The meniscus lens is a mongrel version of the positive and negative lenses. The plano-concave lens is also a negative lens. Such lenses would be known as ‘unequal biconcave’ lenses. Again, biconcave lenses more frequently have a different curvature on their two faces. ![]() It is shown as having both surfaces equally curved. In figure 1 the biconcave lens is a negative lens. Such lenses are called NEGATIVE LENSES because they have a negative effect upon the size of the text seen through them. If you look at a page of text through such a lens the text will appear smaller and farther away than without the lens. Lenses that are thinner in the middle than at the edge cannot be used as magnifying glasses. ![]() The word ‘plane’ as a noun means ‘an accurately flat surface’.) You can use a plano convex lens as a magnifying glass. (In optics the word ‘plane’ as an adjective means ‘it is flat’. The ‘plano’ refers to the fact that one surface is accurately flat. You can see that the plano-convex lens is also a positive lens. In real life it is often the case that the two faces have different degrees of curvature, so the lens would be described as an ‘unequal biconvex’ lens. In the diagram we have shown the curvature of both faces as being equal, so such a lens would be described as being an ‘equal biconvex’ lens. Its name derives from the fact that both faces of the lens have been ground and polished to give a convex shape. In figure 1 you can see that the biconvex lens is a positive lens. Such lenses are called POSITIVE LENSES because they have a positive effect upon the size of text seen through them. If you look at a page of text through such a lens the text will appear bigger and closer than without the lens. Lenses that are thicker at the middle than at the edge can be used as magnifying glasses. You can ignore this long section of text if your interest is confined to astronomy. Once we have dealt with the astronomical telescope we shall describe the theory and practice of building a TERRESTRIAL telescope – a telescope that gives a view that is the right way up and the right way round. We shall describe first the straight-through astronomical telescope and then we shall describe briefly how to build a right-angle telescope (an elbow telescope) which can be more convenient for looking at objects that are high in the sky. We will describe how to design and build an ASTRONOMICAL telescope, which is the simplest type of telescope to build it gives a view that is upside down and back-to-front. We shall not describe the REFLECTING telescope because that type of telescope gathers light by means of a hollow-curved mirror. We shall describe how to design and build a REFRACTING telescope – one that gathers light by means of a lens at its front end. In these pages of text we are going to describe the types of glass lenses that you would most frequently encounter and we will explain how they affect light that shines through them. Experimental work: Measuring the magnification of a telescope.Varying the magnification – making a pancratic telescope.Making a tripod-mounted terrestrial telescope.How to make a terrestrial telescope – our recipe.Choosing relay lenses – keeping it simple.The problems that relay lenses can cause and how to resolve them.The difference between a terrestrial and an astronomical telescope.Making your own eyepieces – the Plössl design.Making an elbow (right-angled) telescope.Incorporating a star diagonal into your design.Doing the scale drawing of the telescope.Why does the lens give an inverted image?. ![]()
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