408 REFRACTION OF LIGHT. 



EXERCISES. 



1. (.) What is refraction of light ? (&.) State the laws governing 

 the same, and (c.) give an illustrative diagram. 



2. (a.) Name and illustrate by diagram the different classes of 

 lenses. (&.) Explain, with diagram, the action of the burning-glass. 



3. (a.) Explain the cause of total reflection. (6.) Show, with 

 diagram, how the secondary axes of a lens mark the limits of the 

 image. 



4. (a.) Using a convex lens, what must be the position of an 

 object in order that its image shall be real, magnified, and inverted ? 

 (b.) Same, using a concave lens ? 



5. (a.) Show how a ray of light may be bent at a right angle by 

 a glass prism. (6.) The focal distance of a convex lens being 6 

 inches, determine the position of the conjugate focus of a point 

 12 inches from the lens, (c.) 18 inches from the lens. 



6. (a.) The focal distance of a convex lens is 30-c/ra. Find the 

 conjugate focus for a point 15 cm. from the lens. (6.) How may the 

 focal length of a lens be determined experimentally ? 



7. If an object be placed at twice the focal distance of a convex 

 lens, how will the length of the image compare with the length of 

 the object? 



8. A small object is 12 inches from a lens ; the image is 24 inches 

 from the lens and on the opposite side. Determine (by construction) 

 the focal distance of the lens. 



9. A candle flame is 6 feet frpm a wall ; a lens is between the 

 flame and the wall, 5 feet from the latter. A distinct image of the 

 flame is formed upon the wall, (a.) In what other position may 

 the lens be placed, that a distinct image may be formed upon the 

 wall ? (&.) How will the lengths of the images compare ? 



Recapitulation. In this section we have considered 

 the Definition, Index, Laws and Explanation 

 of refraction ; Internal Reflection ; Plates, 

 Prisms and Lenses ; principal and conjugate Foci 

 of lenses ; Construction for conjugate foci and 

 images; Spherical Aberration. 



