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The thin lens equation relates the object and image distances to the focal length of the lens.
The thin lens equation relates the object and image distances to the focal length of the lens. If the object distance can be made very large ( = 0) then the equation reduces to the image, which will be very small, being at the focal length of the lens.
Place the white viewing screen at the far end of the magnetic optical rail. Place the cross arrow on the light source at the other end of the rail.
Place the 75 mm (0.075m) convex lens on a magnetic holder at the far end of the magnetic optical rail near the screen (see figure 7.4). The lens should be centered on the opening in the holder.
Adjust the lens until a sharp very small image is formed. Since the object i.e. the cross arrow target is far away, the distance between the lens and the screen is approximately the focal length of the lens. Record the focal length of the lens and object distance.
Focal length of lens
Object distance
Explain why this approximate focal length method works using the thin lens equations.
Parallel light rays should impinge on the circular side of the cylindrical 'lens'. Using a small white card, look for the 'focus' of the lens. As you move the card back and forth, you should see the rays converge into a single line. (Ignore the rays above the cylindrical lens which do not change the spacing.) Using a ruler, measure the 'focal length' (distance from the 'lens' to the focal point) of this one dimensional lens. Record the value below.
Approximate focal length of cylindrical 'lens'
If you reverse the cylindrical lens so the flat side is facing the light
