Light

Light: A form of energy that gives us the sensation vision to our eyes is called lights.

Lens: The transparent denser medium bounded by one or two curved surface is called lens. Types of lens

1. Convex lens
2. Concave lens

Convex lens: The lens which is thicker at the middle and thinner at the edges is called convex lens. Convex lens is of following types.

1. Biconvex lens
2. Plano-convex lens
3. Concavo-convex lens

Concave lens: The lens which is thinner at the middle and thicker at the edges is called concave lens. Types of concave lens

1. Biconcave lens
2. Plano concave lens
3. Convexo-concave lens

Q. Which lens is also called converging lens? Why?

Convex lens is called converging lens because parallel rays of light passing through it converge at a paint after refraction.

Q. Which lens is also called diverging lens? Why?

Concave lens is also called diverging lens because parallel rays of light passing through it appears to diverge from a point of after refraction.

Lens as prism / prismatic behaviour of lens

Lens is made by combination of many small angle prisms. In case of convex lens the base of each prism lies towards its edge. In case of concave lens, when light passes through prism, it deviates towards its base. Light deviates towards its centre in convex lens but light deviates towards its edges in concave lens. This is called prismatic behaviour of lens.

Terminologies used in lens:

Centre of curvature (c): The centre of sphere a part of which forms lens is called centre of curvature (c)

Radius of curvature (r): The radius of sphere a part of which forms lens is called radius of curvature (r)

Principal axis: An imaginary line passing through centres of curvatures of lens is called principle axis

Principal focus (F): A point in principal axis where parallel rays of light converge or appear to converge from after refraction is called principal focus.

Optical centre (o): Geometrical centre of lens is called optical centre (o)

Focal length (f): The distance between optical centre and principal focus of lens is called focal length.

Unit of focal length: Its S.I. unit is meter (m).

[Note: The focal length of convex lens is positive (+ve), focal length of concave lens is negative (-ve)]

Power of lens (P): The converging and diverging capacity of lens is called power of lens (P). It is measured as reciprocal of focal length.

ie Power = 1/focal length P=1/f

Unit of Power: Its S.I unit is Dioptre (D)

Rules:

1. When light passes parallel to principal axis it always refracts through principal focus and vice versa.

1. The ray of light passing through optical centre doesn’t deviate.

Ray diagram of image formed by convex lens.

1. Object at infinity

Nature of image formed

1. The image is formed at F.
2. The image is highly diminished.
3. It is real and inverted.

2. Object beyond 2F

Nature of image formed

1. The image is formed between F and 2F.
2. Image is diminished.
3. Image is real and inverted

3. Object at 2F.

Nature of image formed.

1. Image is formed at 2F
2. Size of image is same of object.
3. Image is real and inverted.

4. Object between F and 2F.

Nature of image formed

1. Image is formed beyond 2F.
2. Image is magnified.
3. Image is real and inverted

5. Object at F

Nature of image formed

1. Image is formed at infinity.
2. Image is highly magnified.
3. Image is real and inverted.

6. Object between F and lens.

Nature of image formed

1. Image is formed at same side of object.
2. Image is magnified.
3. Image is virtual and erect.

Summary of ray diagram:

Ray diagram of image formed by concave lens

Nature of image formed

1. Image is formed between F and lens at same side.
2. Image is diminished.
3. Image is virtual and erect.

Q. Differentiate between real and virtual image.

Magnification (m): The ratio of size of an image to size of object is called magnification (m).

i.e. magnification = size of image / size of object

m = I / O

It is also defined as  ratio of image distance to object distance.

i.e. magnification = Image Distance / Object Distance

Q. Magnification of image is greater than 1. What does it means?

It means that size of image is greater than size of object.

Uses of Convex lens:

1. It is used to correct long sightedness of eye.
2. It is used as magnifying glass.
3. It is used in optical instruments like camera, telescope, microscope etc.

Uses of Concave lens:

1. It is used to correct short sightedness of eye.
2. It is used in telescope.

Human eye

An organ that helps us to see around is called human eye.

1. Sclera: It is the outermost layer of the eye
2. Choroid: It is the middle layer of eye which is dark in colour.
3. Retina: It is the innermost layer of eye formed of nerve cells where image is formed
4. Optic nerve: It is the nerve fibre that carries impulse of vision from eye ta brain.
5. Cornea: The front part of eye which is transparent is called cornea.
6. Ciliary muscle: It is the muscle that supports lens in eye and makes it thicker and thinner as per need.
7. Lens: It is the convex lens formed of crystalline protein.
8. Iris: It is the muscle that control amount light entering the eye.
9. Pupil: It is the gap between iris.
10. Aqueous humor: It is the transparent liquid in eye in front of lens.
11. Vitreous humor: It is the transparent liquid in eye behind the lens.

Accommodation of eye: The ability of eye to adjust focal length of lens so that the images of objects are exactly formed at retina is called accommodation of eye.

Near point: The closet point from an eye where an object ie seen clearly is called near point (25 cm). Far point It lies at infinity

Range of vision: The region between near point and far point where an eye can see the objects clearly is called range of vision.

