Geometrical OpticsThe cameraThe simple magnifierThe compound microscope The telescopeIllumination and Brightness in Optical Instruments

Cameras

Basic parts of a camera:

• Lens

• Light-tight box

• Shutter

• Film or electronic sensor

Lens (or lenses) is for focusing the image.Diaphragm controls the amount of light (exposure).Shutter lets the light through briefly. Its speed also controls the exposure.Film or sensor detects the image.

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Camera adjustments:

• Shutter speed: controls the amount of time light enters the camera. A faster shutter speed makes a sharper picture.

•Focusing: this adjusts the position of the lens so that the image is positioned on the film.

•f-stop: controls the maximum opening of the shutter. This allows the right amount of light to enter to properly expose the film, and must be adjusted for external light conditions. f-stop = f/D

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The intensity I of the light reaching the film is proportional to the area of the lens. Because this area is proportional to D2, we conclude that I is also proportional to D2. Light intensity is a measure of the rate at which energy is received by the film per unit area of the image. The intensity is also proportional to 1/f 2, and thus I D2/f 2. The brightness of the image formed on the film depends on the light intensity, so we see that the image brightness depends on both the focallength and the diameter of the lens.

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The Simple Magnifier (Magnifying Glass)

A magnifying glass (simple magnifier) is a converging lens. It allows us to focus on objects closer than the near point, so that they make a larger, and therefore clearer, image on the retina.

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The power of a magnifying glass is described by its angular magnification:

If the eye is relaxed (object is at focal point and image at infinity) :

If the eye is focused at the near point (image at near point):

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(N is the near point distance and f the focal length)

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A compound microscope has an objective and an eyepiece; it is different from a telescope in that the object is placed very close to the objective lens.

Compound Microscope

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(The objective lens is closest to the object, and the eyepiece is closest to the eye.)

The magnification is given by:

where l is distance between lenses.

v = l- fe is only true if eye is relaxed and image I1 is at Fe, final image at .

where N = 25 cm (normal eye)e

e f

NM Then,

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Eg. The distance between the eyepiece and the objective lens in a certain compound microscope is 23.0 cm. The focal length of the eyepiece is 2.50 cm and that of the objective is 0.400 cm. What is the overall magnification of the microscope?

Astronomical Telescopes (Keplerian)

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TelescopesA refracting telescope consists of two lenses at opposite ends of a long tube. If eye relaxed and eyepiece adjusted so I2 is at infinity, then real image I1 is at focal point Fe’, and distance between lenses is

l = fo + fe

Since ≈ tan ≈ h/fo and ’ ≈ h/fe, total magnifying power

(angular magnification) is

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Eg. The desired overall magnification of a compound microscope is 140. The objective alone produces a lateral magnification of 12.0. Determine the required focal length of the eyepiece.

A terrestrial telescope, used for viewing objects on Earth, should produce an upright image.

Terrestrial Telescopes

Galilean

Field lens

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Illumination and Brightness The amount of light per unit area reflected or emitted by an object being photographed is called its brightness or luminance B. The amount of light per unit area falling on a photographic film or plate is called the illuminance E. The illuminance E depends upon three factors: the brightness B of the object, the area of the entrance pupil of the lens, a2/4, and the focal length of the lens f .

The light entering a camera is proportional to the brightness of the object, is proportional to the area of the entrance pupil, and is inversely proportional to the square of the focal length. As an equation, where k is a proportionality constant and a is the diameter of the entrance pupil. For any given object to be photographed, we can write Badrul Hisham Nov13 15