Four points on Behavior of wavefronts in prisms,Lenses, and Mirrors ??
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1. Consider a plane wave passing through a thin prism,
Clearly, the portion of the incoming wavefront which
travels through the greatest thickness of glass has been
delayed the most. Since light travels more slowly in
glass, this explains the tilt in the emerging wavefront
2. Similarly, the central part of an incident plane wave
traverses the thickest portion of a convex lens and
is delayed the most. The emerging wavefront has a
depression at the center. It is spherical and converges to
a focus.
3. A concave mirror produces a similar effect. The center
of the wavefront has to travel a greater distance before
and after getting reflected, when compared to the edge.
This again produces a converging spherical wavefront.
4. Concave lenses and convex mirrors can be understood
from time delay arguments in a similar manner. One
interesting property which is obvious from the pictures
of wavefronts is that the total time taken from a point on
the object to the corresponding point on the image is the
same measured along any ray. For example
when a convex lens focuses light to form a real image,image seem that rays going through the center are shorter.
But because of the slower speed in glass, the time taken
is the same as for rays travelling near the edge of the
lens.
Clearly, the portion of the incoming wavefront which
travels through the greatest thickness of glass has been
delayed the most. Since light travels more slowly in
glass, this explains the tilt in the emerging wavefront
2. Similarly, the central part of an incident plane wave
traverses the thickest portion of a convex lens and
is delayed the most. The emerging wavefront has a
depression at the center. It is spherical and converges to
a focus.
3. A concave mirror produces a similar effect. The center
of the wavefront has to travel a greater distance before
and after getting reflected, when compared to the edge.
This again produces a converging spherical wavefront.
4. Concave lenses and convex mirrors can be understood
from time delay arguments in a similar manner. One
interesting property which is obvious from the pictures
of wavefronts is that the total time taken from a point on
the object to the corresponding point on the image is the
same measured along any ray. For example
when a convex lens focuses light to form a real image,image seem that rays going through the center are shorter.
But because of the slower speed in glass, the time taken
is the same as for rays travelling near the edge of the
lens.
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