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Lenses
There are two types of lenses:
1) Convex lens or converging lens
2) Concave lens or diverging lens
Convex lens:
A convex lens converges parallel beams of light
Optical centre: is the centre of the lens. Any light ray passes through the optical centre without bending
Principal axis: A line that is perpendicular to the lens and passes through the optical centre
Focal point: The point at which parallel beams of light converge
Focal length: The distance between the optical centre and the focal point
Ray diagram
The properties of the image from a convex lens can be determined by drawing ray diagrams.
Two rays are needed:
1) A ray that passes through the optical centre without bending
2) A ray that is parallel to the principal axis and then bends to pass through the focal point after reaching
the lens
3) A third ray can also be used. It passes through the focal point until it meets the lens and after the lens
it becomes parallel to the principal axis
Note:
Rays for virtual images are drawn using dotted lines
Virtual images cannot be captured on the screen
Properties of image based on object distance from lens
Linear magnification:
Magnification is the ratio of image size and object size
Magnification = image size ÷ object size
Applications of lenses
A convex lens can be used as a magnifying glass. In order to obtain a large upright image the distance
between the Lens(magnifying glass) and object should be less than f
A convex lens is used in a photographic enlarger. In order to obtain an image that is larger and real the
distance between the object and lens should be between f and 2f
A convex lens is used in a projector. In order to obtain an image that is larger and real the distance
between the slide and lens should be between f and 2f
To obtain a clear image for close by objects using a camera, the distance between the screen and the
lens is slightly greater than f
To obtain a clear image for far away objects using a camera, the distance between the screen and the
lens is equal to f
Concave lens
A concave lens is a diverging lens as it diverges parallel beams of light. The rays appear to be diverging
from the focal point
The image formed by a concave lens is virtual, upright and smaller than the object
Eye correction
A long sighted person can see objects far away but cannot see nearby objects because his eye lens
cannot become thick enough to bend the light rays to meet at the retina. To correct this problem a
convex lens is used so that the rays converge to meet at the retina
A short sighted person can see nearby objects but cannot see objects far away because his eye lens
cannot become thin enough to make the light rays meet at the retina. To correct this problem a concave
lens is used so that the rays diverge a little and then converge at the retina after passing through the eye
lens