Ch. 50 Flashcards
(53 cards)
What is refractive index?
The refractive index is a number that tells us how much slower light travels in a material compared to air. Light travels fastest in air at about 300,000 kilometers per second. If light travels at 200,000 km/sec in a material like glass, then the refractive index is 300,000 ÷ 200,000 = 1.50. Air has a refractive index of 1.00, so anything higher than 1.00 means light travels slower in that substance than in air.
What happens when light changes material?
When light passes from one material to another (e.g. from air into glass), it changes speed. If the surface is straight or perpendicular to the light, the light slows down but keeps moving in the same direction. The wavefronts get closer together due to the slower speed. If the light hits at an angle, it bends. This bending is called refraction.
Why does light bend at an angle?
Light bends when it enters a new material at an angle because different parts of the light beam enter the material at different times. For example, if light moves from air (index 1.00) to glass (index 1.50), the bottom part of the beam enters first and slows down, while the top part is still in the air and moves faster. This uneven speed causes the beam to change direction and bend toward the glass.
What affects the amount of bending in refraction?
Two things affect the amount of bending: (1) the ratio between the refractive indices of the two materials, and (2) the angle between the surface and the light beam. Greater difference in refractive index and more slanted angles result in more bending.
What does a convex lens do?
A convex lens (bulges outward) bends parallel light rays toward each other so they meet at one point, called the focal point. In the center, light passes straight through. At the edges, light hits at an angle and bends inward. Half the bending happens when entering, and half when exiting. If the curve is correct, all rays meet at the same focal point.
What does a concave lens do?
A concave lens (curves inward) spreads light rays apart (diverges them). Light at the center passes straight through. Light near the edges enters sooner and bends outward. As a result, rays move away from each other and don’t meet at a point.
What is a cylindrical lens?
A cylindrical lens bends light in only one direction (or plane), unlike a spherical lens which bends in all directions. For example, a convex cylindrical lens bends rays from the sides but not from the top or bottom, forming a focal line instead of a point. A water-filled test tube in sunlight shows this effect. A concave cylindrical lens spreads light in one direction. Placing two cylindrical lenses at right angles (one vertical, one horizontal) makes them behave like a spherical lens—focusing all light rays at one point.
What is focal length?
Focal length is the distance from the lens to the point where light rays come together after passing through it. Parallel light rays (from distant objects) meet at a specific focal point. Diverging rays (from nearby sources) take longer to meet, so the focal point is farther. A more curved (convex) lens can bend rays more sharply, bringing diverging rays to the same focal point. Changing the lens shape changes how and where light is focused.
How does a convex lens form an image?
Light from a point on an object spreads in all directions. A convex lens focuses this light. Rays through the lens center go straight; rays at the edges bend inward and meet on the other side. Each object point forms a focused light point. A screen held at the correct distance shows a clear image, which will be upside down and reversed left-to-right. This is how camera lenses work—they capture images by focusing light to form a complete inverted image.
Q
A
What is a diopter?
A diopter (D) is the unit of refractive power and tells how strongly a lens bends light. It is calculated as: Diopter = 1 ÷ focal length (in meters). Examples: 1 m focal length = +1 D, 0.5 m = +2 D, 0.1 m = +10 D. Convex lenses have positive diopters and converge light. Concave lenses have negative diopters and diverge light. A -1 D concave lens can cancel a +1 D convex lens, resulting in 0 D—light passes without focusing.
What is cylindrical lens power?
Cylindrical lenses have power in diopters like spherical lenses, but they focus light into a line, not a point. A +1 D cylindrical lens focuses light into a line 1 m away. A -1 D lens diverges light as much as a +1 D lens converges it. They also have an axis: 0° = horizontal focus line; 90° = vertical focus line. For example, +2 D at 90° means vertical line focus at 0.5 m from lens.
How does the eye bend light?
The eye works like a camera, bending light to focus it on the retina. Light bends at 4 main places: (1) air to front surface of cornea, (2) back of cornea to aqueous humor, (3) aqueous humor to front of lens, (4) back of lens to vitreous humor. Refractive indices: air = 1.00, cornea = 1.38, aqueous humor = 1.33, lens = 1.40 (average), vitreous humor = 1.34.
What is the reduced eye model?
