Unit-5 | LIGHT | Class-10

Overview: Light is a form of energy that gives us the sensation of vision. It is because of light we can see this world. If there would be no light in this universe, we could see nothing. In this notes, we will discuss about light, lenses, diseases, remedy, object and image.

Light:

Light is a form of energy that gives us the sensation of vision. All the objects that can produce light are called sources of light. Light always travels in a straight path in any medium.

Lens:

Lens is a piece of glass or any other transparent refracting medium with curved sides to either diverse or converse the rays of light.

There are two types of lenses:
  • Convex lens or converging lens
  • Concave lens or diverging lens


Convex lens or converging lens:

Convex lens is that type of lens which bulges out at the centre and is thin at the sides. This lens is called converging lens because it converges the beam of light falling to it to a particular point after refraction. Example: magnifying glass, human eye lens, etc.

Concave lens or diverging lens:

Concave lens is that type of lens which is thinner at its centre and thick at its sides. It is called a diverging lens because it diverges the beam of light falling to it from a fixed point.

Terms related to lens:
  • Centre of curvature: (c or 2F):
The centre of the two spheres that form a lens is called the centre of curvature. It is represented by c or 2F.
  • Optical centre:
The central centr of a lens is called the optical centre.
  • Principal axis:
The straight line that passes through the two centre of curvature and the optic centre is called principal axis.
  • Focus:
It is a point on the principal axis at which the parallel beam of incident light meet or appear to meet after refraction through the lens.
  • Focal length (f):
The distance between the focus (F) and optical centre (O) of a lens is called the focal length.
  • Radius of curvature (R or 2f):
The distance between the optical centre (O) and centre of curvature of the lens is called the radius of curvature.
R = 2 f        , 'f' represents focal length.


Note: Real image is inverted and virtual image is errect.


Formation of images by a convex lens:

When the object is at infinity:


Characteristics of image:

Image is formed at focus (F).
Image is inverted, real and highly diminished.


When the object is beyond 2F:


Characteristics of image:

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


When the object is at 2F:


Characteristics of image:

Image is formed at 2F. The image is same real, inverted and of safe size of the object.


When the object between F and 2F:



Characteristics of image:

Image is formed beyond 2F. The image is real, inverted and enlarged.


When the object is at F:


Characteristics of image:

Image is formed at infinity. The image is real, inverted and highly enlarged.


When the object between F and O:


Characteristics of image:

Image is formed behind the object of same side. The image is virtual erect and enlarged.

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Formation of images by a concave lens:

Object in between infinity and focus:


Characteristics of image:

Image is formed between focus and optical centre on the same side of the object. The image formal is virtual, erect and diminished.

Object at infinity:


Characteristics of image:

Image is formed at focus (F). The image is virtual, erect and highly diminished.

Power of a lens:

The ability of a concave lens to diverge and that of convex lens to converge the rays of light falling on it is called its power.
Mathematically, Power (P) = 1/ focal length(f)

The S.I. unit of power of lens is Dioptre (D). The power of a convex lens is always positive and power of concave lens is always negative.

Magnification:

The magnification is the process of enlarging the apparent size of object, not the physical size of object.
Magnification produced by lens(m) = size of image (I) / size of object (O)

We have,
m = I/ O = v/ u
Lens formula is 1/f  = 1/u + 1/v
Here, the representation of the symbols are as follows:
f = focal length
u = object distance
v = image distance

Optical Instruments:

The instruments which can produce image of an object with the help of lenses, prisms and mirrors are called optical Instruments.

Camera:

It is an optical instrument used to take the photographs of various object on the photographic film. It produces real image. It used convex lens. It used a diaphragm to detect light.

Human eye:

Human eye is a natural optical instrument that consists of a natural convex lens. It produces real image. It used live cells to detect light.

Real image:

 This type of image is inverted and is formed by convex lens. It can be obtained on a screen.

Virtual image:

This type of image is erect and is formed by concave lens. It cannot be obtained on a screen.


Power of accommodation:

Power of accommodation is the process by which the eye changes its optical power ( or adjusts focal length) to maintain a clear image as the distance increases or decreases.

The minimum distance at which the objects can be seen by our eyes distinctly without any pressure or strain to the eye is called near point of the eye. It is about 25 cm.

The maximum distance at which the objects can be seen by our eyes clearly without any pressure or strain is called far point of the eye. This distance is infinity for a normal eye.


Defects of vision:

The human eye may loose the power of accommodation sometimes. This state is called the defect of vision where a person cannot comfortably see the objects.
There are two main common defects of vision:
  • Myopia or near sightedness or short sightedness
  • Hypermetropia or long sightedness or far sightedness
Source: Shutterstock.com


Myopia:
Myopia is a defect of vision in which a person can see near objects distinctly but cannot see the distant objects.

Causes:
1. Shortening of the focal length
2. Elongation of the eyeball

Remedy:
To correct this defect, the patient should use a concave lens of suitable power. This will bring the image back onto the retina.

Hypermetropia:
Hypermetropia is a defect of vision in which a person can see far distant clearly but cannot see near object distinctly.

Causes:
1. Elongation of the focal length
2. Shortening of eyeball

Remedy:
The patient should use glasses with concave lens of required power to provide additional focusing power required to form the image on retina.


Microscope:

Microscope is an optical instrument which is used to magnify the smaller objects.
There are two types of microscope:
1. Simple microscope:
2. Compound microscope

Telescope:

Telescope is an optical instrument which is used to see the distant objects clearly.
There are two types of telescopes:
1. Terrestrial telescope
2. Astronomical telescope

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Numerical problems:

1. A convex lens has a focal length 0.2m. what is the power of the lens?
Solution:
We know,
Power of the lens(P) = 1/f
= 1/ 0.2
= 5 Dipotre

2. Determine the size of the real image of an object of 2cm high placed 40cm away from convex lens. The real image is formed 80 cm away from the lens. 
Solution:
Since the image formed is real, we have,
Height of image/ height of object = -v/u
or, Hi / 2 = -80/40
or, Hi / 2 = -2
So, Hi = -4cm
So, the size of real image of the object is -4cm.


3. Prem wears spectacles of power +1.5D, calculate the focal length and find the lens used.

Solution:
We know,
P = 1/f
or, 1.5 = 1/f
So, f = 0.66m

Therefore, the lens used is convex lens because the focal length of the convex lens is always taken positive.

#PracticeYourself
An object is placed at a distance of 29m from a convex lens of focal length 15 cm. 
a) find the image distance.
b) is the image real or virtual?
c) is the image erect or inverted?




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