CHAPTER 20 Optical instruments

• Explain with the aid of diagrams how a simple camera, slide projector, microscope and telescope work.
• State the function of each part of the eye. Compare the structure and function of the eye with that of a camera.
• Explain how the human eye forms images of objects and is able to focus close and distant objects.
• Distinguish between long sight and short sight. Explain how each of these conditions can be corrected.
• Describe various defects of the eye.
• Explain how a slide projector, microscope, and magnifying glass work.
• Compare the optics of refracting, reflecting and Cassegrain telescopes.

SCIENTIFIC PROCESSES

• Draw and label the main parts of the human eye.
• Distinguish fact from opinion about optical problems.
• Write a report about the construction of an optical device.

COMPLEX REASONING PROCESSES

• Compare the uses of mirrors and prisms in optical instruments
• Propose evidence that would be required to confirm or refute a theory or belief about optical phenomena.

CHAPTER 20 SUMMARY

• The main parts of the human eye are the: cornea, pupil, iris, lens, ciliary muscles, retina and optic nerve.
• Images are produced on the light sensitive retina by the refraction of light by the cornea and the lens.
• The lens has the ability to change its shape, by the contraction and relaxation of the ciliary muscles, so as to focus distant and close objects onto the retina. This ability of the eye is called accommodation.
• The distance of the furthest object that can be seen sharply is called the far point. The closest distance - the near point.
• The optic nerve carries signals from the retina to the brain.
• The retina contains two types of cells - cones and rods. The cones are sensitive to colour and bright light. The rods respond better in dim light.
• Hypermetropia (long-sightedness) is when the eye can see far objects clearly but not close objects. It can be corrected by using convex lenses.
• Myopia (short sightedness) is when near objects can be seen sharply but far objects can not. It can be corrected by concave lenses.
• Presbyopia is a mixture of hypermetropia and myopia and requires bifocals to correct this defect.
• Astigmatism occurs when the cornea is not spherical causing objects in some directions not being in focused while other are.
• The main parts of a camera are the lens, the aperture, the shutter, and the film.
• The speed at which the shutter opens and closes and the size of the aperture controls the light entering the camera and the subsequent photographic exposure.
• The selected shutter speed is determined on the basis of the movement of the object, the speed of the film, and the lighting conditions.
• The lens of a camera is moveable so as to produce sharp images of objects at various distances on the film. Zoom lenses allow a variety of focal lengths.
• Refracting telescopes consist of two convex lenses - a long focal length objective lens and a short focal length eyepiece lens which together produce a large inverted virtual image.
• The magnification of a telescope is given by the formula:
  M = f_o/f_e ..... Where f_o is the focal length of the objective lens, and f_e is the focal length of the eyepiece lens.
• Terrestrial telescopes use a third lens to produce an upright image.
• Galilean telescopes use concave lenses as the eyepieces to produce upright images.
• Reflecting telescopes use large concave mirrors instead of the objective lenses. They are less expensive and can collect more light than refracting telescopes.
• A simple microscope is a single convex lens. Its magnification is given by the formula:
  M = 25/f ,Where 25 cm is the near point of the eye and (f) is the focal length of the lens.
• A compound microscope consists of two convex lenses which together produce a large inverted image at the near point of the eye.
• Projectors and enlargers use a combination of mirrors and lenses to produce an image of a photographic slide or negative onto a screen or light sensitive photographic paper.

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