9/22/20
Light (Lens)
Syllabus
h)
describe the action of a thin lens (both converging and
diverging (P)) on a beam of light
i)
define the term focal length for a converging lens
j)
draw ray diagrams to illustrate the formation of real and
Lens
In general there are two types of lens:
– Converging lens- thicker in the middle than at edge – Diverging lens- thinner in the middle
Complete the diagram to show how the rays of light bend
as it move through the lens.
Lens
Points to note
– A thin converging lens will converge parallel rays of light to a point.
– A ray of light will pass through the center of the lens without
deviation.
(Think about the rectangular glass block)
– Rays from a point on a distance object can be regarded as a
Definition
Optical center (C): the center of the lens
Principal axis: the line which passes symmetrically through the optical
center of the lens.
Focal point (F1 / F2): a point which all rays parallel to the principal axis
will converge to.
Focal length (f): the distance between the focal point and the optical
center.
Focal plane (FF’): vertical plane which passes through F and F' and
perpendicular to the principal axis.
F C converging lens f F’ focal length parallel rays of light F C parallel
rays of light converging lens
principal axis
3 Principal Rays of Light
Incident ray parallel to the principal
axis passes thorough the focal point, F1.
The incident ray which passes
through the optical center is not deviated.
The incident ray passing through
– Pick each point on the object and draw the ray diagram to locate the
position of that image.
– Most objects is a straight line, hence the image is also a straight line – Usually the base of the object is on the principal axis.
– Think about what if the object is not on the principal axis.
Ray Diagram
Steps for Ray Diagram
Pg. 244
1. Draw a line XY, representing the principal axis.
2. Mark position of lens with a vertical line drawn across the
principal axis a point C, with two arrow heads. Light rays will bend at this line.
3. Mark two principal foci, located at equal distance from C. 4. Draw object OA.
5. Draw one ray of light from the top of the object, A straight through the optical center.
6. Draw a second ray of light from the top of the object, parallel to XY, to the lens. Ray will bend and pass through F1.
Practice
1. Given that the lens has a focal length of 2cm. The height of the object is 1cm. Draw the following cases :
a. Object is 3cm from the lens. b. Object is 4cm from the lens.
c. Object is 1cm from the lens. (will the 2 rays meet?)
(Align the position of the lens together in order to compare the characteristics of the image)
2. An object of height 5cm will result in a real image of size 10cm. The distance between the object and the image is 40cm. By drawing a scaled diagram:
a. Find the distance of the lens from the object. b. Find the focal length of the lens.
Make your own notes on the six cases
– Pg. 250
Magnification Ratio
Linear Magnification:
m = height of image height of object
m = distance of image from lens (v) distance of object from lens (u)
An object 3cm high is placed 12cm from a converging lens. A real
image is found to be formed 36cm from the lens. What is the height of the image?
– m = ht. of image/ ht. of object
ht. of image = (v / u) x ht. of object = (36 / 12) x 3
= 9cm
3. For 1a and 1b, find the magnification ratio using the difference in height. 4. For 1c and 2, find the magnification ratio using the difference in
Practice
An object 3.0 cm high is placed 12.0 cm from a converging lens. A real
image is found to be formed 36.0 cm from the lens.
a. Draw a scaled diagram and locate the focal length of the lens. b. What is the height of the image?
c. If the location of the object and screen is fixed, but the image
needs to be 48.0 cm in height, suggest a suitable focal length of the lens to be used.
Application
Slide projector
– Concave mirror – Condenser lens – Projection lens – Slide
Photographic enlarger
Light
Reflection Refraction
undergoes
mirror
Laws of reflection Laws of refraction
Challenge
2. a. A slide projector is used to enlarge a 20cm x 20cm slide onto a screen of dimension 1.6m x 1.6m. The lens of the projector is fixed at 30cm from the slide and 2m from the screen. In order to obtain a sharp image on the screen, a lens with a suitable focal length must be used. Using a ray diagram, find the suitable focal length of the lens. Hence state the maximum size of the sharp image. [6]
MCQ
.Using a light source, the image of an object “F” is focussed onto
a screen through a thin converging lens.
Which of the following images will you see, when you stand
behind the screen?
screen lens
object