. â€‹â€‹Â

Using the chosen scale, draw the object in position along the principal axis. The object is drawn as a vertical line from the principal axis.Â â€‹â€‹

Locate the position of the image by drawing rays from the object to the mirror. Use the rules for drawing ray diagrams to draw the reflected rays.Â â€‹â€‹

At the point of intersection of the reflected rays, draw the image in positionÂ â€‹â€‹

Image formed in Curved mirrorÂ â€‹â€‹

Terms used to describe the images formed by curved mirrors: â€‹â€‹

Positionâ€‹â€‹Â

Real imageâ€‹â€‹Â is on the same side of the mirror as the object. â€‹â€‹Â

Virtual imageâ€‹â€‹Â is on the opposite side of the mirror compared to the object. â€‹â€‹Â

Natureâ€‹â€‹Â â€‹â€‹Â

Upright imageâ€‹â€‹Â has the same orientation as the object. â€‹â€‹Â

Inverted imageâ€‹â€‹Â is oriented in an upside down position compared to the object. â€‹â€‹Â

Sizeâ€‹â€‹Â â€‹â€‹Â

Enlarged imageâ€‹â€‹Â is bigger than the object. â€‹â€‹Â

Diminished imageâ€‹â€‹Â is smaller than the object â€‹â€‹Â

Images formed by Concave mirrorsâ€‹â€‹Â â€‹â€‹Â

The following are the characteristics of images formed by concave mirrors: â€‹â€‹

2. Exampleâ€‹â€‹Â

â€‹â€‹Â An object 5 cm tall is placed 34 cm from a concave mirror of focal length 20 cm. By means of an accurate graphical construction, determine the position, size and the Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹Â â€‹â€‹â€‹â€‹Â nature of the image formed.â€‹â€‹Â