Properties of Waves

Lenses

1. What are the main two types of lenses? 
   Convex and Concave lenses
   
2. What is meant by the principle focus of a convex lens?
   The principle focus of a convex lens is the point at the axis that passes through the optical center of the lens and where the incident rays parallel to the axis are refracted passing through it.
   
3. What is meant by the principal focus of a concave lens?
   The principal focus of a concave lens is the point at the principle axis that passes through the optical center of the lens and where the incident rays parallel to the axis are refracted so that their extensions pass through it.
   
4. What is meant by the principal axis lens?
   Is a straight line that passes through the optical center and the focus of the lens
   
5. How could you find the focal length of convex lens rapidly?
   
6. How could you find the focal length of convex lens accurately?
   
7. How could you distinguish by structure between a convex lens and concave lens?
   Convex lenses are thick in the middle while concave lens are thin in the middle
   
8. What is the relation between the focal length and the curvature of a lens? inverse relation
   
9. Give two reasons why in practice a convex lens may not bring parallel rays to an exact point focus.
   1- Convex lenses refract light of different colors by slightly different amount.  So they will not converge in one point
   
   2- Convex lens converge parallel ray in one point if and only if they are very close to the principle axis. 
   
   
10. Explain how do we draw the image of an object placed in front of a convex lens?
    1- Draw a straight ray from the top most point in the object that passes through the optical center.
    
    2- Draw a straight ray from the most top point in the object parallel to the principle axis and then refract it so it passes in the focus 
    
    3- Draw a straight ray passing in the focus and refract it parallel to the principle axis
    
11. Explain how do we draw the image of an object placed in front of a concave lens?
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12. An object is placed beyond the focal length of a convex lens.  Draw a ray diagram to show how the image is formed and describe the image nature?
    
13. An object is moved from beyond the focal length of a convex lens toward its optical center, what effect will occur on the image size, and position?
    
14. State where an object should be placed if the image formed by a convex lens is to be:
    a- large, upright, and virtual. u<F
    b- real and smaller than the object.   u>2F
    c- real and larger than the object.   2F>u>F
    d- real and the same size as the object.  u=2F
    e- real and at the minimum possible distance from the object.  u=F
    
15. What is the minimum possible distance between an object and its image under a convex lens.  It is 4 times the focal length.
    
16. Describe the lenses formula and the magnification factor.
    
    u: distance between object and optical center
    v: distance between image and optical center
        + (positive) real image, inverted 
        - (negative) virtual image and upright
    f: focal length.  
        + (positive) for convex lenses 
        - (negative) for concave lenses
    L': image length
    L: object length
    m: magnification
    
17. Define "real image" and "virtual image"?
    Real image is an image that can be put on screen
    Virtual image is an image that cannot be put on screen.
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Problems

1. A 3cm high object is placed 24 cm away from a convex lens of focal length 8 cm.  Calculate the position, height and nature of of the image.  
   given: L=3cm, u=24 cm, f=+8 cm 
   required: v?, L'?, image properties?
   Solution:
   1/u+1.v=1/f ,  1/24+1/v=1/8, 1/v=1/8-1/24=2/24=1/12,  v=12 cm
   m=L'/L=v/u,  L'/3=12/24, L'=1.5 cm
   image:  real, inverted, smaller
   
2. A 3cm high object is placed 3 cm away from a convex lens of focal length 8 cm.  Calculate the position, height and nature of of the image.  
   given
   given: L=3cm, u=3 cm, f=+8 cm 
   required: v?, L'?, image properties?
   Solution:
   1/u+1.v=1/f ,  1/3+1/v=1/8, 1/v=1/8-1/3=-5/24=1/12,  v= -24/5= - 4.8 cm
   m=L'/L=v/u,  L'/3=4.8/3, L'=4.8 cm
   image:  virtual, upright , larger 
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