Daniel Vlk
Department of Biophysics
Vision
The eye ball is a sensory organ serving for our vision. It is almost a sphere covered
with an outer white layer called the sclera. It is elastic and relatively rigid. In the
front part of the eye, the sclera becomes transparent, and it is called the cornea.
The optical media of the eye are the following: cornea, aqueous humour (a liquid
between the cornea and the lens), crystalline lens, and vitreous humour (a gel-like
substance which fills the eye ball). After passing through these optical media, the
light rays fall on the innermost tissue layer of the eye – the retina – and form a real
inverted image there.
Human eye
Anatomy of human eye
Anatomy of human eye
The resolution threshold (the ability to distinguish two points at a small distance from
each other) of the human eye is given by a viewing angle of 1’ (one minute of arc).
The iris regulates the amount of light entering the eyeball.
Human eye
In the retina, there are photosensitive cells called cones (responsible for colour vision)
and rods (ensuring vision in darkness). The cones are concentrated in the central
part of the retina, which is called the yellow spot. The retina has specific cones for
red, green and blue colours. The excitation of the photosensitive cell is due to the
photochemical disintegration of substances called visual purples – rhodopsin (in
rods) or iodopsin (in cones).
Human eye
Accommodation is the ability of the eye to change its dioptric power. It is ensured by
a change of lens curvature due to the activity of the ciliary muscles. The near point is
the minimum distance at which an object can be seen sharply, i.e. at which a sharp
image is formed on the retina. Similarly, the far point is the maximum distance of an
object allowing sharp vision. In a normally seeing eye, the far point lies at infinity. A
frequently used term is the distance of most distinct vision (also “convention visual
distance”) which value is 25 cm.
Accommodation of the eye. To see sharply the near point P (lat. punctum proximum),
the curvature of the lens is maximal. R – far point (lat. punctum remotum).
Human eye
The two most important optical aberrations of an ametropic (i.e. not normally seeing)
human eye are called myopia (nearsightedness) and hyperopia (farsightedness). An
eye capable of normal vision is called emmetropic.
In the case of myopia, the dioptric power of the cornea and/or crystalline lens is higher
than the normal value, and „the image is formed in front of the retina“. This
aberration can be corrected by a diverging lens.
Human eye
Human eye
A hyperopia, the dioptric power of the cornea and/or crystalline lens is lower than the
normal value, and the image would be formed „behind the retina“. This aberration
can be corrected by a converging lens.
Human eye
Human eye
Color vision
Human eye
Color vision
Trichromats
Dichromats
monochromates
Human eye
A magnifier or magnifying glass is any converging lens whose focal distance is
smaller or equal to 25 cm, i.e. dioptric power equal or greater than 4 dpt.
The image formed by the magnifier is virtual, magnified and erect. (See – Hyperopia
corrected by a converging lens).
Suppose that the observed object is placed within the focal distance of the magnifier.
Then:
where γ (gamma) is the angular magnification (= ), τ’(tau-prime) is the virtual angle
of vision, τ (tau) is the real angle of vision, d (25 cm) is the distance of the most
distinct vision, and a is the object distance (≈ f).
Optical instruments
a
d
=γ
τ
τ′
Telescopes
Telescopes
Telescopes
Apart of the mechanical movable and supporting parts, the microscope is formed by an
objective and an eyepiece. The simplest objective is a converging lens that forms a
real, magnified and inverted image. The eyepiece is a converging lens used as a
magnifying glass for observation of the image formed by the objective. Therefore,
the resulting image is magnified, virtual and inverted.
The total magnification of an optical microscope (in practice max. about 2000-times) is
given by the product of the objective and eyepiece magnification:
M = MobjMep,
where Mobj is the magnification of the objective, and Mep is the magnification of the
eyepiece.
Microscope
Microscope
F – focal points, f – focal distances, y - object, y' – real image of the object
formed by the objective, y'' – virtual image seen in the eyepiece, D – optical
interval of the microscope.
Microscope
Single lens reflex camera
Camera