LENSES. A lens is any glass, plastic or transparent refractive medium with two opposite faces, and at least one of the faces must be curved.

Size: px

Start display at page:

Download "LENSES. A lens is any glass, plastic or transparent refractive medium with two opposite faces, and at least one of the faces must be curved."

Deirdre Black
6 years ago
Views:

1 1 LENSES A lens is any glass, plastic or transparent refractive medium with two opposite faces, and at least one of the faces must be curved. Types of Lenses There are two types of basic lenses: Converging/ Convex Lenses These lenses are thicker in the middle than at the edge, and cause parallel beams of light to converge. Diverging/ Concave Lenses These lenses are thinner in the middle, than at the edge and cause parallel beams of light to diverge.

$Principal Focus of a Convex Lens This is the point on the principal axis to which parallel rays converge after passing through the convex lens (after refraction).$

2 DEFINITIONS 2 ' Principal Axis This is an imaginary line perpendicular which passes through the optical centre. Principal Focus (F):. Principal Focus of a Convex Lens This is the point on the principal axis to which parallel rays converge after passing through the convex lens (after refraction). (F is real) Principal Focus of a Concave Lens This is the point on the principal axis from a parallel beam of light to the principal axis appears to come on leaving the lens. (F is virtual)

3 3 Focal Length (f) :, This is the distance between the optical centre and the principal focus. Since lit can come from either side of a diverging or converging lens they have two principal foci and two focal lengths, one on either side of the lens. a, "' \. FocalPiane This is the plane which is perpendicular to the principal axis and passes through the principal focus. If a parallel beam oflight passing through the lens is not parallel to the principal axis, it will still be brought to a focus; however the focus would not be on the principal axis, but will be on the focal plane of the lens. Linear Magnification (m)._. -': This is the factor by which the size of the object has been magnified by the lens in a direction which is perpendicular to the axis of the lens. Linear magnification can be calculated by using: m= image height = I object height O OR m= image distance object distance u

4 4 Images Magnification Effect on Image Compared to the Object! greater than 1 equal to 1 less than 1 - Difference between Real and Virtual Images Virtual Image Real Image The image is observed at a point from which rays seem to come from (no light rays actually pass through tl}is imaginary point).. No image is obtained on a screen placed at this point.. \ \ k ' >P,,J.( t , \,

5 5 Virtual Image Produced by Real Images Focused on l A magnifying glass placed closed to an object. ( l,,.,, ( Plane mirror. Spectacle used for correcting long and short sight. c /)f \ ( (. ' ' ( t f- ' 1\. i Differences between Real and Images formed by Lenses Virtual Images Real Images Cannot be formed on a screen. Are not formed by the intersection of real rays. They are erect. Are on the same side of the lens as the object.

6 6 Ray Diagrams Constructive Ray Diagrams for Convex (Converging) Lens Diagram A Light rays that pass through the optical centre of the convex lens are undeviated. Diagram B Light rays that are parallel to the principal axis converge at the principal focus after passing through the convex lens. Diagram C Light rays that pass through the principal focus emerge parallel to the principal axis after passing through the convex lens.

7 Constructive Ray Diagrams for Concave {Diverging) Lens Diagram A Light rays that pass through the optical centre of the concave lens are undeviated. Diagram B Light rays that are parallel to the principal axis appear to come from the principal focus after passing through the concave lens. Diagram C Light rays that appear to pass through the principal focus emerge parallel to the principal axis aftr passing through the concave lens.

8 8 1. An object 6cm high is placed 20cm away from a converging lens of focal length Scm. Find a scale drawing the position, size and nature of the image; the object should be drawn at right angles to the principal axis. 2. An object 2cm high is placed 5cm away from a converging lens of focal length 3cm. Draw a ray diagram of how the image is formed, and find the position and height of the image. k \ _ An object 3cm tall is placed 25cm away from a converging lens, with a focal length of 15cm. Find by a scale drawing the position and size of the image formed An object 4cm tall is place 40cm away from a converging lens, with a focal length of 20cm. Use a ray diagram to describe the image formed. Read and make notes on the following: PFC Page Rays entering the eye Structure and the Action of the Eye (Include a fully labeled diagram) Vision and Diseases (Long, shortsightedness and other defects) Opitical Instruments (Lens and Pinhole camera) Compare the Eye to the Cameras Magnifying glasses Projectors D. Wfliteha!l

9 9 FORMULAS Linear Magnification (m) This is the factor by whicn the size of the object has been magnified by the lens in a direction which is perpendicular to the axis of the lens. Linear magnification can be calculated by using: m= image height = I object height o OR m= image distance = v object distance u Power of Lens (F) When a lens is powerful, it deviates rays more precisely. It will converge (or diverge) parallel rays to (or from) a focus in a short distance (a powerful lens will have a short focal length). We can calculate the power of a lens by using: F = 1 f (always in meters) The power of lens is measured in dioptre (D). [NB. 1D is the power of the lens of focal length 1 metre. Diverging lens have a negative power.] Lens Formula We can use the lens formula to determine the focal length of a lens. OR 1 f = 1 u + 1 v f = uv u + v When using the lens formula distances of real objects and images are given as positive values, whereas distances of virtual objects are given as negative values.

10 1 Examples 1. A building is 6m high, and it is 80m from a converging camera lens. If the camera forms an image which is 6mm high, (a) What is the magnification? (b) How far must the camera film be behind the lens for the image to be formed? 2. A film projector is used to produce a real image on a screen. The screen is 30m away from the lens, and 3.Ocm from the lens of the projector. Calculate (a) the magnification (b) the height of the image on the screen if the object on the film is 5mm high.

11 11 3. A converging lens with a power of D is used in a pair of spectacles. Calculate: (a) The focal length of the spectacles (b) The position of the image formed, if an object is 25cm from its optical centre. (c) fully describe the image formed. 4. A diverging lens has a power of -2.0 D, calculate: (a) the focal length of the diverging lens (b) the position and describe the nature of the image it forms, if an object is 2m away from the lens.

Lenses. A lens is any glass, plastic or transparent refractive medium with two opposite faces, and at least one of the faces must be curved.

$Lenses. A lens is any glass, plastic or transparent refractive medium with two opposite faces, and at least one of the faces must be curved.$ PHYSICS NOTES ON A lens is any glass, plastic or transparent refractive medium with two opposite faces, and at least one of the faces must be curved. Types of There are two types of basic lenses. (1.)