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# Refraction through lenses

## LENSES:

Lenses are used as magnifying glasses.  They are also used in microscopes, telescopes, cameras and projectors.  The human eye has a natural lens and which enables people to see clearly.  There are two types of lenses: Converging and Diverging lenses.

The converging lens brings light rays together while the diverging lens spreads light rays apart.  A converging (convex) lens bulges at the centre while diverging lens gets thinner at the centre.

TERMS

Terms which are commonly used in lenses include, principal axis of a lens, the principal focus of a lens, optical centre of a lens, and focal length of a lens.  The principal axis of a lens is the line joining the centre of curvature of the two surfaces of the lens, and passing through the middle of the lens.

The principal axis of a lens is the line joining the centre of curvature of the two surfaces of the lens, and passing through the middle of the lens.

The principal focus of a lens is the point on the principal axis to which all rays parallel and close to the axis converge or diverge, after refraction the lens.  The principal focus of a converging lens is real, while that of a diverging lens is virtual.  The optical centre of lens is defined as the centre of the lens which is a point on the principal axis of the lens.  Rays of light which pass through the optical centre are undeviated.  The focal length of a lens is the distance between the optical centre and the principal focus of the lens.

FORMATION OF IMAGES IN LENSES

## CONVERGING LENS

To produce the image of an object by a converging lens, two major rays are required:

(1)        A ray from the top of the object incident on the middle, c, of the lens and passes through the lens undeviated.

(2)        A ray from the top of the object parallel to the principal axis, incident on the lens, and refracted through the principal focus, F.  At the point where these two rays interact, the image of the object is formed.

OBJECT AT INFINITY

When an object is very far from the lens i.e at infinity, the image is real, inverted and formed at the focus of the object beyond 2f1.

OBJECT BEYOND 2F1

When an object is very far from the les i.e at infinity, the image is real, inverted and formed at the focus of the object beyond 2f1.

OBJECT BEYOND 2F1

When an object is placed beyond 2F1, the image of the object is formed between F and 2F and is real, inverted and smaller than the object (diminished).

POWER OF A LENS

The power of a lens is the reciprocal of the focal length of a lens in metres.

P = 1/f

## THE SIMPLE MICROSCOPE OR MAGNIFYING GLASS

A complex lens gives an enlarged upright virtual image of an object placed inside the principal focus.  This constitutes a simple microscope.  It is used for reading and studying biological specimens.

General revision

1. A uniform meter rule 0f mass 90g is pivoted at the 40cm mark. If  the rule is at equilibrium with an unknown mass m, placed the 10cm mark and a 72g mass at the 70cm mark determine m.
2. A wire is gradually stretched until it snaps .Sketch a load – extension graph for the wire and on the graph indicate the (a)Elastic limit (b) Yield point (c) Maximum load (d) Breaking point

Weekend Assignment

1. The reciprocal of a lens is called …………….
2. Principal axis    (b) power  (c ) bi-convex   (d) optical centre
3.  ………….. is the distance between optical centre and the principal focus of a lens
4. Optical centre     (b) focal length   (c ) principal axis  (d) radius of curvature
5. An object placed 36cm from a converging lens of a focal length 24cm, forms a real image which is 6cm high. What is the height of the object.
6. 4cm            (b) 3cm            (c) 2cm             (d) 12cm
7. An object is placed 20cm from a converging lens.  If the real image formed is 80cm. find the focal length of the lens?
8. 15cm          (b) 30cm          (c) 10cm           (d) 16cm

5.  A candle of height 5cm is placed 30cm from a convex lens of vocal length 10cm. The      height of the candle’s image is

(a) 10cm            (b) 7.5cm        (c) 5cm           (d) 2.5cm

THEORY

1. The screen of a pinhole camera is a square of side 0.16m and it is 0.15m behind the pin hole. The camera is placed 11m from a flag staff and positioned so that the image of the flag staff is formed centrally on the screen.  The image occupies three quarters of the height of the screen.  What is the height of the flag staff.
2. A converging lens of focal length 15cm forms a virtual image at a point 10cm from the lens. Calculate the distance of the object from the lens