lens placode

Examples of lens placode in the following topics:

• Development of Vision

  • The epithelium thickens to form the lens placode.
  • The lens differentiates and invaginates until it detaches from the epithelium.
  • This vesicle is subsequently invaginated by further inductions from the chordamesoderm, and induces the ectoderm that thickens (lens placode) and further invaginates to a point that detaches from the ectoderm and forms a neurogenic placode.
  • The lens placode is triggered by the chordamesoderm to invaginate and form the optic cup, composed of an outer layer of neural retina and inner layer of pigmented retina that unite and form the optic stalk .
  • Some cells in the lens vesicle will form the cornea and the lens vesicle will develop completely to form the definitive lens.

• The Lensmaker's Equation

  • A lens whose thickness is not negligible is called a thick lens.
  • If the lens is biconvex, a beam of light travelling parallel to the lens axis and passing through the lens will be converged (or focused) to a spot on the axis, at a certain distance behind the lens (i.e. the focal length).
  • In this case, the lens is called a positive or converging lens.
  • If the lens is biconcave, a beam of light passing through the lens is diverged (spread); the lens is thus called a negative or diverging lens.
  • Diagram of a negative (diverging) lens.

• The Compound Microscope

  • It is made of two convex lenses: the first, the ocular lens, is close to the eye; the second is the objective lens.
  • The first lens is called the objective lens and is closest to the object being observed.
  • The objective lens creates an enlarged image of the object, which then acts as the object for the second lens.
  • The distance between the objective lens and the ocular lens is slightly shorter than the focal length of the ocular lens, fe.
  • where m is total magnification, mo is objective lens magnification, me is ocular lens magnification.

• Refraction Through Lenses

  • Such a lens is called a converging (or convex) lens for the corresponding effect it has on light rays.
  • The concave lens is a diverging lens, because it causes the light rays to bend away (diverge) from its axis.
  • The distance from the center of the lens to the focal point is again called the focal length f of the lens.
  • The more powerful the lens, the closer to the lens the rays will cross.
  • Compare the effect of a convex lens and a concave lens on the light rays

• Thin Lenses and Ray Tracing

  • An ideal thin lens has two refracting surfaces but the lens is thin enough toassume that light rays bend only once.
  • Another way of saying this is that the lens thickness is much much smaller than the focal length of the lens.
  • A thin symmetrical lens has two focal points, one on either side and both at the same distance from the lens.
  • The treatment of a lens as a thin lens is known as the "thin lens approximation. "
  • (Ray 2 lies on the axis of the lens. ) The distance from the center of the lens to the focal point is the lens's focal length f.

• The Thin Lens Equation and Magnification

  • How does a lens form an image of an object?
  • A ray entering a converging lens parallel to its axis passes through the focal point F of the lens on the other side.
  • The third ray passes through the nearer focal point on its way into the lens and leaves the lens parallel to its axis (rule 4).
  • The thin lens equation is:
  • Ray tracing is used to locate the image formed by a lens.

• Aberrations

  • An aberration is the failure of rays to converge at one focus because of limitations or defects in a lens or mirror.
  • This aberration happens when the lens fails to focus all the colors on the same convergence point .
  • Since violet rays have a higher refractive index than red, they are bent more and focused closed to the lens. shows a two-lens system using a diverging lens to partially correct for this, but it is nearly impossible to do so completely.
  • Spherical aberrations are a form of aberration where rays converging from the outer edges of a lens converge to a focus closer to the lens, and rays closer to the axis focus further.
  • The apparent effect is that of an image which has been mapped around a sphere, like in a fisheye lens.

• Combinations of Lenses

  • A compound lens is an array of simple lenses with a common axis.
  • In contrast to a simple lens, which consists of only one optical element, a compound lens is an array of simple lenses (elements) with a common axis.
  • An achromatic lens or achromat is a lens that is designed to limit the effects of chromatic and spherical aberration.
  • In the most common type (shown in ), the positive power of the crown lens element is not quite equaled by the negative power of the flint lens element.
  • Calculate focal length for a compound lens from the focal lengths of simple lenses

• The Magnifying Glass

  • A magnifying glass is a convex lens that lets the observer see a larger image of the object being observed.
  • A magnifying glass is a convex lens that lets the observer see a larger image of the object under observation.
  • The lens is usually mounted in a frame with a handle, as shown below .
  • The highest magnifying power is obtained by putting the lens very close to the eye and moving both the eye and the lens together to obtain the best focus.
  • When the lens is used this way, the magnifying power can be found with the following equation:

• Development of Hearing and Balance

  • The human inner ear develops during week four of embryonic development from the auditory placode, a thickening of the ectoderm that gives rise to the bipolar neurons of the cochlear and vestibular ganglions.
  • As the auditory placode invaginates towards the embryonic mesoderm, it forms the auditory vesicle or otocysts.