INTRODUCTION TO PHYSIOLOGICAL OPTICS 400 



virtual image and the mirror will be greater than 

 the distance between the mirror and the lumi- 

 nous point. As the luminous point approaches 

 the mirror, its virtual image will also approach. 



2. Hold a small object nearer the mirror than 

 its principal focal distance. 



Note that the image is virtual, upright, and 

 larger than the object. 



Construction of Image from Concave Mirrors. 

 Determine by construction the length of the 

 image of an arrow 2 cm. long, placed 10 cm. from 

 the middle point of a concave mirror of 5 cm. 

 radius of curvature. 



Draw a horizontal line. With any convenient 

 point on this line as a centre describe an arc 

 of 5 cm. radius, that shall intersect the line. 

 This arc will be the section of a concave mirror. 

 The horizontal line will be the principal axis, 

 and the intersection of the principal axis and 

 the arc, the middle point of the mirror. The 

 principal focus of the mirror will lie halfway 

 between the centre of curvature and the middle 

 point. At right angles to the principal axis 

 and 10 cm. from the middle point draw a vertical 

 arrow 2 cm. long. Determine first the position 

 of the image of the point of the arrow. Draw 

 from the point to the mirror an incident ray 

 parallel to the principal axis. This parallel ray 



