A concise Introduction to thin lenses

Other topics

Introduction to thin lenses involves general description about properties of thin lenses.

A lens can be defined as a piece of curved glass or plastic that makes things look larger, smaller or clearer when you look through it.

In human eye, one component is a lens and so we can also define lens as the transparent part of the eye, behind the pupil, that focuses light so that you can see clearly.

The idea behind lens operations is that when a light ray passes from air which is more optically denser than the lens material, it is refracted.

When many rays passes through the lens, they all refracted the same way and so they meet at a common point. Sometimes they don’t meet but instead they are scattered after refraction but they are seemed to be spreading from a common point.

Lenses are usually made of glass, transparent plastic or perspex.

common application of lenses includes cameras, spectacles,telescopes, microscopes, film projectors and the human eye.

A thin lens means a lens whose thickness is negligible compared to the radius of curvature of the lens surfaces.

Types of lenses

The basic two types of lenses are convex and concave lenses.

Convex lenses are also called converging lenses as they cause the rays that passes through it to meet at a point. Convex lenses are thickest at the middle and they thin in as you move towards their edge.

In this lesson we will be talking about biconvex lenses meaning that it is symmetrical if we cut it long it’s edges. Both sides of it’s services at the center are bulging outwards and the edges are curved inwards uniformly on both sides.see the figure below

Concave lenses are also called diverging lenses as they cause the rays passing through them to be spreading from a common pint. Concave lenses are thinnest at the middle and they they become thicker as you move towards the edges.

There are variations of convex and concave lenses as illustrated in figures below

Effects of lenses on Parallel rays of light

A cardboard with parallel slits is placed between the mirror and a bi-convex lens as in figure below

The mirror is set such that it reflects the sun rays so that the rays passes through the slits before they reach the lens.

After making observations, the bi-convex lens is replaced with a concave lens

Observation

when a convex lens is used, the rays are converged at a point on the paper and then diverge as they continue as shown.

When concave lens is used , the rays diverge as if they were from the focal point in front of the lens as shown.

Investigating convergence and divergence of light using a ray box

A ray box acts as a source of parallel beam. A spot light can also be used.

A parallel beam is directed incident to the the lens as shown

A white paper is placed on the other side of the lens and it’s position adjusted until a sharp point is observed.

observation

When a convex lens is used, the rays are converged at a point on the paper and then diverges as they continue as shown below

If convex lens was replaced with concave (diverging) lens, the rays will be observed diverging as if they are coming from a point on the other side of the lens. see the diagram below.

Explanations

Light is usually refracted when it passes through a glass prism. A lens can be considered as an assembly of many tiny prisms where each prism refracts light as in figure below.

Please note that, the middle part of the prism is like a rectangular glass prism and a ray that is incident to it at a perpendicular angle passes through without being refracted. As we may see in other lessons, a ray of light that passes normally through the geometrical center of the lens, passes through undeviated.

The figure below shows representation of concave lens as an assembly of prisms.

conclusions

Rays of light that passes through a lens converges at a fixed point from the lens if the lens is a converging lens or diverge from a common imaginary point if the lens is a diverging lens.

The point at which the rays emerging from the lens converge or seems to diverge from is referred to as the principal focus.

A convex lens has a real principal focus while a concave lens has a virtual (imaginary) principal focus.

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Related Topics

• Focal length by non-parallax method
• Estimating focal length
• focusing a distance object
• The lens formula
• Linear magnification
• Ray diagrams
• Image formation by thin lenses
• Introduction to thin lenses
• Finding focal length by displacement
• Albert Einstein

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References:

• Secondary Physics Student’s Book Four. 3rd ed., Kenya Literature Bureau, 2012. pp. 1-42.
• Abbot A. F. (1980), Ordinary Level Physics, 3rd Edition, Heinemann Books International,
  London.
• Nelkon M. and Parker P., (1987), Advanced Level Physics, Heinemann Educational
  Publishers, London.
• Tom D., and Heather K. Cambridge IGCSE Physics. 3rd ed., Hodder Education, 2018, https://doi.org/978 1 4441 76421. pp. 106-142.

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