Optical instruments: Applications
Microscopes: one of the applications of light waves is seen in the operation of a simple microscope or magnifying glass. It is also seen in the compound microscope.
Simple microscope or magnifying glass
The compound microscope
Telescopes (astronomical telescope): Astronomical telescope is the simplest type of telescope, used in viewing distant objects, such as stars and planets.
The astronomical telescope
The Galilean telescope in normal adjustment.
The terrestrial telescope
The simple camera
The slide projector
The human eye
The eye is one of the most sensitive instruments devised by nature. The essential parts of the eye and their various functions are described below:
Optical features of the eye
Optical features of the eye
- The cornea is the transparent front part of the eye, that serves as a protective covering and also partly focuses light entering the eye.
- The iris acts as a stop or diaphragm of variable size, while the pupil is a circular aperture in the iris.
- The aqueous humour is the transparent liquid between the lens and the cornea. The vitreous humour is a jelly like liquid between the lens and the rest of the eyeball.
- The retina is the light-sensitive surface at the back of the eye.
- The eye lens is supported by the ciliary muscles and its function is to focus light entering the eye onto the retina.
How we see with the eye
The optical system of the eye consists of the cornea, the aqueous and vitreous humour and the lens. They form a real, diminished and inverted image of an external object on the retina. The retina transmits the impression created on it by this 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.
Normal vision and defects of vision and corrections
The nearest point at which an object is clearly seen by an eye is known as the near point and the farthest point of clear vision is known as the far point.
Long sight (or hypermetropia)
A long sighted person can see objects at a far distance but cannot see close objects clearly, this is caused by the eyeball being too short or the eye lens not being sufficiently convergent.
It is corrected by a suitable converging lens being placed in front of the eye for near vision.
Note that the focal length of a lens =
I/f = 1/v + 1/u
F = uv / U+V
Short sight (myopia)
A short sighted person cannot see distant objects clearly as rays from such objects are focused in front of the retina instead of at the retina. Such a person has eyeballs that are too long, or lenses that are too convergent. This defect of vision is corrected by the use of suitable diverging lenses. The diverging lens makes objects at infinity appear to be at the person’s far point.
Similarities between the human eye with the camera
I) The pupil in the human eye performs the same function as aperture in camera.
II) Both have converging lens systems to focus light from an external object.
III) The human eye has the retina which is light sensitive and the camera has a film which is also light sensitive.
IV) The amount of light entering the eye/camera are being controlled respectively by iris in the human eye and diaphragm in camera.
V) The human eye is impregnated with black pigment within, and the camera consists of a light tight box painted black inside.
Differences between the human eye with the camera
I) The human eye has a variable focal length, while that of the camera is fixed.
II) The eye is a biological organ while the camera is a mechanical device, leading to other differences.
III) The eye suffers defects of vision while the camera does not suffer any of such defects.
IV) The distance between lens and the retina in the human eye is fixed, while the distance between the lens and the film in a camera can be varied.
1. A patient with a sight defect has at least a distance of distinct vision of 150cm. For him to be able to read a material placed at a distance of 25cm, what is the focal length of the glasses he should wear? (Jamb 1997)
Solution: F= uv/u+v = 25×150/ 25+150 = 21.4cm
2. A man wears convex lens glasses of focal length 30cm in order to correct his eye defect. Instead of the optimum 25cm, his least distance of distinct vision is? (Jamb 1998)
Solution: f= uv/u+v, f= -ve, v= -ve
-30= 25×(-ve)/25+(-v); v=150cm
3. In a projection lantern of focal length f, the object distance u, is such that? (Jamb 1994)
4. If the focal length of a camera lens is 20cm, the distance from the film at which the lens must be set to produce a sharp image of an object 100cm away is? (Jamb 1993)
Answer: f= uv/u+v
20= 100×v /100+v
5. A magnified and virtual image of a near object is produced by? (Wassce 2004)
A. Prism binoculars
B. Astronomical telescope
D. Simple microscope
6. Draw a clearly labelled diagram to illustrate how two converging lenses may be arranged to form a compound microscope. (Wassce 2002)
7. In a compound microscope, the image formed by the objective lens is at a distance of 3.0cm from the eye lens. If the final image is at 25.0cm from the eye lens, calculate the focal length of the eyes lens. (Wassce 2002)
Solution: u= 3, v=25
1/f = 1/u + 1/v; 1/f = ⅓ + 1/25
= 25+3/75 = 28/75; f= 75/28 =2.68~2.7
8. An astronomical telescope has objective eyepiece lenses of focal length 3.5m and 5cm respectively. Determine the magnifying power of the telescope when in normal adjustment. (Wassce 2016)
Solution: (focal length of objective lens = f0 = 3.5m and focal length of eyepiece= Fe =5cm = 0.05m
= F0 / Fe = 3.5/0.05 = 7.0