Lenses in science and technology

TECHNOLOGY

By MICHAEL JOHN UGLO
WELCOME and our ninth lecture as per our schedule is on lenses in our Science in Action Series for today. If you come across the word lens in science, then it may remind you of the hand lens which is a form of a magnifying glass known as a convex lens used to concentrate sunlight to light a cigarette or make a fire using the sunlight. That is the instrument you have rightly thought of. Lens may also remind you of the two convex lenses used in a microscope as an eyepiece and an objective lens to multiply the tiny specimen on the stage for viewing. This is also the correct picture of the lens and its uses. You also have concave lenses which diverges or spreads out rays of light directly opposite to the function of the concave lenses. Your eye balls also contain a lens called eye lens to vary in shape and size by a type of muscles called cilliary muscles to control the light entering the pupil of your eyes to take care of what is called the power of accommodation to appropriately focus the light from different distances on to the retina for the optic nerve to transmit it to the brain for processing.
In this lecture, I will attempt to relay to all you need to know about lenses
A lens in physics is a device. It is an instrument or device that can transmit a ray of light. Light in physics can be any electromagnetic ray which includes a visible light ray. Lenses are made from grounded and polished glasses and plastics. Lenses can refract and is they can bend light. When they refract light they can bring the light which travel in parallel rays to focus or converge light rays. This will form a positive focal length behind the lens. That particular lens is called a positive lens or a convex lens.
The refraction can diverge or spread out the parallel rays of light that fall on the surface or incidents on the surface. This is normally done by a negative lens or a concave lens. The focal point is formed in front of the convex lens. It will be read in negative value while the convex lens’ focal length value is in positive values. The refraction happens for any material or medium that the incident light is in contact with. Any material has its own refractive index. For instance, water has a different refractive index to air, then sea water has different refractive index to a glass and so on.
Different types of lenses are, known and these are double sided convex lenses are called biconvex lenses. One sided convex and one-sided plane lenses are called plano-convex lenses. Likewise, double sided concave lenses are called biconcave and one-sided concave and one-sided plane lenses are known as plano-concave lenses. One sided convex and the other concave sided lenses are called meniscus lenses or concave-convex lenses.
These lenses function to correct aberrations in visions. These visions can be magnified as in magnifying glasses and microscopes or telescopes with unreal images known as virtual images. They can also be reduced to smaller sizes and form real images as in camera lenses. The lenses can be used to correct defects in the eye as health problems to see correct visions. These problems are called myopia for short sightedness which results in people having long eye balls that the ray of light to the retina is not correctly focused and the focal point is not found on the retina to transmit the impulse to the brain via the optic nerve. A convex lens is used to correct this. Hypermetropia is the other health condition for long sightedness resulting from people with short eye balls. The focal point cannot reach the retina when the eye lens focuses it because the focal length falls short. Thus to extend it in the corrective exercise is to use a concave lens to bring the focal point to land on the retina for the optic nerve to bring the impulse to the brain to detect or form the image.
In the case of another health condition called the astigmatism, the curvature of the lens and cornea of the eye is not conducive to form the correct image because the focal lengths are affected. Lenses with corrective curvatures can be tried and tested to help correct this health condition. The important equation to make the different kinds of lenses are
1/f=(n-1)[1/R1-1/R2+(n-1)d/nR1R2] where;
f is the focal length of the lens
n is the refractive index of the lens material
R1 is the radius of curvature of the lens closer to light source
R2 is the radius of curvature of the lens further from the light source
The optical power of the lens is obtained from the reciprocal which is 1/f of the focal length f of the lens.
A regular arrangement of a lens or more than one lenses can refract the light to form any image form the electromagnetic spectrum. Electrons beams, radio beams and infra radiations and even microwaves can be refracted to focus for the formation of images. Bigger convex lenses can be used to concentrate solar energy on photovoltaic cells to harvest solar energy from the sun. In radar and radioastronomy together with radio communications use lenses to refract waves that cannot be seen with the eye into usable forms for users.
Lenses in camera as well as microscopes, telescopes and kaleidoscopes have different arrangements of these refracting surfaces of lenses and mirrors for their different uses.
Microscope lenses are convex lenses known as an optical lens found at the eye piece. The other convex lens is the objective lens. Both lenses are used to magnify specimen for the eye to see a bigger image of what cannot be seen because it is so tiny.
A telescope is an astronomical device also made of convex lenses to bring to near point the image of an object that is far off in space. Kaleidoscopes an instruments which have reflecting surfaces of mirrors facing one another to see a repetition of the rays of light from an object. Mirrors have a silvery backing to reflect light while a mirror is transparent and can refract light.
Diffraction of light is used to refer to light that bends around the edge of an object. Diffraction and refraction are both used to mean bending of light rays. Diffraction is bending of light rays around the edge of an object while refraction is bending of light as it travel from one medium to another.
When light ray travels from a more, dense medium to a less dense medium, it bends away from the normal. The light travels that from a less dense medium to a more, dense medium it bends towards the normal. At an angle, when the angle of incidence is more than the critical angle, total internal reflection occurs when all the light rays reflected back into the medium.
My prayer for PNG today is in this hymn; “What a friend we have in Jesus, all our sins and pains to bear, What a joy it is to carry, take it to the Lord in prayer”

Next week: Radioactivity and nuclear energy.

Michael Uglo is the author of the science text book Science “Science in PNG, Pacific, Asia and Caribbean”and a lecturer in Avionics, Auto-Piloting and Aircraft Engineering. Please send comments to l: [email protected]