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 Uses of lenses on optical devices

Microscopes

Simple microscopes use lens systems to magnify very small objects,  it uses magnifying glass where the image appears clearest at about 25 cm from the eye. This distance is known as the least distance of distinct vision (D) or near vision.

An object (S1) too small to examine with the naked eye is placed just outside the focus Fo of the objective lens in the microscope above.

Tracing the infinite and central rays as we have before, we can find the location of the image I1. We know that a source within the focal length of a converging lens will result in an enlarged virtual image; hence if we position the eyepiece of the microscope so that its focus overreaches I1, the final image seen (I2) will be a magnified image.

You will notice that our simple microscope gives an inverted image, which could be inconvenient for certain situations. This problem can be solved by using a different lens arrangement

Magnification in a simple microscope

Magnification produced depends on the focal length of the lens. Lens of short focal give greater magnification than those of long focal length.

The angle ϐ subtended by the image at the eye is much greater than α which is the angle that the object would subtend at the eye when viewed without the lens.

The ratio of the ϐ toα is known as angular magnification or magnifying power of an instrument. The angular magnification is equal to linear magnification.

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 Uses of a simple microscope

1. To study the features of small animals in biology

• 2. To look closely at small print on a map
• 3. To observe crystals in physics and chemistry
• 4. For forensic investigation by the police

2. Compound microscope – It consists of two lenses with one nearer the object called the objective lens and the other nearer the eye called the eyepiece lens.

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Uses of compound microscope

1. Used to observe Brownian motion in science
2. To study micro-organisms and cells in biology
3. Analyze laboratory tests in hospital.
4. Telescope –

Telescopes serve much the same purpose as microscopes; both magnify what the user wishes to observe. The difference is that microscopes are supposed to be used to examine small objects that are close to the objective lens, while telescopes are supposed to be used to examine objects that are very far away

Telescope consists of two lenses; objective and eye-piece lenses. The objective lens has a large focal length while the eye-piece lens has a much shorter focal length.

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• 3. The camera –Cameras work on similar principles as the Magnifying Glasses.   They consists of a converging lens system, clicking button, shutter, diaphragm and a mounting base for the film all enclosed in a light proof box.
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The distance is adjusted to obtain a clear focus.

• The diaphragm has a hole called the aperture with an adjusting control knob to control the amount of light entering the camera. The shutter opens to allow light and close at a given time interval.
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 Uses of a camera

1. The sine camera is used to make motion pictures
2. High speed cameras are used to record movement of particles
3. Close circuit television cameras (CCTV) are used to protect high security installations like banks, supermarkets etc.
4. Digital cameras are used to capture data that can be fed to computers.
5. Human eye – It consists of a transparent cornea, aqueous humour and a crystal-like lens which form a converging lens system. The ciliary muscles contract or relax to change the curvature of the lens.

 Though the image formed at the retina is inverted the brain ‘sees’ the image as upright.

 For distant objects ciliary muscles relax while near objects it contracts to control the focal length and this is known as accommodation. When at 25 cm away an object appears clearest and this is known as least distance of vision or near point.

Common eye defects

1. Short sightedness or hypermetropia– the eyeball is too large for the ‘relaxed focal length’ of the eye. The defect is corrected by placing a concave lens in front of the eye.

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2. Long sightedness or myopia – images are formed beyond the retina. The defect is corrected by placing a converging lens in front of the eye.

3. Presbyopia – this is the inability of the eye to accommodate and this occurs as the eye ages due to the weakening of the ciliary muscles. It can be corrected by the use a pair of spectacles.
4. Astigmatism – this is a defect where the eye has two different focal lengths as a result of the cornea not being spherical. Corrected by the use of cylindrical lens.
5. Colour blindness– caused by deficiency of colour detecting cells in the retina.

  Power of lens

 The power of a simple lens is given by the formula: Power = 1 / f. The unit for power of a lens is diopter (D).

  Example

  Find the power of a concave lens of a focal length 25 cm.

Solution

 Power = 1 / f = 1 / -0.25 = -4 D.

Examples

1. Calculate the height of a building 300 m away from a pinhole camera which produces an image 2.5 cm high if the distance between the pinhole and the screen is 5.0 cm.

 Solution

 Object distance = 300 m, image height = 2.5 cm, image distance = 5.0 cm. Object

 height/ image height = object distance/ image distance

 Object height = (30,000 × 2.5) / 5.0 = 15,000 cm = 150 m.

2. The length of a pinhole camera is 25.0 cm. An object 2.0 cm is placed 10.0 m from the pinhole. Calculate the height of the image produced and its magnification.

 Solution

Image height = (image distance × object height) / object distance

 = (25 ×200) / 10 = 500 cm or 5 m. Magnification = image distance / object distance = 25 /10 = 2.5