Optical Instruments

• The Compound Microscope
• The Astronomical Telescope
• The Human Eye

THE COMPOUND MICROSCOPE

The compound microscope produces a greater magnification than the simple microscope.  It has two lenses, the objective lens which has a short focal length and the eye piece used as the magnifying glass to view an image formed by the objective lens.

The image formed by the objective lens is within the principal focus of the piece.  So a final image is formed at the least distance of distinctive vision from the eye.

THE ASTRONOMICAL TELESCOPE

An astronomical telescope is used for viewing distance objects like stars and planets. The astronomical telescope uses two convex lenses; the objective lens and the eye piece.

The objective lens has a long focal length and forms a real image of a distant object at its focal plane.  The position of the eyepiece and the objective lens must coincide along the principal focus so that the final image is at infinity.  The astronomical telescope gives an inverted image which can be tolerated when looking at the stars but is at a disadvantage on the earth.

THE HUMAN EYE

The optical system of the eye consist of the cornea, the aqueous, the vitreous humour and the lens.  They form a real and inverted image of an external object on the retina.  The retina transmits the impression created on it by the image through the optic nerve to the brain.  The brain then interprets the impression.  The amount of light entering the eye through the pupil is regulated by the iris.)

(a) A long sighted person can see objects at a distance but cannot see close objects clearly.  His near point is more than 25cm which is the near point of the normal eye.  It is caused by the eye ball being too short so that rays from object at 25cm from the eye are brought to focus behind the retina.  It is corrected by converging lens placed in front of the eye for near vision.

(b) A short sighted person cannot see distant objects clearly as rays from such objects are focused in front of the retina.  His far point is less than the normal far point which is at infinity.  It is corrected by the use of diverging lens.  The diverging lens makes the object at infinity to appear to be at the person’s far point.

CLASSWORKS 3&4

1. Explain these eye defects with their corrections (a) Hypermetropia (b) Myopia (c) Astigmatism
2. Write short note on these: (a) Accommodation (b) near point (c) far point
3. Give 3 differences between the camera and the eye
4. Highlight 3 similarities of the eye and the camera

ASSIGNMENT 3&4

SECTION A

1. For correcting long sight defects in the human eyes, we require ……….. (a) Converging lens (b) diverging lens (c) microscope (d) periscope
2. A magnified and virtual image of a near object is produced by (a) prism binocular (b) astronomical telescope (c) periscope (d) simple periscope
3. When an astronomical telescope is in normal adjustment, the focal length of the objective lens is 50cm and that of the eye piece is 5.0cm. What is the distance between the lenses (a) 10.0cm (b) 30.0cm (c) 45.0cm (d) 55.0cm
4. Which of the following optical instruments does not make use of a lens? (a) projector (b) periscope (c) eye (d) microscope
5. The ability of the eye to focus object at different distances is called …….. (a) Power (b) accommodations     (c) normal vision  (d) long sight
6. What part of the camera corresponds to the iris of the eye? (a) diaphragm (b) film (c) lens (d) shutter
7. Binocular vision (a) Restricts the field of view (b) Enables a person to see further (c) Enables objects to be seen in relief (d) Enables objects to be seen clearly

SECTION B

1. Illustrate with diagrams how these eye defects can be remedied by the use of suitable lenses
2. Draw the optical arrangement of an astronomical telescope in normal adjustment showing the positions of the principal foci of the lenses used and the path of two rays from the top of a distant object through the instrument to the observer’s eye.
3. Explain these terms: binocular vision, persistence of vision

MID-TERM PROJECT

Using a white cardboard, draw the dispersion of white light and the colour wheel for the mixtures of primary colours