5.4 Understanding Lenses

  There are 2 types of lenses
(1) Convex lenses
(2) Concave lenses 

Differences between convex and concave lenses 

(a)convex lenses

    ~Convex lenses are also known as converging lenses or positive   lenses.
    ~Convex lenses are thicker in the centre than at the edge.
    ~Cross-sectional views of common types of convex lenses:  

    ~Convex lenses refract incident rays of light,which are parallel to the principal axis,so that they converge to  a point on the principal axis.This point is called the real principal focus.

(b)concave lenses

    ~Concave lenses are also known as diverging lenses or negative lenses.

    ~Concave lenses are thinner in the centre than at the edge.

    ~Cross-sectional views of common types of concave lenses:

    ~Concave lenses refract incident rays of light , which are parallel to the principle axis ,so that they appear    

      to diverge from a point located on the incident side of the principal axis. This point is called the virtual    

      principal focus.

Common terminology

(a) convex lens

(b) concave lens

Power of a lens

*The power of a lens is a measure of the ability of the lens to converge or diverge rays of light.

*The formula for the power of a lens is:

                                                            Power(D)=1/Focal length(m)

                                                                            =1/f

*unit of the power of lens is dioptres or D.

*A convex lenses has a positive focal length and power.

*A concave lenses has a negative focal length and power.

Ray Diagrams

*to determine the position and characteristics of images formed by lenses for any position of the object.

*can be completed using any two out of the three rays of light.

Images Formed by Convex Lenses

(a) Distant object , u = infinity

  Image distance: v = f

  *Real

  *Inverted

  *Diminished

  *On the opposite side of the object

(b)Object distance is more than 2f, u>2f

Image distance: f<v<2f

*Real

*Inverted 

*Diminished

*On the opposite side of the object

(c)Object distance is equal to 2f ,u=2f

Image distance : v = 2f

*Real

*Inverted

*Same size as object 

*On the opposite side of the object

(d)Object distance is between f and 2f , ,f<u<2f

Image distance:v>2f

*Real

*Inverted

*Magnified

*On the opposite side of the object

(e)Object distance is equal to f, u=f

Image is at infinity

*virtual 

*Upright

*Magnified

*On the same side as the object

(f)Object distance is less than f ,u<f

Object distance:v>u

*virtual

*upright

*magnified

*On the same side as the object

Images formed by Concave Lenses

*do not depend on the position of the object with respect to the lens

*an image formed by a concave lens is a always

(a)virtual 

(b)upright

(c)smaller than the object 

(d)located between the lens and the object

(e)the image distance is less than the focal length

Linear Magnification

*the ratio of the size of the image to the size of the object.

Linear magnification, m=size of image/size of object

The Use of Lenses in Optical Devices 

Differences  between microscope and telescope

(a) Microscope

*A compound microscope is an optical instrument used to view very small or fine objects.

*A compound microscope consists of two convex lenses of high power.

  (a) The lens nearer the object is called the objective lens , with focal length,f。

  (b) The lens nearer the eye is called the eyepiece , with focal length,fe'

*The objective les has a higher power as it has a shorter focal length compared to the eyepiece.

(b)Astronomical Telescope

 *an astronomical telescope is an optical instrument that is used to view object at a great distance , such as 

  planet and stars.

*an astronomical telescope consists of two convex lenses. The objective lens whose power is low has along  focal length,fo.The eyepiece whose power is high has a short focal length fe.

*the distance between the objective lens and the eyepiece is (fo+fe).