Defect of vision: The inability of eye either to see an object at near clearly or to see an object at far clearly is called defect of vision. Types of defect of vision:

1. Myopia (Short sightedness)
2. Hypermetropia (Long sightedness)

Myopia

The defect of vision in which an eye can see an object at near clearly but not the object at far is called myopia. Causes

1. Due to thick lens (short focal length).
2. Due to elongation of eye ball.

Correctness/remedy of myopia.

1. This defect can be corrected by using concave lens of suitable power in-front of eye.

Hypermetropia

The defect of vision in which an eye can see an object at far clearly but not the object of near is called hypermetropia. Causes:

1. Due to thin lens (long focal length).
2. Short-eye ball

Correctness/remedy of hypermetropia.

1. This defect can be corrected by using convex lens of suitable power in-front of eye.

Solved exercise

a. What is the nature of image formed by concave lens in terms of size and orientation.

Image formed by concave lens is diminished and virtual.

b. When does a convex lens form virtual image?

Convex lens forms virtual image when object lies between F and lens.

c. What is the meaning of saying “The power of a lens is one diopter” ?

Meaning of “The power of a lens is one diopter” is that the focal length of lens is one meter.

d. How do we find out whether the size of an image is bigger or smaller than size of an object?

It is found by taking ratio of size of image to size of object.

e. Why is the power of concave lens negative?

The power of concave lens is negative because it is the diverging lens so that light appears to diverge from a point called principle focus.

f. where do we get the image from convex lens when the object is kept at infinity?

We get the image at Principal focus.

g. How do we see an object with the help of your eyes?

When the light from an object enters our eye though lens, its image is formed by lens on the retina which produces impulses of vision. This impulse is carried up to brain by optic nerve and our brain sees the objects.

h. Write two differences between convex and concave lens.

i. How does the use of lens correct the defect in vision?

The lens used to correct the defect of vision either converges or diverges the rays of light passing through it so that image of the object is exactly formed at retina. In this way, use of lens correct the defect in vision.

j. Write two differences between image formed by convex and concave lens.

k. What type of defect is shown the figure? Write its causes.

Myopia/Short sightedness is shown in figure. Its causes are:

1. Due to thick lens
2. Due to elongation of eye ball.

l. A girl sitting at the last bench in a classroom cannot read clearly anything written on black board. What kind of defect does she have? How can it be corrected?

She has short sightedness. It can be corrected by using concave lens of suitable power in front of eye.

m. A convex lens has a focal length of 20 cm. What is the power of the lens?

f = 20cm = 20/100 m =0.2 m

P = ?

We know,

P = 1/f

P = 1/0.2

P = 5 D

Therefore, Power of lens is 5D

n. What is the function of vitreous humor?

Functions of vitreous humor are:

1. It makes the eyeball rigid.
2. It acts as refractive medium.

o. Apex complains that he can’t see the letters written on blackboard when he sits on the last bench of his class.

i. What type of defect of vision does he have?

He has myopia.

ii. What are its causes?

Its causes are:

1. Due to thick lens.
2. Due to elongation of eye ball.

iii. How can it be corrected? Draw the necessary diagram.

It can be corrected by using concave lens of suitable power in front of eye.

p. Apex can read letters written on board only when she sits on last bench. Answer the questions.

i. What is the defect of her vision?

Defect of her vision is long sightedness.

ii. Mention the causes of defect.

The causes of defect are:

1. Due to thin lens.
2. Short eyeball.

iii. What type of lens must she use in her spectacles for correction. Why?

She most use convex lens of suitable power in her spectacles for correction which creates virtual image of nearby object of farther place after refraction so that person suffering from long sightedness can see clearly.

q. What type of lens has -1.5 D power? For what type of defect of vision is it used to correct? Draw defected and corrected figures. Also mention the causes of such defects.

Concave lens has -1.5 D power. It is used to correct short sightedness. Its cause are:

1. Due to thick lens.
2. Due to elongation of eye ball.

r. Differentiate between Short and long sightedness.

s. Convex lens is called positive lens.

Convex lens is called positive lens because it converges actual rays of light after refraction.

t. People suffering from hypermetropia ore advised to wear spectacles of suitable positive power.

Image of object at near forms behind the retina of eye suffering from hypermetropia and spectacles of suitable positive power converges rays of light so that image of that object forms exactly at retina and object is seen clearly. Therefore, people suffering from hypermetropia are advised to wear spectacles of suitable positive power.

u. Solve the numerical.

1. A convex lens has focal length of 5O cm. Calculate its power.

f = 50cm = 50/100 m =0.5 m

P = ?

We know,

P = 1/f

P = 1/0.5

P = 2 D

Therefore, Power of lens is 2D

2. A concave lens has focal length of 40 cm. Calculate its power

f = -40cm = -40/100 m =-0.4 m

P = ?

We know,

P = 1/f

P = -1/0.4

P = -2.5 D

Therefore, Power of lens is -2.5 D