Learning objectives By the end of this chapter, you should be able to:
} distinguish between concave and convex lenses and be able to sketch optics diagrams for each
} draw the formation of images in lenses and give uses for each type
} apply the correct formulae to each lens situation
} understand the concept of lens power and how to calculate the power of a combination of lenses
} describe the structure of the eye, its main parts and the corrective solutions of using lenses.
Types of lens
Lenses are optical devices usually made of glass or plastic that transmit and refract light. For the Leaving Certificate Physics course, we will study two types: convex and concave.
• A convex lens (Fig. 15.1) is designed in such a way that the centre of the lens is wider than the sides. It seems to bulge out at the centre. A convex lens is also known as a converging lens: any rays of light coming into the lens will emerge converging together.
• A concave lens (Fig. 15.2), ‘caves in’ at the centre and is wider at the sides. This is also known as a diverging lens as any rays coming in will emerge diverging apart.
NOTE
Remember: convex = converging.
FF F F
FIG. 15.1 In a convex lens, the centre is wider than the sides