To simplify calculations, the eye is modeled as the ‘reduced eye’—a single curved surface about 17 mm in front of the retina. Total refractive power = 59 diopters for distant vision. About 2/3 of this power comes from the cornea due to the large difference in refractive index between air and cornea. The lens adds ~20 diopters (1/3) and is important because it can change shape to focus—this ability is called accommodation.
How does the eye form an image?
The eye focuses light on the retina just like a glass lens focuses light on paper. The image is upside down and flipped left to right. The brain is trained to interpret it correctly and flips it upright in your mind, so you see the world properly oriented.
What is accommodation?
Accommodation is the process by which the eye increases its refractive power to focus on near objects. In young people, the lens is soft and flexible. Normally, it is flatter due to tension from suspensory ligaments. The ciliary muscle has two types of fibers: (1) meridional (pull ligaments inward), (2) circular (squeeze around lens). When these muscles contract, ligament tension reduces and the lens bounces into a rounder shape. This increases curvature and refractive power. Controlled mainly by parasympathetic nerves from the oculomotor nerve. In children, the lens can increase its power from 20 D (far vision) to 34 D (close-up), a 14 D increase. Sympathetic control is weak and not important for normal focusing.
What is presbyopia?
Presbyopia is the age-related loss of lens flexibility, reducing its ability to accommodate (focus on near objects). It happens because lens proteins denature and harden. In children, accommodation power = ~14 D. By age 45–50: <2 D. By age 70: almost no accommodation—lens is stiff. The eye remains focused at a fixed distance. People with presbyopia often use bifocal glasses: top part for distance vision, bottom part for reading or close work.
What is visual acuity?
Visual acuity is how sharp or clear your vision is, or how well your eye can distinguish two separate objects. In an ideal eye, light from a single point focuses into a tiny dot (~11 µm wide) on the retina. Cones in the fovea are ~1.5 µm wide, allowing us to see two points if they’re at least 2 µm apart. Normal vision can distinguish two points forming a 25 arc second angle, like two 1.5–2 mm dots at 10 m. Sharp vision only happens in the fovea (<0.5 mm wide); outside it, rods/cones converge onto fewer nerve fibers, reducing clarity.
How is visual acuity measured clinically?
Visual acuity is tested using a letter chart from 20 feet away. 20/20 vision means you can read at 20 feet what a normal person can also read at 20 feet. 20/200 means you can only read at 20 feet what a person with normal vision can read at 200 feet.
What are the 3 main ways the brain judges distance (depth perception)?
- Size of known objects on the retina – The brain estimates distance from how large familiar objects appear. 2. Moving parallax – With one eye, moving your head causes closer objects to shift more than distant ones. 3. Stereopsis – With both eyes, binocular disparity (2.5 inches apart) lets the brain compare angles for depth. It works well for close objects but is less useful beyond 50–200 feet.
How is aqueous humor formed?
Aqueous humor is made by the ciliary processes at 2–3 µL/min. The ciliary body has folds (~6 cm² surface area) with epithelial cells over blood vessels. 1. Na⁺ is pumped out of blood into intercellular space. 2. Cl⁻ and HCO₃⁻ follow for electrical balance. 3. Water follows osmotically. 4. Nutrients like amino acids, vitamin C, and glucose are transported into the aqueous humor.
What is the path of aqueous humor drainage?
It flows from posterior chamber through the pupil into the anterior chamber. From there: iridocorneal angle → trabecular meshwork (sponge-like) → canal of Schlemm (a vein-like circular channel) → aqueous veins. These veins usually carry only aqueous humor, not blood.
What is normal intraocular pressure (IOP) and how is it measured?
Normal IOP is ~15 mm Hg (12–20 mm Hg is normal range). Measured using a tonometer: 1. Cornea is numbed. 2. A flat footplate gently presses the cornea. 3. Resistance to indentation gives a pressure reading.
How is intraocular pressure regulated?
IOP is regulated by drainage of aqueous humor through trabecular meshwork into canal of Schlemm. The channels (2–3 µm wide) offer resistance. At ~15 mm Hg, outflow rate matches formation at ~2.5 µL/min. Phagocytic cells in the meshwork remove debris, while iris epithelium helps clear proteins and particles.
