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### Transcript of 5.4 Lenses

Understanding Lenses

Understanding LensesLenses are made of __________ transparent material such as glass or clear plastics. They have two faces, of which at least curved one is ______

Types of lenses:(a) ______ lens, also known as Convex thicker converging lens. It is ________ at the centre of the lens.

Biconvex

Plano convex

Converging meniscus

(b) ________ lens, also known as Concave thinner diverging lens. It is _________ at the centre of the lens.

Biconcave

Plano concave

Diverging meniscus

Focal Point and Focal Length of a lens (convex lens)

1. When light rays which are parallel and close to the principal axis inwards strikes on a convex lens, they are refracted ________ and converge to a focal point point, F on the principal axis. This point is a _____ _______ of the convex lens.

Focal Point and Focal Length of a lens (concave lens)

2. When light rays are parallel to the principal axis fall on a concave lens, outwards they are refracted ___________ and appear to diverge from the focal point, F, on the principal axis.

F

F f

f

3. The focal point, F is a point on the principal axis where all rays are close converge and parallel to the axis that _________ to it after passing through a convex diverge lens, or appear to ________ from it after passing through a concave lens. 4. The focal length, f is the distance between the focal point and the optical centre ____________________.

5. The optical centre, C is the geometric centre of the lens. It is the point deviation through which light rays pass through without __________.

6. The principal axis is the line passing through C and is _____________ to perpendicular the lens.

The power of the lens is equal to the reciprocal of its focal length in metres. The power of lens is related to its focal length by the expression: Power of lens =1 f ( meter )

Diopter

convex Power for ___________ lens is positive, and power for _________ lens is negative. concave1 f ( meter )

Example 1:

The focal length of a concave lens is given as 30 cm. What is the power of the lens?Solution:

Power of lens =

1 f ( meter )

=

1 0 .3

= -3.33 D

Example 2:

The power of a lens is labeled as -10D. Name the type of the lens and determine the focal length of the lens.Solution: Type of lens: Concave

1 Focal length = Power=

1 10

= -0.1 m = - 10 cm

Complete the table below.

Power of the lens/ D +20- 6.67

Focal length/ cm5

-15- 20

-55

+20

Images Formed by LensesConstruction rules for image formed by Convex LensRule 1: A ray parallel to the principal axis is refracted through the focal point, F Rule 2: A ray passing through the focal point is refracted parallel to the principal axis. Rule 3: A ray passing through the optical centre, C travels straight without bending.

The point of intersection is the position of the image. The images formed by a convex lens depend on the object distance, u.

Construction rules for image formed by Concave LensRule 1: A ray parallel to the principal axis is refracted as if it appears coming from the focal point, F which is located at the same side of the incident ray. Rule 2: A ray passing through the focal point is refracted parallel to the principal axis. Rule 3: A ray passing through the optical centre, C travels straight without bending.

The point of intersection is the position of the image. The images formed by a concave lens are always virtual, upright and diminished.

A ______ image can be captured on a screen. real virtual A ______ image cannot be captured on a screen

Magnification, mThe size of an image formed by lens varies with the position of the object. The linear magnification, m is defined as:Size of image, hi Size of object, ho m=

=

Image distance, v Object distance, u

hi ho

=

v u

The image is __________ if m > 1 magnified The image is __________ if m < 1 diminished the same as the object The image is ______________________ if m = 1

Characteristic of image formed by a convex lens(u = ) Characteristics of image:

Object at infinity

- Real- Inverted - DiminishedF 2F F

- Position of image:At F (v = f)

Characteristic of image formed by a convex lensObject beyond 2F (u > 2f) Characteristics of image: - Real - InvertedF 2F F 2F

- Diminished - Position of image: Between F and 2F ( f < v < 2f)

Characteristic of image formed by a convex lensObject at 2F (u = 2f) Characteristics of image: - Real2F 2F F F

- Inverted- same size as the object (m=1) Position of image: At 2F ( v = 2f)

Characteristic of image formed by a convex lens

Object between F and 2F (f < u < 2f) Characteristics of image: - Real - Inverted2F F F 2F

- Magnified (m > 1) Position of image: Image beyond 2 F ( v > 2f)

Characteristic of image formed by a convex lensObject at F (u = f ) Characteristics of image: - Cannot be determined

F

F

Position of image: Image at infinity.

Characteristic of image formed by a convex lensObject Between F and C (u 2f) Characteristics of image: - virtual - erect2F F F

- diminished Image on the same side of the object.

Characteristic of image formed by a concave lensObject between F and C (u