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APPLICATIONS OF OPTICS

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2.52 The Human Eye

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2.51 Microscopes & Projectors

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The Human Eye

The main function of the human eye is to form images on the retina. Light enters the eye through the transparent cornea and passes through the lens system to be focussed on the light-sensitive retina, which responds by sending signals to the brain via the optic nerve.

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Aqueous Humor

Source: http://hyperphysics.phy-astr.gsu.edu/hbase/vision/eye.html

Cornea

Retina

Iris

Vitreous Humor

Crystalline Lens

The amount of light entering the eye through the pupil is controlled by a diaphragm, the iris, which reacts to the amount of light available. It can dilate to as much as 6mm and contract to as little as 2mm and gives a person their distinct eye colour.

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The light rays are focussed through the lens system onto the retina which is composed of light sensitive cells called rods and cones. The rods react to the level of brightness (these give you night vision) whereas the cones react to colour as well as brightness (general vision). Cone cells are concentrated near the centre of the retina whereas the rod cells are concentrated at the outer edges of the retina.

Short-sightedness (Myopia)

– able to focus on near objects but distant objects appear blurred.

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Eye Defects

Far–sightedness (Hyperopia)

– able to see distant objects but unable to focus on near objects.

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Optical Instruments

• An optical instrument is a device that uses lenses to create an image of an object
• Microscope, camera, telescope, magnifying glass, movie projector, binocular, eyeglasses,

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• Use a ray diagram to sketch the passage of light through any one of the following optical technologies!

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The Camera: converging lens

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The Lens of a Camera

• When in proper focus, the position of the film coincides with the position of the real image
• To focus: move lens closer to film for a distant object
• Image distance increases as the object distance decreases

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Projectors

A slide projector consists of a system of mirrors and lenses and produces a real, magnified, inverted image of a piece of photographic slide on a screen.

The Microscope

• Similar to Magnifying Lenses
• Object between F’ and lens
• Enlarged, virtual, image beyond 2F’

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The magnification of the magnifying glass is expressed as the ratio of the near point distance to the focal length of the lens. In general, the near point is taken as 25cm.

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Microscopes

A simple microscope consists of a single convex lens or magnifying glass. When the object is placed inside the focal length, a large upright image is produced. This occurs when the image is at the near point of the eye.

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Microscopes (cont.)

A compound microscope consists of a converging lens of short focal length and an eyepiece lens of a longer focal length. This produces a large inverted real image just inside the focus of the eyepiece lens.

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TELESCOPES

• Similar to a microscope but the object is much farther away. Two lenses: objective and eyepiece.
• Objective: produces a real image
• Eyepiece: acts like a magnifying glass – produces a virtual image

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